Vertical Seismic Profiling

VSP dan HSP• Di dalam survey seismik geophone dan sumber seismik terletak di (dekat) permukaan (jenis HSP).

• Tetapi pada survey VSP, geophone-nya berada di dalam sepanjang lubang sumur dan sumber seismiknya berada di permukaan dan jarang berpindah.

• Dengan demikian VSP akan merekam pertama kali adalah gelombang-gelombang transmisi DGW (down going wave) dan berikutnya adalah gelombang-gelombang pantul dari reflektor yang ada UGW (up going wave).

Teknik Lapangan VSP

VSP Wavefields

The VSP records seismic motion due to

energy travelling both downward and

upward past the geophone position. As

would be expected the arrivals at the

downhole receiver are categorised

primarily by their direction of arrival into

Downgoing:

P1

P2

Or Upgoing:

VSP Wavefields

The VSP records seismic motion due to

energy travelling both downward and

upward past the geophone position. As

would be expected the arrivals at the

downhole receiver are categorised

primarily by their direction of arrival into

Downgoing or upgoing events.

Downgoing multiples

Depth0

0

TD

Time

P2

P1

Downgoing direct

arrival

These arrange themselves within the VSP

data set as events with different apparent

slopes in the data. The co-ordinate system

for this display is depth horizontal and time

vertical. Again we have :

Downgoing wavefields.

VSP Wavefields

The VSP records seismic motion due to

energy travelling both downward and

upward past the geophone position. As

would be expected the arrivals at the

downhole receiver are categorised

primarily by their direction of arrival

into Downgoing or upgoing events.

These arrange themselves within the VSP

data set as events with different apparent

slopes in the data. The co-ordinate system

for this display is depth horizontal and time

vertical. Again we have :

Downgoing wavefields

and upgoing wavefields

Upgoing primaries

Upgoing multiples

Depth0

0

TD

Time

P2

P1

Downgoing direct

arrival

VSP WavefieldsThe VSP records seismic motion due to

energy travelling both downward and

upward past the geophone position. As

would be expected the arrivals at the

downhole receiver are categorised primarily

by their direction of arrival into Downgoing

or upgoing events.Upgoing primaries

Upgoing multiples

Downgoing multiples

Depth0

0

TD

Time

P2

P1

Downgoing direct

arrival

These arrange themselves within the VSP data set

as events with different apparent slopes in the

data. The co-ordinate system for this display is

depth horizontal and time vertical. Again we have

:

Downgoing wavefields

and upgoing wavefields

and both classes together

VSP WavefieldsIt is fairly obvious from this diagram, that an upgoing multiple ceases to be

evident in the data when the geophone is below the last reflector involved in

its generation.

Recorded just above a reflector, the primary and all the multiple reflections in its

“tail” each have their last “bounce” at that reflector. This means that once the

geophone is just below the reflecting interface, the primary and its multiple tail can

no longer be recorded.

This reverberant is then manifest in the downward travelling waveform below this

point. The termination of an upgoing event within the body of the data allows the

recognition of a multiple and the determination of its origin. As a corollary to this,

the continuation of an upgoing event to the first arrival curve (time-depth curve)

defines it as a primary reflection which can then be accurately labelled in both depth

and time.

VSP WavefieldsA part of a sample VSP dataset is shown on

the right. For clarity on the computer

screen only a small section of the display

has been included although it is quite easy

to see the wavefields discussed in the

preceding slides. For example there are:

downgoing direct arrivals

upgoing primaries

downgoing multiples

upgoing multiples

Two-Way Time Transformation

The display on the previous slide is fine for examining the data in isolation but the display is

not suitable for direct comparison with any surface seismic data. Surface derived data is

displayed at two-way time, that is the events are positioned at the time corresponding to the

transit time from surface to the reflector and back to the surface. The VSP geophone is

considerably closer to the reflecting interface than the surface geophone and therefore the

time at which the reflector appears is somewhere between the transit time from the surface

to the reflector and the full two-way time of the surface seismic event.

However the situation can be approximate to that in the diagram on the next slide. In

this case the equivalence between the surface seismic data and the VSP are shown

with respect to the travel paths of the reflected energy

Two-Way Time TransformationWith the source positioned coincident

with the wellhead, the VSP travel path

reduces to the first part of that for the

near-field surface seismic trace. If the

path is extrapolated to surface it can be

seen that the only difference between

the two travel paths is the additional

time between the downhole detector

and back to the surface.

Geophone spread Source

Well

GeophonesIf this additional time is added in to the

arrival times for the geophones, it is possible

to correct to the surface seismic arrival

times. With the source at the wellhead, this

correction to be applied is equal to direct

arrival time; hence doubling the arrival time

shifts to surface seismic time co-ordinates.

Two-Way Time Transformation

Once the arrival times have been doubled it is

possible to display the VSP data in a manner similar

to surface seismic although there are significant

differences between this data type and the VSP.

These are discussed in the VSP interpretation topic.

It is obvious from the panel to the right

that the new co-ordinate system lends

itself far better to the comparison

between the two data types than does

the recorded time data panel.

Upgoing primary

Downgoing direct arrivals

Conclusion

These characteristics of the VSP in a sense constitute all that is required to

understand the basic “rig source” or “zero offset” survey type when shot into a

vertical well. As can be seen in the section on interpretation, this type of VSP is

capable of providing much information but is very much limited to the direct

environs of the subject well with regard to the extent of subsurface illumination it

provides .

More complex VSP survey types allow for additional information to be gathered but

introduce complications into the processing sequence.

For more further discussion of survey types and processing sequences please refer to

the appropriate sections from the master presentation.

Perambatan Gelombang Seismik Dalam VSP

Simulasi Numerik

Sintetik seismogram VSP

Ada banyak informasi yang dapat kita peroleh dari menganalisa DGW saja. Misalnya : distribusi cepat

rambat gelombang seismik di sekitar sumur pemboran yang dapat mencerminkan "compaction" dan

porositas batuan, analisis "attenuasi" yang mencerminkan derajat saturasi fluida di dalam batuan, dan

analisa spectral. Dikenal bahwa lapisan gas senang sekali menyerap frekuensi tinggi. Penurunan spektral

yang menyolok antara gelombang-gelombang yang sempat melalui reservoar gas dan yang tidak dapat

dijadikan indikasi adanya gas.