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Application Software and Monitoring Tools InterSism Seismic refraction interpretation and Down-Hole seismic measurements User Guide GeoSoft by Engineer Giorgio Scioldo Corso G. Matteotti n. 12 10121 Torino (TO) tel. 0115624689 - fax 0115175007 www.geoandsoft.com – [email protected]

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Page 1: Application Software and Monitoring Tools › manuali › english › InterSism.pdf · 2018-07-23 · Application Software and Monitoring Tools . InterSism Seismic refraction interpretation

Application Software and Monitoring Tools

InterSism Seismic refraction

interpretation and Down-Hole seismic measurements

User Guide

GeoSoft by Engineer Giorgio Scioldo

Corso G. Matteotti n. 12 10121 Torino (TO)

tel. 0115624689 - fax 0115175007 www.geoandsoft.com – [email protected]

Page 2: Application Software and Monitoring Tools › manuali › english › InterSism.pdf · 2018-07-23 · Application Software and Monitoring Tools . InterSism Seismic refraction interpretation

ISOMAP & ROTOMAP - User Guide Summary • i

Summary

1. Introduction to programme 1

Introduction to InterSism programme .................................................................................. 1

2. Programme installation 5

Requirements for programme running................................................................................. 5 Programme installation........................................................................................................ 5

3. Programme protection 9

Programme protection......................................................................................................... 9 Protection key on USB port ......................................................................................................9

4. Use of the programme in the local network 11

Use of the programme in a network .................................................................................. 11

5. Programme use 13

Data type definition............................................................................................................ 13 Surface seismic ................................................................................................................. 13

“Project Definition” Button.......................................................................................................13 "First Arrivals Time Search" Button ........................................................................................18 "First Arrivals Time Table" Button...........................................................................................22 “Travel Time Graph Calculation” Button .................................................................................22 “Depth Calculation” Button......................................................................................................25 “Printout Production" Button ...................................................................................................26

Down-hole seismic ............................................................................................................ 30 “Project Definition” Button - DH ..............................................................................................30 "First Arrivals Search" Button - DH .........................................................................................35 "First Arrivals Table" Button - DH ...........................................................................................39 “Travel Time Calculation” button - DH ....................................................................................39 “Depth Calculation” Button - DH .............................................................................................41 “Printout Production” Button - DH...........................................................................................42

6. Theoretical notes 44

Seismic refraction.............................................................................................................. 44 The seismic behaviour of P waves .........................................................................................44 Applications.............................................................................................................................45 Problems and limitations of seismic refraction .......................................................................46 Interpreting results ..................................................................................................................46

7. Bibliography 51

Main references................................................................................................................. 51

Contacts 52

Contacts ............................................................................................................................ 52

Page 3: Application Software and Monitoring Tools › manuali › english › InterSism.pdf · 2018-07-23 · Application Software and Monitoring Tools . InterSism Seismic refraction interpretation

INTERSISM - User Guide 1. Introduction to programme • 1

1. Introduction to programme

Introduction to InterSism programme InterSism is a software programme designed to follow the entire processing of seismic data using refraction seismic method acquisition. The programme can be acquired in two different versions: one for surface seismic refraction only and the other including the treatment of down-hole seismic measurements. This software is able to follow every developmental phase completely automatically, leaving the user the possibility of intervening manually to ensure complete control of the calculation process. In the surface seismic survey interpretation, the InterSism programme extracts field measurements directly from different format files, requesting possible completion if some information, for example geophone or shooting, were not stored in the instrument. The first phase of processing consists of determining the First Arrivals. The programme uses differing methodologies in an integrated manner, from cross-correlation to wavelet-analysis, repeating the procedure to refine results obtained with continuous checks of compatibility between times identified and those derived by the interpolation of adjacent geophones. The First Arrivals can then be verified and eventually corrected manually, operating directly on original signals.

The second phase involves the calculation of travel times. InterSism can process up to nine shots, three of which are internal to spreading, and uses an analytic minimum procedure to ensure recognition of travel times, which by far offer the best correlation coefficient with the previously determined first arrival times.

Page 4: Application Software and Monitoring Tools › manuali › english › InterSism.pdf · 2018-07-23 · Application Software and Monitoring Tools . InterSism Seismic refraction interpretation

INTERSISM - User Guide 1. Introduction to programme • 2

It should be noted that, in order to properly perform later processing, the shooting carried out at the array end contains travel time segments relative to all layers crossed. In the first phase, the programme works on single travel times, assuming that the number of layers is that chosen by the user, and determines every possible distribution of the break point. It then compares the results obtained on different travel times in order to identify, among all possible break point combinations, the one that minimises the mean square deviation of the speed of each layer, thus assigning travel time segments to the correct refractors. Also in this case, it is possible to intervene manually both in terms of number of related layers from each shoot and in terms of break point position. Following the travel time construction and the determination of the propagation speed of the seismic signal in different ground layers, InterSism passes to the Generalized Reciprocal Method (GRM) to identify the refractor pattern. The software uses an automated procedure which, starting from a value of XY of an attempt equal to zero (which obtains the conventional time-depth function) and from the presumed depth (previously calculated beneath each geophone), experiments with different XY values until it determines optical distance XY; that is, the distance for which the direct and inverse rays emerge in the vicinity of the same point on the refractor. In the case in which only one or two shots have been performed, the programme uses the intercept method to determine the refractor pattern.

Page 5: Application Software and Monitoring Tools › manuali › english › InterSism.pdf · 2018-07-23 · Application Software and Monitoring Tools . InterSism Seismic refraction interpretation

INTERSISM - User Guide 1. Introduction to programme • 3

Down-hole seismic Down-hole seismic survey is among the simplest and inexpensive methods, and is therefore the most utilised. The seismic source is positioned on the surface at a fixed distance from the mouth of the borehole. The travel time of the first arrivals of the generated seismic waves are measured at regular depth intervals by means of a geophone moved gradually inside the hole. In the seismic survey interpretation in the hole, InterSism extracts field measurements in SEG2 format files only, then requiring completion of missing data (distance of the energetic source and geophone depth in correspondence to different readings). Also in this case, the first processing phase consists of determining the First Arrivals, both on wave P and on wave S, whose entries are automatically grouped by the software in three different files. The programme uses differing methodologies in an integrated manner, from cross-correlation to wavelet-analysis, repeating the procedure to refine results obtained with continuous checks of

Page 6: Application Software and Monitoring Tools › manuali › english › InterSism.pdf · 2018-07-23 · Application Software and Monitoring Tools . InterSism Seismic refraction interpretation

INTERSISM - User Guide 1. Introduction to programme • 4

compatibility between times identified and those derived by the interpolation of adjacent geophones. The First Arrivals can then be verified and eventually corrected manually, operating directly on original signals. The second phase consists of the calculation of the travel time. Using formulations similar to those previously described, the software identifies the travel time and determines the speed and thickness of encountered layers. Following the construction of the travel times and determination of the seismic signal propagation speed in different ground layers, InterSism calculates the characteristic elastic modules, having received indication from the user of the density of lithotypes encountered during drilling, expressed in Kg/m3. As is well known, the compression waves P produce a vibration in the ground in the direction of the vibration itself, while in the shear waves S, the vibration is transverse with respect to the direction of propagation. While wave speed P is due to compressive strength, shear wave speed depends on the shear strength of the material. Knowing, therefore, the speed of waves P, of waves S and the density of the ground, it is possible to determine the dynamic elastic modules: Deformability modulus (E): also known as Young's modulus or elasticity. Expresses the relationship between stress σ and corresponding deformation ε generated along the application direction of the same stress:

εσ

=E

Shear modulus (G) or rigidity modulus: expresses the relationship between the shearing force exerted parallel to the cutting surface and corresponding deformation

ατ

=G

Poisson's Ratio : expresses the relationship between the deformation perpendicular to the direction of stress application and longitudinal direction

ν 'εε

εε

=ν'

Volume modulus K: expresses the relationship between hydrostatic pressure and the deformation undergone by the material. It is also called compressibility modulus or Bulk modulus. Assuming a homogeneous, isotropic and perfectly elastic medium, the following relationships between the four modules exist:

GK3KG9)21(K3)1(G2E+

=ν−=ν+=

)1(2)21(K3

EK9EK3

)1(2EG

ν+ν−

=−

=ν+

=

)21(3)1(G2

)EG3(3EG2

)21(3EK

ν−ν+

=−

=ν−

=

)GK3(2G2K3

K6EK3

G2G2E

+−

=−

=−

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INTERSISM - User Guide 2. Programme installation • 5

2. Programme installation

Requirements for programme running. • Microsoft® Windows® XP, Windows® Vista, Windows® 7, Windows® 8 • 100 MB of available hard-disk • Printer: a driver for at least one printer, even if not connected to the PC, must be installed

for correct software functioning.

Programme installation To install, start installation programme INTERSISM32SETUP.EXE from the CD-ROM or from the folder in which it was downloaded.

Press NEXT to start installation.

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INTERSISM - User Guide 2. Programme installation • 6

Press NEXT after reading the license agreement.

Select the folder in which you want to create the INTERSISM directory and click NEXT.

Press NEXT to create the group "INTERSISM32" in the Start menu.

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INTERSISM - User Guide 2. Programme installation • 7

Choose whether to create the programme icon on the Desktop.

Press INSTALL to proceed with installation...

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INTERSISM - User Guide 2. Programme installation • 8

After installing the files, press NEXT after reading the instructions

Press FINISH to exit from installation and start up the programme.

Next, to start up INTERSISM32, click Start, then choose the group INTERSISM32 from the Programmes menu and click INTERSISM32.

Page 11: Application Software and Monitoring Tools › manuali › english › InterSism.pdf · 2018-07-23 · Application Software and Monitoring Tools . InterSism Seismic refraction interpretation

INTERSISM - User Guide 3. Programme protection • 9

3. Programme protection

Programme protection The programme will operate only if the key is connected to the port and if installation procedures have been performed successfully. If automatic installation procedures were not successfully performed, you can attempt to manually install drivers, according to the procedures described below.

Protection key on USB port

Installation warnings: since protection key management requires the installation of a driver that is copied to a programme sub-folder during installation, THE PROGRAMME MUST BE INSTALLED BEFORE CONNECTING THE PROTECTION KEY TO THE USB PORT.

SmartKey USB installation in Windows systems is integrated with the Add New Hardware Wizard. The installation procedure is therefore as follows: • Install the programme, which will install the drivers. • Insert the SmartKey USB in the computer USB. • Select Next from the first Add New Hardware Wizard installation window. • Select End from the second Add New Hardware Wizard installation window. If automatic installation procedure is not successfully completed (for example because the hardware key was connected before having completed programme installation), you can manually perform installation, proceeding as follows:

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INTERSISM - User Guide 3. Programme protection • 10

1. Start up Windows and LOG IN as ADMINISTRATOR. 2. Select the EUTRON folder in the installed programme folder and start up the SDI.EXE programme (SmartKey Driver Installer). SDI offers three options for three types of installation and uninstallation: SmartKey Parallela, SmartKey USB and Global Security System. The SDI graphic interface offers three options:

• Install: installs the selected type of drivers • Uninstall: uninstalls the selected type of drivers • Delete: uninstalls drivers without checking for any dependencies.

3. Select the "USB" board and click <Install>. 4. The completion of driver installation is signalled by the appearance of a window: click on <OK> to exit. If installation has not been performed successfully, SDI will open a pop-up window which will detail which operations failed and the number of operating system errors.

When connecting the protection key, if the operating system starts the installation of new hardware anyway, you must indicate the EUTRON folder as the location for finding necessary drivers. For further information regarding protection key hardware, visit http://www.eutron.com.

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INTERSISM - User Guide 4. Use of the programme in the local network • 11

4. Use of the programme in the local network

Use of the programme in a network Programmes must be used in a local network without having to move the hardware key from one computer to another. It is also possible to purchase multiple licenses for simultaneous programme use at multiple locations. "Servers" are those processor units to which the hardware key is physically connected. "Clients" are the processor units that remotely require authorisation from the server in order to operate in non-demo mode. The programme must first of all be installed on the Server (including hardware key drivers) and on each Client (in demo mode). The KEYSERVER.EXE hardware key management programme must therefore be installed on the server, using the installation programme available on the CD-ROM or from our website at address: http://www.geoandsoft.com/download/KeyServerSetup.exe Once KEYSERVER.EXE starts up, the hardware key management programme window will appear.

Click on “Create Program Configuration File” and then browse the network, searching for the programme for which you want to enable network use. The path could for example be \\COMPUTER03\C\PROGRAMS\INTERSISM32\INTERSISM32.EXE Obviously, in order to carry out this operation, the INTERSISM programme folder on the Client must be temporarily read/write shared. The KeyServer can be disconnected after configuration operations have been completed. Since the KeyServer programme simply creates a text-only file in the INTERSISM32 file, contained in the IP address of the location acting as Server, alternatively to the previously explained procedure, you can also perform this operation manually: create a text document with the name IPKEYSERVER.TXT, then type the Server IP address in the document and save. At this point, you can copy this file on a disc and transfer it into the programme folder on the Client computer.

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INTERSISM - User Guide 4. Use of the programme in the local network • 12

From this point on, and if the KEYSERVER.EXE programme is started up, the INTERSISM32.EXE programme installed on the "computer03" PC will be fully operational. NOTE: the simultaneous use of programmes on more than one location is subject to the purchase of multiple licenses. Otherwise, programmes can be carried out in non-demo mode on one single processor unit at a time.

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INTERSISM - User Guide 5. Programme use • 13

5. Programme use

Data type definition When the programme opens, you must first of all choose the type of data to be analysed, clicking on <Surface> or <Down-Hole>. Necessary information will be requested depending on the type selected.

Surface seismic Click on <Surface> to access the programme section dedicated to the processing of seismic refraction surveys carried out on surfaces.

“Project Definition” Button

Click on this button to access the window to create a new project or to open an existing project.

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INTERSISM - User Guide 5. Programme use • 14

Create a new project

Allows the user to open a new project, eliminating any data in the memory. The programme does not offer a specific command for saving data, since data is automatically saved at the end of each operation. To start a new project, it is first of all necessary to give it a name. Click on <Create a new project> and select the type of data to be treated: SEG-2, SU (Unix Seismic), SEG-Y, DTA or else first arrival data entered directly into the table.

In the event that you are using a programme version provided by the manufacturer of the instrument together with the seismograph, the data type selection window will not be presented, as the programme will be automatically set to process data produced by the instrument itself. These versions, in fact, are designed to allow hardware manufacturers to supply their devices with a functional version of the software for use with the hardware itself.

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INTERSISM - User Guide 5. Programme use • 15

Once the type of data to be treated has been selected, click "Open" to give the project a name. File entry After having identified the folder containing the data, you can now enter files containing recordings in the project. Click on the first button, <Add>, and select the first file.

Click on the next <Add> buttons to individually enter all files corresponding to performed shooting.

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INTERSISM - User Guide 5. Programme use • 16

If files containing recordings of different shots should have the same name and are diversified only by the last character, made up of a progressive number (for example: demo1.dat, demo2.dat, demo3.dat, etc.), you can simply double click on the first and the programme will automatically load all following files. Shot point coordinates Height marks of shot points are entered in the <X shoot> and <Z shoot> boxes. The programme uses shots performed at the array ends in height calculations, while the others are ignored as they introduce an offset that is, however, automatically compensated when the travel time graphs are shifted. Seismograph position The field "<" to the right of the <X> (Delete) button, if checked, indicates that the corresponding recording was carried out moving the seismograph to the opposite array end and that, therefore, the order of geophones is inverted. Geophone and shoot position entry Very often, data files incorporate information relative to geophone geometrical structure. In the event that not all necessary information is present in files, you will have to complete missing information in the tables inside the window. Remember that both geophone and energetic point coordinates must be of the same origin, even if InterSism poses no constraints regarding the selection of such origin.

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INTERSISM - User Guide 5. Programme use • 17

The <Auto-complete> button allows us to automatically assign the abscissae and ordinates of geophones, which are interpolated automatically after the user has entered at least the first and last pair of coordinates. The presence of intermediate values is however taken into consideration in interpolation, giving rise to a broken line. Abscissae and ordinates are independent from one another.

Open an existing project

This control allows the user to open an already existing project, therefore loading previously processed information in the programme memory. Commands already performed will be listed in green with a checkmark in the programme menu bar. Commands that can currently be performed will be shown in green without a checkmark and commands which have not yet been carried out, but at the moment cannot be selected because additional information or intermediate calculation phases are necessary, will be shown in red.

Save the project with a name

Allows the user to duplicate an entire set of both input and output data, to facilitate processing based on the same initial data.

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INTERSISM - User Guide 5. Programme use • 18

"First Arrivals Time Search" Button

The selection of first arrivals is performed automatically. It is possible to later manually refinish obtained results.

In the event of shooting inside geophone spreading, the InterSism programme treats the two branches of the travel time as independent and analyses them separately. This allows the programme optimal use of the particular algorithms characterising the automatic first arrival search. For every internal shoot, the list of files will consequently present the two branches as separate entities. These are identified by the prefix [LEFT] and [RIGHT] which by convention identify, respectively, the geophones found between the shoot point and the arbitrary origin of spreading and those found instead on the other side. This was done intentionally, for the purpose of obtaining seismograms with time increasing from left to right. This is useful in terms of overlapping the visual impression when the user wishes to compare direct and inverse shoot times, especially for verification of the coincidence of times on the last geophone (which in the shots at the array end should in theory be the same).

The <View> button allows the user to view the seismic traces and, if they have already been defined, the first arrival times, but without being able to make any changes.

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INTERSISM - User Guide 5. Programme use • 19

The buttons <Zoom + > and <Zoom - > allow the user to view only the time interval in question among the first arrivals, or else total acquisition. The user can instead use the buttons <Overall normalization> and <Individual normalization> to digitally amplify signals, using the absolute maximum or, individually, the relative maximum for each trace as a reference value. Select the checkbox next to "Filtering" and the programme will remove the highest frequencies in order to eliminate the most noticeable noise. Select the checkbox next to "Background" to highlight the negative and/or positive half-waves. Finally, use the "Scale" field to type in a scale factor, variable from –5 to +5. In addition to scaling tracing, negative values invert the direction of waves. Important note Checkbox: a box in the shape of small square with a label on the side that describes its function. The user can activate a checkbox by clicking on it or pressing the space bar when the checkbox is selected. If the checkbox is checked, this means that the relative function is active. If deactivated, it remains empty. Select the <Automatic> button and the programme will start up automatic determination of first arrivals, iteratively using a sequence of algorithms based on a min-max and cross-correlation analysis which is followed by a finishing procedure of times based on wavelet-analysis, if no phase inversions in the first arrival seismic signal are detected.

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INTERSISM - User Guide 5. Programme use • 20

The buttons <Zoom + > and <Zoom - > allow the user to view only the time interval in question among the first arrivals, or else total acquisition. The user can instead use the buttons <Overall normalization> and <Individual normalization> to digitally amplify signals, using the absolute maximum or, individually, the relative maximum for each trace as a reference value. Select the checkbox next to "Filtering" and the programme will remove the highest frequencies in order to eliminate the most noticeable noise. Select the checkbox next to "Background" to highlight the negative and/or positive half-waves. Finally, use the "Scale" field to type in a scale factor, variable from –5 to +5. In addition to scaling seismograms, negative values invert the direction of waves. Important note Checkbox: a box in the shape of small square with a label on the side that describes its function. The user can activate a checkbox by clicking on it or pressing the space bar when the checkbox is selected. If the checkbox is checked, this means that the relative function is active. If deactivated, it remains empty. Click <Continue> to return to the main window, where you can select <Manual> to manually refinish the first arrival position, double clicking directly on the seismic traces inside the window showing the seismic traces.

Page 23: Application Software and Monitoring Tools › manuali › english › InterSism.pdf · 2018-07-23 · Application Software and Monitoring Tools . InterSism Seismic refraction interpretation

INTERSISM - User Guide 5. Programme use • 21

The buttons <Zoom + > and <Zoom - > allow the user to view only the time interval in question among the first arrivals, or else total acquisition. The user can instead use the buttons <Overall normalization> and <Individual normalization> to digitally amplify signals, using the absolute maximum or, individually, the relative maximum for each trace as a reference value. Select the checkbox next to "Filtering" and the programme will remove the highest frequencies in order to eliminate the most noticeable noise. Select the checkbox next to "Hat" to highlight the negative and/or positive half-waves. Finally, use the "Scale" field to type in a scale factor, variable from –5 to +5. In addition to scaling tracing, negative values invert the direction of waves. Important note Checkbox: a box in the shape of small square with a label on the side that describes its function. The user can activate a checkbox by clicking on it or pressing the space bar when the checkbox is selected. If the checkbox is checked, this means that the relative function is active. If deactivated, it remains empty. Click on <Lens> to open a virtual magnifying lens, allowing the user to verify the choice of first arrivals on the seismograph in further detail. The magnifying lens can be moved with the mouse and automatically aligns to the single seismic traces. Press "ESC" to close the lens.

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INTERSISM - User Guide 5. Programme use • 22

You will have to perform at least one automatic recognition of first arrivals for each travel time branch. Once this sequence of operations in the main menu has been completed, the <Travel Time Calculation> button will be activated.

"First Arrivals Time Table" Button

You can view the first arrival times that have been determined using the <First Arrivals Time Table> button.

Use, respectively, button combinations CTRL-C and CTRL-V to copy the table of values (after having positioned your mouse on the first box on the upper left) and then paste them in programmes such as Excel or Word. The programme will automatically recognise if data has been entered in seconds or milliseconds. In the latter case, upon confirmation of entered data, data will be converted into seconds. You can therefore either manually enter the data in seconds or milliseconds. To modify the contents of cells, use <Canc> to completely empty the current cell, <Ins> to restore an accidentally cancelled value, thus making it faster to edit cell contents. Inside of said cells, you can also use the <arrow> button to move, and the <Enter> button is to be interpreted as "pass to the next cell."

“Travel Time Graph Calculation” Button

Before calculating travel time, you must estimate the number of layers being investigated, choosing a number of 2, 3 or 4 layers with buttons. For proper calculation development, it is in fact necessary that the two shots carried out at the array end "see" all the expected refractors. It is in fact exactly on these two shots that the number of layers can be estimated. Click <Travel Time Graph Calculation> to start travel time calculation.

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INTERSISM - User Guide 5. Programme use • 23

Automatic travel time calculation is based on an analytic minimum procedure that allows the identification of break point arrangement, so that the resulting travel time presents the absolute best correlation coefficient with the first arrival times. Based on the number N of layers that were set, this procedure is applied in three different ways: • For shots carried out near the array end, a break point number is set equal to N-1, in order

to obtain a travel time formed by N segments that correspond to the N layers set. • For internal shots, configurations for a number of break points, varying from 0 (single layer)

to N-1 (N layers) are determined and recorded. The first segment is always assigned to the first layer.

• For internal shots, configurations for a number of break points, varying from 0 (single layer) to N-1 (N layers) are determined and recorded. The first segment can be assigned to a layer other than the first.

The apparent speed resulting in the shots described in points 2) and 3) are compared with the reference speed obtained from shots described in point 1). A number of break points is selected for each shoot, giving rise to the travel time whose segments better correlate to the reference travel time in terms of apparent speed. The implementation of a more complex algorithm, which would operate on the overall possible configurations of break points, is under development. It would select optimal distribution not only based on apparent speed but also in correlation to the lines superimposed by different travel times. InterSism makes it possible to intervene manually both in terms of number of related layers from each shoot and in terms of break point position. In this way, it is possible to verify calculation results automatically and to review the distribution of break points. For each break point configuration, the analytically optimal travel time is however utilised. If the travel time calculation has not yet been carried out, raw data will be presented. You must click on <Calculate Travel Time Graphs> to start automatic procedure.

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INTERSISM - User Guide 5. Programme use • 24

<Direct> and <Inverse> - these two boxes allow shots in the direct and inverse direction to be hidden, improving legibility of travel time speed. Once the calculation procedure has been completed, you can view the apparent speed of travel time segments by pressing and holding the left mouse button over it.

<Add Break Point> – click on any point of the travel time to be modified and the programme will perform calculations again, automatically searching for the optimal travel time for the new break point number. <Remove Break> – click on any point of the travel time to be modified and the programme will rerun calculations, automatically searching for the optimal travel time for the new break point number. <Move Break Point> – use this button to move the break point to a precise position, clicking on the point to be moved and then clicking on the new break point position. The type of situation that is obtained is therefore the following: The purpose of manual intervention is mainly to check congruency between the partially superimposed features of the line of travel times obtained by different shots. Observing the following picture, you can note how the travel times identified by the letters A and B present a good parallel up to the vertical green line, while, starting from segments highlighted with red circles, the travel times divide sharply. This means that the correct position of the break point is

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INTERSISM - User Guide 5. Programme use • 25

slightly different from the one that resulted from the analytical interpretation and, therefore, it is best to manually perform displacement. After checking the number and position of the break points, click on <Continue> to return to the main menu, where the <Depth Calculation> button will be active.

“Depth Calculation” Button

Click <Depth Calculation> and the programme will automatically carry out all relative calculation procedures, without any need for further manual interventions.

Intersism uses different methods depending on the type of survey conducted. A single shoot at the array end Using the intercept times method, the programme estimates the speed of layers and depth of refractors, that are considered to be ideally flat and horizontal. This method is generally not advised as it causes the apparent speed of ground to be assimilated to the actual speed. The presence of sloped stratigraphic discontinuities can cause significant errors. Therefore, the use of a single shoot should be reserved only for very approximate surveys, for which large errors can be considered acceptable. Two shots at the array end Using the intercept times method and comparing the direct and inverse travel times, the programme determines the speed of surveyed ground as well as the depth and slope of refractors, that are considered to be ideally flat. Three or more shots Having defined the number and the position of break points, the programmed shifts travel time segments on each shoot belonging to the same refractor, so as to align them with an iterative procedure to attribute greater weight to the segments having a higher number of geophones in common. To assess the progress of travel times in the lines not covered by recordings, the programme determines the average slope of known travel time lines in the two directions, using the previously calculated average slopes, weighed with respect to the number of actual segments.

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InterSism then passes to the Generalized Reciprocal Method (GRM) to identify refractor pattern. The software uses an automated procedure which, starting from a value of XY of an attempt equal to zero (which obtains the conventional time-depth function) and from the presumed depth which was previously calculated beneath each geophone, experiments with different XY values until it determines optical distance XY; that is, the distance for which the direct and inverse rays emerge in the vicinity of the same point on the refractor. After the calculating procedure has been completed, the <Printout Production> button on the main menu will be active.

“Printout Production" Button

Use this command to access the window for selecting and configuring printouts to be printed.

You can enter up to three lines of titles in the window. Double click on the coloured boxes next to the <Travel Time Graphs> button and the colour selection window will appear. From this window, you can assign a colour to each layer and each refractor. Double click on the coloured boxes next to the <Section> button and the colour and texture window will appear. From this window, you can assign a colour and texture to each layer. The automated selection of textures and their printing characteristics (texture colour, background colour and scaling) can be managed from the "Textures Selection" window. From the first line of the palette, you can choose the texture colour, clicking on the desired colour checkbox. From the second line, choose the desired background colour. Scaling is selected clicking on the desire scale factor. The colours present on the palette can be modified by double clicking on them. To use a single texture, double click on the selected texture. To use two superimposed textures, click on the first (which will become highlighted in red) and then double click on the second. The programme will write the number of selected textures, complete with scale, followed by the colour code. Using the automatic selection procedure, you can attribute a single scale for both superimposed textures. To choose which type of document to produce, simply click on the buttons: <Travel Time>: the document produced will contain both the original travel times and the shifted travel times graph. <Section>: the programme will present the stratigraphic section in the form of circles of depth and curves. The <Minimum height> field allows you to extend the sections at depths of up to the indicated height, or at a slightly higher depth (for graphic purposes). If the field is left empty, the minimum

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INTERSISM - User Guide 5. Programme use • 27

height is automatically calculated. In any case, if the minimum calculated automatically is less than that required, that calculated height prevails. The <X Scale> field allows you to define the horizontal scale with which the section will be drawn. If this box is left empty, the programme will automatically determine the optimal horizontal scale in relation to A4 paper dimensions. The <Z Scale> field allows you to define the vertical scale with which the section will be drawn. If this box is left empty, the programme will automatically determine the optimal vertical scale in relation to A4 paper dimensions. The <A> and <B> fields allow you to enter brief text alongside the spreading line. <First Arrivals>: manages normalized seismograph printing with first arrivals highlighted. <Print All Graphs>: click on this button to send all produced printouts from the programme to the pre-set printer: Travel time, Stratigraphic section, First Arrivals. <Generate all GIFs>: click on this button and all printouts produced by the program will be created in GIF format in the folder where the project has been processed. <Printout Preview>: click on this button to open a window in which all printouts produced by the programme will be displayed. <Export Printouts>: click on this button to export printouts produced by the programme in DOC for Microsoft Word format (which will be automatically opened at the end of export).

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Print preview window

The toolbar is displayed in the print preview window, along the top of the window in place of the menu bar. This allows the user quick access to different controls using the mouse. To select one of the controls within the toolbar, click on the corresponding icon. The toolbar contains the following controls:

• Zoom +: click on the first icon to activate the Zoom function, which can only be used with the mouse. This function allows part of the graph to be magnified. This function remains active until the Zoom - icon is selected. To magnify a part of the graph: click on the icon, then select the area to magnify, clicking on the beginning point in the magnifying window and then dragging the mouse until the rectangle delimits the area to be enlarged contains all desired elements. At this point, release the button. Note: Given the low screen resolution, text may appear slightly different depending on the zoom level. This will not in any way affect the quality of final output printing.

• Zoom -: click on the second icon and the graph will go back to its original size. • Arrows: click on the arrow-shaped icons after having activated the "Zoom +" option and the

graph will scroll on the screen. • Print: click on this icon and the graph will be sent to the pre-set printer. • Resize printing: click on this icon and the graph will be sent to the pre-set printer, and will

automatically be resized so that it can be printed in a single piece of paper. • Create DXF: click on this icon to access a dialogue window where you can assign a name

to the file in DXF format, which in default will be the same name as the data file. • Create EMF (W): click on this icon to access a dialogue window where you can assign a

name to the file in EMF format (Enhanced Windows Metafile), compatible with Microsoft Word import filters.

• Create EMF (D): click on this icon to can access a dialogue window where you can assign a name to the file in EMF format (Enhanced Windows Metafile), compatible with Corel Draw import filters.

• Create BMP: click on this icon to access a dialogue window where you can assign a name to the file in BMP format.

• Create GIF: click on this icon to access a dialogue window where you can assign a name to the file in GIF format.

• Create JPG: click on this icon to access a dialogue window where you can assign a name to the file in JPG format.

• Exit: click on this icon to close the window and return to the programme.

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Down-hole seismic Click on <Down-hole> to access the programme section dedicated to the processing of borehole seismic surveys with the down-hole method.

“Project Definition” Button - DH

Click on this button to access the window to create a new project or to open an existing project.

Create a new project

Allows the user to open a new project, eliminating any data in the memory. The programme does not offer a specific command for saving data, since data is automatically saved at the end of each operation.

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To start a new project, click on <Create a new project> and select the type of data to be treated: SEG-2, DTA, SG2 files. Once the type of data has been selected, click "Open" to give the project a name. File entry After having identified the folder containing the data, you can now enter the files containing recordings in the project. Click on the first button, <Add>, and select the first file.

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Important note: since each point where a measurement is carried out with a borehole geophone corresponds to a single data file, you must give names following a specific format, in such a way as to allow the programme to properly reconstruct the sequence of acquisition. In particular, the last two characters of the file name must be numbers, ordered from 01 to 99 (as a maximum number). File named p5001.dat, p5002.dat … p5013.dat, p50014.dat will therefore be correct files, files called p1.dat, p2.dat (missing a 0 in front of the last number) or testA.dat (alphabetic sequences not recognised) are INCORRECTLY named. Once you have not followed the previously described rules of numbering, you can then enter files, clicking the <Add> button and manually selecting each file to be added. Use the <Remove> button to delete incorrectly selected files from the list.

Once you have completed selected, confirm by clicking <Confirm>.

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The window shown must indicate to which recording channel the vertical geophone corresponds and what order was used in recordings. By definition, depth should be at regular intervals: test01.dat (-2m) test02.dat (-4m) test03.dat (-6m) are correct combinations, while INCORRECT combinations include test01.dat (-10m) test02.dat (-15m) test03.dat (-20m) (missing the reading at 5m of depth) or test01.dat (-2m) test02.dat (-5m) test03.dat (-9m) (intervals are not constant). After having confirmed geophone configuration, the programme will then verify the congruity of entered data and will return to the main window and delete the <Confirm> button. Important note: the InterSism programme can process acquisitions carried out with three or five channel geophones. In the “Down-Hole Geophone Configuration” window, you will have to indicate by means of which channel P waves, SX waves and SY waves were acquired. As the shear waves have a lower speed with respect to compressional waves, they reach the geophone when the first compressional wavefront has already passed, causing a disturbance in the measurement of transverse waves. To improve the ratio between P waves and S waves, in order to facilitate the identification of the latter, it is always preferable to create a double horizontal energization with an opposite direction. This permits you to subtract the waveforms relating to these two acquisitions, significantly reducing the component made from P waves present in the signal. The programme, however, allows you to also process acquisitions carried out with one or two shots: in the first case, energization will have to be considered vertical; in the second, the procedure of subtraction of the waveforms will not be performed. THOUGH PART OF THE PROGRAMME, THESE TWO OPTIONS ARE NOT RECOMMENDED. Shoot offset In the <Shoot offset> box, enter the distance of the energization point from the vertical of the borehole. Entry of this data is extremely important for correct interpretation of reading, as the energetic source is often set at a distance from the borehole axis that is not negligible and, therefore, correction of the arrival times that takes account of this distance is required. Geophone position In this field, you will have to enter the ground level depths at which recordings were made. The recording depth must always be at constant intervals, normally between 2 to 4 m. The <Auto-complete> button allows us to automatically assign the depths at which geophone is located, which are interpolated automatically after the user has entered at least the first and last pair of coordinates. The presence of intermediate values is however taken into consideration in interpolation, giving rise to a broken line.

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INTERSISM - User Guide 5. Programme use • 34

Open an existing project

This control allows the user to open an already existing project, therefore loading previously processed information in the programme memory. Commands already performed will be listed in green with a checkmark in the programme menu bar. Commands that can currently be performed will be shown in green without a checkmark and commands which have not yet been carried out, but at the moment cannot be selected because additional information or intermediate calculation phases are necessary, will be shown in red.

Save the project with a name

Allows the user to duplicate an entire set of both input and output data, to facilitate processing based on the same initial data.

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"First Arrivals Search" Button - DH

The selection of first arrivals is performed automatically. It is possible to later manually refinish obtained results.

The programme subdivides readings relative to P waves and S waves into three files with the suffix "_PW", within which all P wave readings are grouped, and "_SX" and "_SY", within which all readings carried out on the two horizontal geophones, orthogonal between them, relative to S shear waves.

The <View> button allows the user to view the seismic traces and, if they have already been defined, the first arrival times, but without being able to make any changes.

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The buttons <Zoom + > and <Zoom - > allow the user to view only the time interval in question among the first arrivals, or else total acquisition. The user can instead use the buttons <Overall normalization> and <Individual normalization> to digitally amplify signals, using the absolute maximum or, individually, the relative maximum for each trace as a reference value. Select the checkbox next to "Filtering" and the programme will remove the highest frequencies in order to eliminate the most noticeable noise. Select the checkbox next to "Background" to highlight the negative and/or positive half-waves. Finally, use the "Scale" field to type in a scale factor, variable from –5 to +5. In addition to scaling tracing, negative values invert the direction of waves. Important note Checkbox: a box in the shape of small square with a label on the side that describes its function. The user can activate a checkbox by clicking on it or pressing the space bar when the checkbox is selected. If the checkbox is checked, this means that the relative function is active. If deactivated, it remains empty.

Select the <Automatic> button and the programme will start up automatic determination of first arrivals, iteratively using a sequence of algorithms based on a min-max and cross-correlation analysis which is followed by a finishing procedure of times based on wavelet-analysis, if no phase inversions in the first arrival seismic signal are detected.

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The buttons <Zoom + > and <Zoom - > allow the user to view only the time interval in question among the first arrivals, or else total acquisition. The user can instead use the buttons <Overall normalization> and <Individual normalization> to digitally amplify signals, using the absolute maximum or, individually, the relative maximum for each trace as a reference value. Select the checkbox next to "Filtering" and the programme will remove the highest frequencies in order to eliminate the most noticeable noise. Select the checkbox next to "Background" to highlight the negative and/or positive half-waves. Finally, use the "Scale" field to type in a scale factor, variable from –5 to +5. In addition to scaling seismograms, negative values invert the direction of waves. Important note Checkbox: a box in the shape of small square with a label on the side that describes its function. The user can activate a checkbox by clicking on it or pressing the space bar when the checkbox is selected. If the checkbox is checked, this means that the relative function is active. If deactivated, it remains empty.

Click <Continue> to return to the main window, where you can select <Manual> to manually refinish the first arrival position, double clicking directly on the seismic traces inside the window showing the seismic traces.

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The buttons <Zoom + > and <Zoom - > allow the user to view only the time interval in question among the first arrivals, or else total acquisition. The user can instead use the buttons <Overall normalization> and <Individual normalization> to digitally amplify signals, using the absolute maximum or, individually, the relative maximum for each trace as a reference value. Select the checkbox next to "Filtering" and the programme will remove the highest frequencies in order to eliminate the most noticeable noise. Select the checkbox next to "Background" to highlight the negative and/or positive half-waves. Finally, use the "Scale" field to type in a scale factor, variable from –5 to +5. In addition to scaling tracing, negative values invert the direction of waves. Important note Checkbox: a box in the shape of small square with a label on the side that describes its function. The user can activate a checkbox by clicking on it or pressing the space bar when the checkbox is selected. If the checkbox is checked, this means that the relative function is active. If deactivated, it remains empty. Click on <Lens> to open a virtual magnifying lens, allowing the user to verify the choice of first arrivals on the seismograph in further detail. The magnifying lens can be moved with the mouse and automatically aligns to the single seismic traces. Press "ESC" to close the lens.

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You will have to perform at least one automatic recognition of first arrivals for each travel time branch. Once this sequence of operations in the main menu has been completed, the <Travel Time Calculation> button will be activated.

"First Arrivals Table" Button - DH

You can view the first arrival times that have been determined using the <First Arrivals Table> button.

Use, respectively, button combinations CTRL-C and CTRL-V to copy the table of values (after having positioned your mouse on the first box on the upper left) and then paste them in programmes such as Excel or Word. The programme will automatically recognise if data has been entered in seconds or milliseconds. In the latter case, upon confirmation of entered data, data will be converted into seconds. You can therefore either manually enter the data in seconds or milliseconds. To modify the contents of cells, use <Canc> to completely empty the current cell, <Ins> to restore an accidentally cancelled value, thus making it faster to edit cell contents. Inside of said cells, you can also use the <arrow> button to move, and the <Enter> button is to be interpreted as "pass to the next cell."

“Travel Time Calculation” button - DH

Before calculating travel time, you must estimate the number of layers being investigated, choosing a number of 2, 3 or 4 layers with buttons. Click <Travel Time Calculation> to start travel time calculation.

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Using the first arrival values, the programme will calculate the speed and thicknesses of layers, using procedures similar to those normally used in seismic refraction. Automatic travel time calculation is based on an analytic minimum procedure that allows the identification of break point arrangement, so that the resulting travel time presents the absolute best correlation coefficient with the first arrival times.

If the travel time calculation has not yet been carried out, raw data will be presented. You must click on <Calculate Travel Time> to start up the automatic procedure. This procedure must be repeated for P waves, for Sx waves and for Sy waves. Once the calculation procedure has been completed, you can view the apparent speed of travel time segments by pressing and holding the left mouse button over it.

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<Add Break Point> – click on any point of the travel time to be modified and the programme will perform calculations again, automatically searching for the optimal travel time for the new break point number. <Remove Break Point> – click on any point of the travel time to be modified and the programme will rerun calculations, automatically searching for the optimal travel time for the new break point number. <Move Break Point> – use this button to move the break point to a precise position, clicking on the point to be moved and then clicking on the new break point position. After checking the number and position of the break points, click on <Continue> to return to the main menu, where the <Depth Calculation> button will be active. Following the construction of the travel times and determination of the seismic signal propagation speed in different ground layers, InterSism calculates the characteristic elastic modules, having received indication from the user of the density of lithotypes encountered during drilling, expressed in kg/m3.

“Depth Calculation” Button - DH

Click <Depth Calculation> and the programme will automatically carry out all relative calculation procedures, without any need for further manual interventions. This procedure must be repeated for P waves, for Sx waves and for Sy waves.

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After the calculating procedure has been completed, the <Printout Production> button on the main menu will be active.

“Printout Production” Button - DH

Use this command to access the window for selecting and configuring printouts to be printed. You can enter up to three lines of titles in the window. Double click on the coloured boxes next to the <Section and Travel Time> button and the colour and texture selection window will appear. From this window, you can assign a colour and texture to each refractor. The automated selection of textures and their printing characteristics (texture colour, background colour and scaling) can be managed from the "Textures Selection" window. From the first line of the palette, you can choose the texture colour, clicking on the desired colour checkbox. From the second line, choose the desired background colour. Scaling is selected clicking on the desire scale factor. The colours present on the palette can be modified by double clicking on them.

To use a single texture, double click on the selected texture. To use two superimposed textures, click on the first (which will become highlighted in red) and then double click on the second. The programme will write the number of selected textures, complete with scale, followed by the colour code. Using the automatic selection procedure, you can attribute a single scale for both superimposed textures. To choose which type of document to produce, simply click on the buttons: <Section and Travel Time>: the documentation processed will contain the wave travel time selected next to it (P, Sx or Sy) and the stratigraphic section with the dynamic elastic module values characterising each different layer: • Deformability modulus (E) • Shear modulus (G) • Volume modulus K • Poisson's Ratio The <Minimum height> field allows you to extend the sections at depths of up to the indicated height, or at a slightly higher depth (for graphic purposes). If the field is left empty, the minimum height is automatically calculated. In any case, if the minimum calculated automatically is less than that required, that calculated height prevails.

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The <X Scale> field allows you to define the horizontal scale with which the section will be drawn. If this box is left empty, the programme will automatically determine the optimal horizontal scale in relation to A4 paper dimensions. The <Z Scale> field allows you to define the vertical scale with which the section will be drawn. If this box is left empty, the programme will automatically determine the optimal vertical scale in relation to A4 paper dimensions. The <A> and <B> fields allow you to enter brief text alongside the spreading line. • <First Arrivals>: manages normalized seismograph printing with first arrivals highlighted. • <Print All Graphs>: click on this button to send all produced printouts from the programme

to the pre-set printer: Travel time, Stratigraphic section, First Arrivals. • <Generate all GIFs>: click on this button and all printouts produced by the program will be

created in GIF format in the folder where the project has been processed. • <Printout Preview>: click on this button to open a window in which all printouts produced

by the programme will be displayed. • <Export Printouts>: click on this button to export printouts produced by the programme in

DOC for Microsoft Word format (which will be automatically opened at the end of export).

Print preview window

The toolbar is displayed in the print preview window, along the top of the window in place of the menu bar. This allows the user quick access to different controls using the mouse. To select one of the controls within the toolbar, click on the corresponding icon. The toolbar contains the following controls:

• Zoom +: click on the first icon to activate the Zoom function, which can only be used with the mouse. This function allows part of the graph to be magnified. This function remains active until the Zoom - icon is selected. To magnify a part of the graph: click on the icon, then select the area to magnify, clicking on the beginning point in the magnifying window and then dragging the mouse until the rectangle delimits the area to be enlarged contains all desired elements. At this point, release the button. Note: Given the low screen resolution, text may appear slightly different depending on the zoom level. This will not in any way affect the quality of final output printing.

• Zoom -: click on the second icon and the graph will go back to its original size. • Arrows: click on the arrow-shaped icons after having activated the "Zoom +" option and the

graph will scroll on the screen. • Print: click on this icon and the graph will be sent to the pre-set printer. • Resize printing: click on this icon and the graph will be sent to the pre-set printer, and will

automatically be resized so that it can be printed in a single piece of paper. • Create DXF: click on this icon to access a dialogue window where you can assign a name

to the file in DXF format, which in default will be the same name as the data file. • Create EMF (W): click on this icon to access a dialogue window where you can assign a

name to the file in EMF format (Enhanced Windows Metafile), compatible with Microsoft Word import filters.

• Create EMF (D): click on this icon to can access a dialogue window where you can assign a name to the file in EMF format (Enhanced Windows Metafile), compatible with Corel Draw import filters.

• Create BMP: click on this icon to access a dialogue window where you can assign a name to the file in BMP format.

• Create GIF: click on this icon to access a dialogue window where you can assign a name to the file in GIF format.

• Create JPG: click on this icon to access a dialogue window where you can assign a name to the file in JPG format.

• Exit: click on this icon to close the window and return to the programme.

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INTERSISM - User Guide 6. Theoretical notes • 44

6. Theoretical notes

Seismic refraction As known, seismic refraction uses acoustic waves in solid bodies, using a striking hammer, a blowback or a small explosive charge for energization. Seismic (sound) waves travel in the sub-soil at a different speed depending on the lithotypes crossed and are subject to phenomenon such as reflection, refraction and diffraction along the geological discontinuity surfaces. The boundary surfaces of the geological layers, characterised by a difference in speed between the upper layer (V1) and the layer below it (V2), produce refracted rays that travel parallel to the discontinuity surfaces (with the same speed as the "fastest" layer), continuously refracting elastic energy upwards (therefore towards the "slowest" layer). One of the main conditions for application of the seismic refraction method is that the propagation speed of the seismic waves increase with depth (V1 < V2). Another essential requirement for proper functioning of this methodology is that layers possess a thickness sufficient for detection. Otherwise, the number of geophones needs to be increased and the spacing decreased. The refracted energy that returns to the surface is measured using the geophones. Through a study of the travel time, and therefore of the speed, you can trace back to the geometric arrangement and the mechanical-elastic characteristics of the lithotypes present in the survey area. It is good practice to compare the energization intensity to the spreading length and, when possible, use the automatic gain control options. Otherwise, the first arrivals could be difficult to recognise, both because of the excessive weakness of their signal (geophones far away from the shot point) and because of possible saturation of closer geophones.

The seismic behaviour of P waves

P waves are always called Primary Waves as they propagate in the medium crossed with higher speed with respect other waves. In P waves, the particles making up the medium crossed are moved in the same direction of the wave propagation; in this case, radially, Therefore, the material extends and compresses with the propagation of the wave. P waves travel through the ground in a similar way that sound waves travel through the air. The speed with which P waves propagate in a medium depends on the physical properties (rigidity, density, saturation) and on the degree of homogeneity of the ground. A "wave train," whose signals will be detected by the receiving stations at intervals of time dependent on the elastic characteristics of the ground, is emanated from the energetic source. The graphic recording of the arriving wave train is called "seismogram." The wavefronts can be spherical surfaces or in any other form. The principle of Huygens, fundamental in the studies of any wave disturbance, establishes however that "at every moment, any point of a wavefront of any form can be considered as a point source of circular waves (or spherical if the perturbation is in three dimensions)". Wave propagation can therefore be interpreted as a continuous generation of circular waves that, interfering with one another, give rise to a wave resulting as macroscopically observable.

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The normal half-lines at wavefronts are called "seismic rays" and indicate the propagation direction of elastic waves. To analyse data obtained by means of seismic refraction, it is usual to consider both the seismic wave images and seismic ray paths. The latter can replace wavefronts with good approximation, allowing processing of only the travel times of the elastic waves and not all the other parameters typical of a wave phenomenon such as amplitude, frequency and phase. When a ray encounters a heterogeneity on the path, for example a lithological contact with another material, the incident ray is transformed into several new rays. The angles that the incident ray, the reflected rays and the refracted rays form with the normal on the contact surface between the materials are linked to each other and the speed of propagation of some relations known as "SNELL's laws." Snell's Laws state that the incident ray, the reflected ray and the refracted ray lie on the same plane. In addition, the following relationships apply between the angles:

for reflection: α = γ for refraction: V1 · sin α = V2 · sin β There is a particular value of the angle of incidence, called the "critical angle" or the "limit angle" (ic) that is particularly significant for the seismology of refraction. In fact, the critical angle is defined as the angle of incidence which makes β=90°. If the angle of incidence is equal to 90°, the refracted wave, according to the principle of Huygens, propagates along the contour of the layer and acts as a source for the new secondary wavefront and for new rays. The path of the seismic rays go out with an angle equal to the critical angle. It is possible to determine the critical angle by calculating the inverse of the sine of the value of the speed of the first layer/speed of the second layer. If the angle of incidence is higher than the limit angle, the refraction phenomenon disappears and we have total reflection without propagation of the wave in the second layer.

Applications

Seismic refraction surveys are normally used for the following issues: Mapping areas formed of disintegrated material Searching for groundwater depth Determination of resistance to excavation Mapping the permeability of gravel layers Mapping layers composed of loose material Location of fracture zones

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INTERSISM - User Guide 6. Theoretical notes • 46

Mapping of bedrock

Problems and limitations of seismic refraction

• The seismic speeds of grounds involved in the survey must increase with depth. • Layers must possess a thickness sufficient for detection. • Surveys taken on backfill or in areas used as landfills and in the presence of strong

anthropogenic noise often lead to unreliable results. • Single, narrow fractures often cannot be identified.

Interpreting results

The tools able to record ground movements are seismometers and geophones. These must be placed on the surface of the ground. Usually, you have seismograms of different geophones, set at a different distance from the source. These can be printed based on time and distance, in what are called "shot records". The Time / Distance variation is called "deviation" (moveout). Arrivals with large deviations are strongly inclined in the shot records and vice-versa. From shot records, we can therefore distinguish the arrival of different seismic waves (direct, reflected, and refracted). At short distances (offset), the first arrival is of direct waves: this arrival has large amplitude and the deviation is constant at all distances. At greater distances, the first arrival begins to be refracted. This arrival is characterised by small wave amplitude and constant deviation, but less than that of the direct wave. The last arrival to each geophone is the reflected wave: its deviation tends to increase with the distance according to a hyperbolic function. The diagram that represents the times of propagation of seismic waves based on the distance from the emission point is defined as the “propagation curve" or “travel time curve”. Defining its progress is the primary task in the interpretation of seismic refraction. What essentially matters for the refraction method is to identify the arrival time of the first wave of each geophone ("first break picking"). This distinction is not always easy. The two segments relative to arrival of the direct, first and then the refracted front waves are not always easily distinguished. The break point, or the point at which the variation of sloping occurs, is called the critical distance ("cross-over distance" ). It is therefore important to observe that the selection of first arrivals must already be considered part of the data interpretation, more than as part of the collection. The operator must therefore be careful to directly verify on-site that the signal/noise ratio is acceptable or, if necessary, repeat acquisition.

Methods of interpretation

The methods of interpretation for seismic refraction can be grouped into three main classes: (1) ITM - Intercept-Time Methods (2) DTM - Delay Time Methods (3) Ray tracing methods The level of calculations required is progressively greater from method to method. The ITM methods essentially require only a calculator. The DTM methods vary, from the simplest versions to the generalized version, and require use of a personal computer. The Ray Tracing methods require significantly higher computational resources and will not be considered in this context ITM - Time-Intercept Methods

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INTERSISM - User Guide 6. Theoretical notes • 47

The travel time, or distance-time curve, in the case of a seismic detection by refraction is expressed on a graph by the equation of a straight line. In the presence of flat and horizontal stratification, the slope of the straight lines of the first arrivals is equal to the reciprocal of the speed of propagation in the medium 1 (V1) or the discontinuity or second medium (V2). From the diagrams of these straight lines, we can determine the thickness of the upper layer. Based on the power of the upper layer, in fact, we will have a straight line with equal slope (it being linked only to speeds) but with an otherwise positioned break point: the more powerful the upper layer, the greater the time taken by the refracted front wave to generate and return to the surface. Therefore, the distance of receipt of the direct and refracted front waves varies. The thickness can be calculated considering the abscissa of the break point ( ), or else considering the intercept time , or rather the value of arrival time of the refracted wave at a distance of 0, obtained by projecting the straight line relative to the speed of propagation in the second half on the time axis.

cx

0t

21

22

120

12

12c

VV

VV2t

h

VVV-V

2x

h

−=

+=

In cases of a single, not horizontal layer Taking into consideration the slope of the layer, calculations become more complicated: • The speeds observed for the second layer are the apparent speeds, varying significantly

with the slope (the higher the actual speed in the up-dip direction, the lower the down-dip direction).

• The depths determined by means of intercept times are the inclined depths and not the actual depths under the shot point.

• Associated spreading is required, as the seismograms obtained in one direction only provide an apparent speed for the second layer.

The equations for calculation of the inclined depth as follows:

αcos2Depth )(1

dip)-(downdipdownitV −⋅

=

αcos2Depth )(1

dip)-(updipupitV −⋅

=

where Depth (down-dip) = the refractor dips from the shoot point toward the geophone Depth (up-dip) = the refractor goes back up toward the shoot point V1 = ground speed Ti (up-dip) = up-dip intercept time Ti (down-dip) = down-dip intercept time

2

21

22

V) V- (V

cos =α

A useful approximation for V2 (that cannot be measured directly from the time curve path) is given by the following equation:

δ+

⋅⋅= cos

VVVV2

Vdip)-2(downdip)-(up 2

dip)-2(downdip)-(up 22

where V2 = approximate speed of the underlying layer

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INTERSISM - User Guide 6. Theoretical notes • 48

V2(up-dip) = apparent speed of the underlying up-dip measured layer V2(down-dip) = apparent speed of the underlying down-dip measured layer δ = estimated slope value The value of angle δ can be calculated using the following formula:

2

VV

sinV

Vsin

dip)-2(up

11

dip)-2(down

11⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟

⎟⎠

⎞⎜⎜⎝

−−

Since the cosine of the angle δ is about 1.0, slopes must be low. It is important to note that the primary assumption in using the ITM method is that the geometrical structure of the contact surface is flat. This assumption allows you to use information derived from observations (arrivals) over the break point distance to derive a depth which is determined in the range of the point of energization. However, these methods are useful for a general evaluation of the depths and to perform a simple check of the results obtained with more sophisticated interpretation techniques. In cases of multi-layered ground Cases of multi-layered ground do not involve particular variations. The calculation principles remain the same, with the overlapping (arrivals in both directions) for all necessary layers. The following equations can be used with horizontal layers:

22

23

21

23

1

212

223

323i2 VV

VVVVh

VV2

VVth−−

⎟⎟⎠

⎞⎜⎜⎝

⎛−

−=

The thickness h1 of the first layer is determined using the case of ground with two layers and the intercept time ti2 of the second line segment or the critical distance Xc2 determined by the first two line segments. This thickness is used when calculating the thickness of the following layer h2. In terms of critical distance, the equation can be rewritten as:

⎟⎟⎟

⎜⎜⎜

−−−+

+−

=22

23

21

232

21

223

1

1

23

233c2

VV

VVVVVVVh

VVVV

2xh

The calculation can be extended to deeper layers using extensions of these general equations. Since the equations in this form contain the thicknesses of less deep layers, the calculation starts with the first layer and progresses downwards. Note that these equations do not include the slope. The equations for sloped layers are used in the GRM method (Palmer - 1980). Reciprocal methods The reciprocal methods include more than 20 methods of interpretation, including methods based on delay times, which may or may not require measurement of reciprocal time. Reciprocal time is the travel time along the refractor from one energetic source to another. There are different versions of this method, which differ in some correction factors. In general, a contact surface that is not excessively sloped and a constant refractor speed are hypothesised. Knowing the travel time from the energetic source S to a particular geophone G and the associated travel time from the source S' to the same geophone G, and the travel time from the energetic source S to a particular geophone G1 (possibly distant from the first) and the associated travel time from the source S' to the same geophone G1, it is possible to calculate the speed of the refractor using the following formula:

'2tt

x2V

Δ+Δ

Δ=

where:

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INTERSISM - User Guide 6. Theoretical notes • 49

Δx = distance to the surface between the geophone g and geophone g1 Δt = difference in direct travel times Δt’= difference in associated travel times The direct travel time t for geophone G can be expressed as per the following formula:

2

gc

1

sg

Vx

icosV

hht +

+=

where hg = minimum depth at the geophone hs = minimum depth at the source xg = distance between the geophone and the shoot Similarly, an equivalent equation can be written for the time taken by the seismic wave to reach the geophone G, starting from the associated source. The calculated times will obviously be different. What instead must be equal is the total travel time between the direct and associated sources. This time is called the end rate:

2

0c

1

'ss

Vx

icosV

hht +

+=

where x0 is equal to the sum of the distance of the geophone from the direct travel time shoot point and of the geophone distance from the associated travel time shoot point. Subtracting the sum of the previously calculated times from the end rate, it is possible to solve the equation with respect to h and obtain the minimum depth. This procedure must be performed for each geophone. The depth h is the radius of an arc of circumference with the centre in the relative position of the geophone: the boundary of the refractor will touch at least one point in each of these arcs and, therefore, can be determined graphically as the envelope of the arcs. Generalized Reciprocal Method - GRM The generalized reciprocal method was developed to overcome some imperfections in simpler methods, which faced difficulty due to the presence of topographic or subsurface, particularly bumpy, surfaces. The Palmer method uses two functions: the analysis-speed function and the time-depth function. The Analysis Speed function The particularity of this method is its use of two-geophone direct and inverse arrivals set at a distance of XY. If the position XY is chosen in such a way that the point of emergence of the refractor is common, the travel time, and therefore the speed calculation, will only depend on the characteristics of the material.

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INTERSISM - User Guide 6. Theoretical notes • 50

The analysis-speed function, referring to the figure below, is defined by the equation:

2)45()24(2)12(2t

)78()57()45()24()12(t)34()45()57()78(t)56()45()34()12(t

2ttt

t

AB

BX

AY

ABBXAY

++=

++++=+++=

+++=

+−=

The value of this function refers to midpoint G between X and Y. Producing graphs of travel times in relation to geophone position, it is possible to derive the speed V2 of the refractor, regardless of the slope and topography. What influences the calculation is therefore the choice of geophones XY. If the position XY is chosen in such a way that the output point of the refractor is common, the travel time, and therefore the speed calculation, will only depend on the characteristics of the material. The Time-Depth function The GRM method now includes the creation of time-depth functions for each geophone. Referring to the previous figure, the generalized time-depth function is defined by the equation:

2)V/XYt(tt

t 2ABBXAY +−+=

Calculation of optimal XY value Evaluation of the optimal XY distance can be obtained using the previously determined speeds and thicknesses of the layer: if the thicknesses and speeds of each layer is known, the value XY for each layer can be calculated directly. Therefore, by applying an iterative procedure, it is possible to determine these values automatically and with considerable precision. Ray Tracing Method The programmes based on this method usually start from a first interpretation, carried out with one of the previously described methods. The expected arrival time to a geophone, based on an initial model, is then calculated. The complexity of the calculation increased depending on the model used. The calculation is carried out in iterative form up to convergence, which in the more complex models must at times be forced. After having determined the arrival times to geophones, you must correct the model, so as to make the calculated times and observed times coincide, as much as possible. You must then proceed once again with the calculation of travel times based on the correct model.

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INTERSISM - User Guide 7. Bibliography • 51

7. Bibliography

Main references • Hawkins, L. V., 1961, The reciprocal method of routine shallow seismic refraction

investigations: Geophysics, 26, 806-819. • Palmer D., 1980 - "The Generalized Reciprocal Method of Seismic Refraction Interpretation"

- Society of Exploration Geophysicists • Palmer D., 1986 - "Refraction Seismic" - Geophysical Press • Redpath B., 1973 - "Seismic Refraction Exploration for Engineering Site Investigation" -

Explosive Excavation Research Laboratory

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INTERSISM - User Guide Contacts • 52

Contacts

Contacts

INTERSISM - Seismic refraction interpretation and Down-Hole seismic measurements

has been produced by: GeoSoft by Engineer Giorgio Scioldo Corso G. Matteotti n. 12 10121 Torino (TO) tel. 0115624689 - fax 0115175007 www.geoandsoft.com – [email protected]