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VII INGEPET 2011 (EXPR-2-BP-29-E)

A PRACTICAL COMPARISON OF VARIOUS LAND SEISMIC SOURCE OPTIONS

Bill Pramik (Geokinetics)

Abstract For many decades, explosives and Vibroseis seismic sources have dominated the land seismic acquisition market. While these sources are extremely effective for acquiring high quality seismic reflection data, they can be expensive, time consuming and don’t always exhibit the environmental and social sensitivity that is sometimes required of us. This is why there have always been continuous attempts to develop new seismic sources that are more cost and time effective, with a lighter environmental footprint, that maintain the same level of seismic fidelity and depth of investigation. Many of these sources have come and gone over the years but only a few persist, working in niche markets with specific applications. And new sources continue to appear on the horizon. In this presentation, we will show the pros and cons of a number of seismic source options and compare representative data samples from each. Explosives generally require the drilling of relatively deep shot-holes and while the holes themselves do not present much of a problem, the equipment to drill them can. Large vehicle access can be tricky in some areas. There is also the concern about how close we can get to buildings and other infrastructure without causing damage and the elevated HSE risk of handling explosives. Vibroseis suffers from some of these same limitations because of the size of the vehicles involved. Accelerated Weight Drop sources are smaller and more agile and typically generate less destructive ground motions, but lack the penetration ability found in Vibroseis and explosives. Other types of land sources are currently being deployed that may bridge the gap between more conventional heavy sources and the lighter, niche sources. A large number of comparison tests between Vibroseis and explosives exist, and a few between other lighter sources. When comparing these different source types, consideration must be given to more than just data quality. Cost, ease of access, environmental and social aspects and acquisition efficiency must all be taken into account. It must be recognized that there are a large variety of targets that need to be imaged when acquiring seismic data. Certainly, there will be cases where the depth of penetration associated with conventional heavy source is required, but there are just as many instances when a lighter, more cost effective solution is appropriate. Having the knowledge and experience with which to base the source selection process on can be invaluable for designing the optimum seismic survey for any given objective. Introduction Buried explosives and Vibroseis as seismic sources have dominated the land seismic market for many years, and for good reason. These sources are extremely effective for acquiring high quality seismic reflection data, are well understood technically and logistically, and typically are readily available. However, they can be expensive, time consuming and do not always provide the environmental and social sensitivity that is sometimes required of us. For this reason, there have been continuous attempts to develop new seismic sources that are more cost effective, have a lighter environmental footprint and maintain the same level of seismic fidelity and depths of investigation. In order to make the best choice of a seismic source for any given project, it is important to consider all the implications of each source type and to have an understanding of how to compare the relative merits of each.

VII INGEPET 2011 (EXPR-2-BP-29-E) 2

Conventional Wisdom The dominant energy sources in the seismic industry have been explosives and Vibroseis for decades. This raises the obvious question: Why? In large part, this is the case because these sources are tried-and-tested, are very reliable, represent a “known quantity” and generally are readily available. If this is the situation, the next obvious question would be, “why should we consider something else?” The answer to this question, to some extent, is due to the fact that these sources have been around a long time. Explosives were really the first reliable production source used in seismic exploration and have been around since before caring for the environment, or how our operations impacted local populations were a significant concern. When Vibroseis was introduced in 1962 it was seen as a safer, friendlier seismic source but the vehicles were still large, loud and created a significant impact on the environment. Today, both explosives and Vibroseis are considered to be expensive, and to have large environmental and social impacts. In addition, even though explosives and Vibroseis are currently considered to be the best seismic sources, there are instances where alternate energy sources may provide superior seismic data quality. It is important to remember that, when it comes to seismic sources, there is no one source that is appropriate for all applications. There are many factors that must be considered when selecting the best source for any given job. These factors include economics, logistics and environmental/social considerations. In addition, we must always remember to consider the geophysical implications of our source decisions. When evaluating the economics of a seismic source, it is important to consider the “real cost” of operating that source. This necessarily includes costs not directly connected to the source: Items like the time and cost of line clearing, the actual time needed to record the data and costs associated with land use restrictions and timing. Agriculture can have a significant effect on the cost of the source, especially if it means a delay in acquisition because of planting or harvesting crops. The logistics of seismic sources have even more considerations. What is required to get the source to and from the project area? Once the source is at the project site, how easy will it be able to move around? This can significantly affect the data quality if access is poor. In some cases, the choice of a seismic source can impact the manpower requirements of a job, for example, line clearing. The choice of a source can also impact how infrastructure must be dealt with for items such as stand-off distances, damages and social impact. Environmental sensitivity is becoming more a part of the way we operate. Some regions have specific mandated environmental restrictions but working in a manner consistent with awareness of the environment and a desire to protect it is always the best course of action. With this in mind, the choice of a seismic source should also take into account its environmental impact. Surface damages and disturbance to flora and fauna are some obvious factors, but we should also consider damage to infrastructure and local water supplies. The social impact of the source can be considered here as well since the local inhabitants are part of the environment. Finally, we must consider the geophysical implications of our choice of a seismic source. We need to ensure that the source provides adequate penetration to reach the objectives and that the bandwidth of the source will achieve the required resolution to meet the exploration requirements. Another geophysical point to consider is the consistency of the source. Is the source signature repeatable? Can the source gain access to the prospect area sufficient to provide uniform and consistent coverage over the prospect without leaving large gaps? Source Options When putting together a list of source options, unconventional sources should be included in the list with the more common sources so that all potential advantages can be considered. The


conventional options consist of explosives and Vibroseis but the list of unconventional sources is far too large to provide a complete list here. However, for the purposes of this writing, three nominal categories of unconventional sources can be described. These would be weight drop sources, accelerated weight drop sources and a generic category we will describe as “other”. Most of these unconventional source types also fit into a classification that can be described as “surface impulsive” sources, or sources that create an impulse (rather than a vibratory sweep) at the surface of the earth (without drilling our burying the source). Classic weight drop sources are essentially obsolete, having been replaced by more powerful accelerated weight drop (AWD). These sources use some type of mechanism to help gravity accelerate a falling mass to increase the amount of energy generated. The generic “other” category mostly consists of sources that are either used in academia, small scale engineering surveys or have become obsolete. These include sources like the Betsy gun, the land air-gun and Dino-Seis. There is, however, one exception in this category. This is an electro-magnetic driven impulsive source (EIS).

Vibroseis Truck Explosives Drill Buggy

Accelerated

Weight Drop

Electromagnetic

Impulsive Source

Figure 1: Some Seismic Source Options

The Decision Process When choosing which seismic source option is best for a particular project, the first question that must be asked is “What should I do?” It is in this step where consideration must be given to the geophysical implications of the source, the environmental and social implications, and the cost/benefit of each source option. Obviously, if a particular source option cannot fulfill the geophysical requirements (penetration, bandwidth, etc.), then it cannot be considered as a viable option. Environmental and social sensitivity can sometimes be mitigated or negotiated so


these may not always be as significant and the cost/benefit analysis will vary with market conditions. Once a decision is made at this point, the second question must be “What can I do?” The answer to this question is sometimes not something we can control. Environmental and/or social restrictions, which are significantly different than environmental and/or social considerations, may eliminate one or more source options. The terrain of the project may make it impossible for some source types to gain access to the area, eliminating those sources from consideration. Finally, if a particular source option is simply not available, that source will also drop from consideration. When the questions of what should and can be done are answered, the final question is “What will I do?” If you are fortunate and have more than one source option left after answering the previous questions, the relative merits of the remaining sources must now be compared. When making this comparison, the three principal desires of any project should be the primary considerations: Better, Cheaper and Faster. Using more modern terminology, these can be replaced with Quality, Cost and Time. The quality of the seismic source relates to its ability to meet the exploration objective and must be the first consideration. If the data gathered is not capable of providing the information necessary for exploration, it really does not matter how cheap or fast the data was acquired; the data’s value to the explorationist is effectively zero. The cost of the source option drives the goal of getting the data as cheap as possible, which generally should be the secondary consideration. This is not just the direct cost of operating the source but includes how the choice of a source impacts the remainder of the acquisition. The choice of a source can have direct and indirect effects on these costs. Time should generally be considered as the third goal when selecting a source, but there are cases where time becomes more important than cost. As a general rule, time equals money, meaning that more time in the field usually means a more expensive operation. There are, however, exceptions to this and sometimes time can be traded for cost. In other words, under certain conditions, if it is more important to have the results of an acquisition project sooner than it is to save on cost, raising the cost of the project can deliver those results sooner. Ultimately, the choice of which source to use will impact many aspects of any seismic project. Regardless of the source, the priority order of the qualifying parameters must be 1) meet the exploration objectives, 2) obtain the data in the most economic manner and 3) obtain the data as quickly as possible. Comparing Some Options It is not always easy to make comparisons between the various seismic source options and there is no simple equation that can be used to provide the answers. Parameters that are common to all source types can vary dramatically as the environment and other factors change between prospect areas, and often within prospect areas. However, it is useful to summarize the various characteristics of some of the different source options as a starting point for future decisions. Source types that will be compared here include explosives, Vibroseis, Accelerated Weight Drops (AWD’s) and Electromagnetic Impulsive Sources (EIS’s). Source Scalability The scalability of the strength of a seismic source, and the options for how that source can be scaled, can have an impact on the geophysical quality of the source and the economics of the source. In general, there are three methods for scaling the strength of a source: Increasing the


strength of an individual source, increasing the number of sources that are initiated simultaneously, or increasing the number of individual shots that are summed together in processing. Of course, various combinations of the three basic methods can also be used to increase the strength of a seismic source. These effects can be seen in a series of equations that are used to calculate the relative effect of scaling a source on the signal-to-noise ratio that is recovered. These equations are,

Explosives 2

1

3

1

)()( smnE n

Vibroseis 2

1

)(tsnkE

AWD 2

1

)(skE

EIS 2

1

)(snkE

Where: E = relative signal-to-noise ratio m = mass of each individual charge (explosives) n = number of simultaneous sources fired k = relative strength of each individual source t = sweep length (Vibroseis) s = number of stacked shots.

Notice that the n and k terms scale linearly in these equations, meaning that if these quantities are doubled, the signal-to-noise ratio will double. All other terms scale by either the square root or cube root of the value. The n term is probably the most useful and direct method for scaling the strength of a seismic source. Because, in all cases, this term scales linearly, it has the largest impact on the strength. A requirement for the inclusion of the n term is that individual sources can be synchronized with each other. Explosives, Vibroseis and EIS’s meet this requirement but AWD’s do not. This is why this term is not included in the scaling equation for the AWD. Generally the k term or strength of a source is a value determined by the make and model of the unit and can only be scaled downwards from some preset maximum. For example, a 60,000 pound vibrator has a maximum k value of 60,000 pounds and 100% drive level but can be scaled down to 48,000 pounds if the drive level is set to 80%. For the AWD and EIS, this term cannot accurately be scaled, even downwards. As such, the k term is not useful for actually scaling the strength of a seismic source. For explosives, the mass of the charges can be used to scale the strength of the source, but because the strength of the source scales as the cube root of the mass, it takes a large change in charge size to make a modest increase in the source strength. Summing shots, and in the case of Vibroseis lengthening the sweep, improves the signal-to-noise ratio as the square root of the increase. While these are effective ways to raise the relative strength of the source, it comes at the expense of operational efficiency since more time is required at each shot point. Penetration The ability of a seismic source to produce enough energy to adequately image the desired subsurface events is a critical parameter in the selection process. Many times, the depth of the objective reservoir horizons is sufficient but often, the underlying structures are also required to gain a complete understanding of the geologic setting. Most seismic sources can be used to reach almost any desired target depth but the cost of the source, the time required for acquisition and the geophysical effect of increasing the source effort must all be considered.


Explosives and Vibroseis have proven their ability to image very deep targets and are easily scaled to optimize the cost and efficiency of reaching them. In the case of explosives, simply using a larger charge size is an easy way to increase the depth of penetration. Similarly, an array of shot holes in a pattern can be more effective with respect to the total amount of explosives required but at the cost of drilling more shot holes. Vibroseis arrays are common and are an easy way to scale up the strength of a Vibroseis source but consideration must be given to the affect this has on the source array size. AWD’s and EIS’s are relatively weaker sources but can be configured to reach deeper targets but the trade-off is time and expense. When considering the EIS, the ability to synchronize multiple sources, increasing the source strength linearly, can have a significant time advantage over summing multiple shots but as in the case of multiple vibrators, care must be taken to ensure that the source array does not become too large. The only option for increasing the depth of penetration of an AWD is to stack shots until the desired penetration is achieved. Bandwidth In many ways, we are at the mercy of nature with respect to the frequency content we recover in seismic acquisition. While low frequencies travel relatively easily through the earth, higher frequencies are attenuated rapidly. Ideally, we would like the source to generate enough low frequency energy for deep penetration and shallow resolution, and enough high frequency energy to overcome the attenuation of the earth. Explosives have limited ability to tune the frequencies they generate. In general, smaller charge sizes provide more high frequency energy compared to larger charges, and deep shot holes generate more low frequency energy than shallow holes. Beyond this, there are no real methods for altering the output frequencies of explosives. Vibroseis, by its nature, has the ability to adjust not only the range of frequencies generated, but their relative amplitudes as well. This provides a unique opportunity to generate a source signal specifically tailored to our exploration needs. However, there are limits to the high frequencies that can be generated related to mechanical and operational limitations of vibrator design and construction, and Vibroseis is historically ineffective at generating very low frequencies. Some relatively recent advances in vibrator design has helped with the low frequency content of Vibroseis data and, if these types of vibrators are available, they should be given consideration in the process. Coverage The selected seismic source must provide a good quality signal to provide good data quality. In addition, the ability of a source to provide uniform and consistent source coverage over the project area can have an equally important effect on the quality of the final data. Gaps in the source effort caused by the inability of a particular source to access certain portions of the project, or restrictions on source placement due to buildings and other infrastructure can create areas of poor data quality. Large vehicles like vibrators and drill buggies can have limited access to areas because of terrain or vegetation. Depending on conditions, additional line clearing may be required resulting in increased time and cost of acquisition. Smaller vehicles of the types used for AWD’s and EIS’s can access many areas that other sources cannot reach, providing improved spatial coverage of the source effort. Another factor that can affect the acquired coverage is the required “stand-off” distance. This is the minimum distance from buildings and other infrastructure that must be maintained to avoid damages to those structures. The measurement used to determine the required stand-off distance is PPV, or peak particle velocity. As a general statement, explosives generate the largest PPV and, as a result, have the largest stand-off distances. Vibrators, AWD’s and EIS’s


generate smaller PPV and can typically get much closed to buildings and structures. This lower PPV level can significantly improve the coverage uniformity obtained is a seismic project which will improve the overall quality of the final product. Cost The cost of any chosen seismic source will vary considerably with terrain and environment and this makes it impossible to provide a definite cost relationship between the various source options. The cost of each source type must be considered on a case by case basis considering all the direct and indirect costs associated with each source type. These would include things such as cost of fuel and materials, line clearing requirements, potential damages, manpower and the time required for preparation and acquisition. As a general statement, time in the field is proportional to cost. In other words, if it takes longer, it will cost more. This is true for a specific source type but cannot be directly applied between source types. For example, while in one situation, explosives might provide faster acquisition than Vibroseis, the associated costs of Vibroseis could be significantly lower than that of explosives, making Vibroseis less expensive over the life of the project. Data Examples The data examples discussed here are not intended to promote one source type over another, but are provided to demonstrate that different source types can be used to acquire comparable data quality in a variety of conditions. All examples provided here are direct comparisons between source types with effectively equal acquisition parameters and processing, even though some comparisons were acquired at different times. Data Example 1: Explosives vs. Vibroseis (mini-vibes) This example is from the Canadian shallow gas province and provides a comparison between shallow explosives with small charges and Vibroseis using mini-vibes. The basic acquisition parameters are shown in Table 1. The processed data are shown in Figure 2 with the explosives shown on the left and Vibroseis on the right. Comparing these two sections, it appears that the explosives have provided better imaging of the deeper events around 400ms while the Vibroseis has better frequency content and reflection continuity around 150ms. In this case, the choice of which source is superior would depend on which of these zones was of primary importance to the exploration objective.

Table 1: Parameters Explosives vs. Vibroseis (mini-vibes)

Source Type Explosives Vibroseis (mini-vibes)

Group Interval 12.5 m 12.5 m

Near Offset 6.25 m 6.25 m

Far Offset 993.75 m 993.75 m

Number of Groups 160 160

Source Interval 12.5 m 12.5 m

Fold 80 80

Source Parameters

Charge Size 0.5 kg

Charge Depth 4 m

Number of Vibrators 1 x 17,000# @ 80%

Sweep Length 8 sec

Number of Sweeps 1

Sweep Frequency 10 – 120 Hz


Explosives Vibroseis

Figure 2: Canadian Data Example: Explosives vs. Vibroseis (mini-vibes) Data Example 2: Shallow Explosives vs. EIS This data example is from Colombia in an area with severe environmental restrictions. These restrictions include limiting access to the area by hand-cut trails not more than 2 meters wide. The comparison is between shallow explosive charges and EIS, and the basic acquisition parameters are provided in Table 2. The processed data are shown in Figure 3 with the explosives on top and EIS on the bottom. The exploration objective is just above the strong reflector at 900ms. Comparing the sections, it is evident that both source options provide high quality seismic data. The explosives appear to have less surface related noise than the EIS, but the EIS appears to have a broader amplitude spectrum. Figure 4 shows comparative amplitude spectra for the two processed sections. From the spectra, it is evident that the EIS provided superior high frequency content, but also provided better low frequencies.


Table 2: Parameters Explosives vs. Vibroseis (mini-vibes)

Source Type Shallow Explosives EIS

Group Interval 12.5 m 12.5 m


Far Offset 743.75 m 743.75 m


Source Interval 12.5 m 12.5 m

Fold 60 60

Source Parameters

Charge Size 0.5 kg

Charge Depth 3 m

Number of EIS 4 (2 x dual)

Number of Stacked Shots 8

Explosives

EIS

Figure 3: Colombian Data Example: Shallow Explosives vs. EIS


Explosives EIS

onSEISExplosives

Figure 4: Colombian Data Example: Shallow Explosive vs. EIS – Amplitude Spectra Data Example 3: Vibroseis vs. EIS This data example is from Pennsylvania in north eastern America and is in the Marcellus shale trend. It provides a comparison between Vibroseis and EIS in an area with reasonably good access. The basic acquisition parameters are shown in Table 3. The processed data are shown in Figure 5 with Vibroseis on the left and EIS on the right. The Marcellus shale package is the series of reflections around 1.25 seconds. Because these are old, fast rocks, this represents a penetration depth of approximately 2400 to 3000 meters. At the Marcellus interval, both source types provide good images of the shale packages.

Table 3: Parameters: Vibroseis vs. EIS

Source Type Vibroseis EIS

Group Interval 25 m 25 m


Far Offset 5987.5 m 5987.5 m


Source Interval 50 m 50 m

Fold 120 120

Source Parameters

Number of Vibrators 4 x 43,000# @ 80%

Sweep Length 8 sec

Number of Sweeps 8

Sweep Frequency 8 – 96 Hz

Number of EIS 6 (3 x dual)

Number of Stacked Shots 16


Vibroseis EIS

Figure 5: Marcellus Data Example: Vibroseis vs. EIS Technical and Economic Contributions By having a better understanding of the technical, logistical, economic and environmental considerations of the various land seismic source options, better decisions can be made as to which source is most appropriate for a given acquisition situation. Taking these considerations into account in the source selection process will provide the best data quality at the most efficient cost with the least impact on the environment. While the best option may not always be the least expensive, it is important to recognize that the principal consideration of any seismic program must be the collection of data with sufficient quality to meet the exploration objectives. If other factors are placed ahead of this, the resulting data may not provide adequate information for the discovery and development of petroleum reserves, rendering the data effectively valueless. When this occurs, the money spent on the acquisition has been wasted, the time lost can never be recovered, and the impact on the environment has produced no beneficial results. Conclusions There is no one seismic source option that is appropriate for all situations. It is also rarely the case that a situation only has one solution to the source decision. The decision ultimately becomes a balance between what should be done and what can be done, which will determine what will be done. Consideration of all the relevant factors associated with seismic sources,


including quality, time, cost and environmental and social impact, will provide the best opportunity to optimize the value of any seismic project. Above all, it is critical to never compromise the ability to achieve the exploration objectives. If the acquired seismic data cannot provide the information required to discover and develop the desired reserves, the money, time and effort spent for that acquisition has been wasted. Bibliography

Yilmaz, O zdog an. Seismic Data Analysis. 2nd ed. Tulsa, OK: Society of Exploration Geophysicists, 2001. Print. Investigations in Geophysics; No. 10. Sheriff, Robert E. Encyclopedic Dictionary of Exploration Geophysics. Tulsa, OK: Society of Exploration Geophysicists, 1991. Print. Coffeen, J. A. Seismic Exploration Fundamentals: the Use of Seismic Techniques in Finding Oil. Tulsa, OK: PPC, 1978. Print.

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