Trends in Seismic Data Acquisition – A Perspective

Many of the technical changes taking place in the oil industry are evident in the office of the explorationist. Desk top processing, workstation based 3D interpretation and visualization packages are familiar tools to the "state of the art" geophysicist.

Changes have also taken place in seismic data acquisition. They involve advances in technology combined with alternative methods of operation. Never before have acquisition parameter selection and program layout been so unconstrained. Productivity, data quality and value for the seismic dollar have all increased. As well, methods more sensitive to environmental concerns have allowed year round operations in some areas and exploration in a few previously restricted areas. These enhancements have altered the way that many oil companies explore.

These changes are summarized here in the following categories: Availability of a Large Number of Low Cost Channels; Software Driven Recording Systems; Huge Instantaneous Dynamic Range of Acquisition; and Low Impact Seismic and Crew Activity.

Availability of a Large Number of Low Cost Channels

Never before have so many channels been available at such a low cost. Practically, this means that earth scientists can increase fold and offset, enhance resolution by decreasing CMP spacing, or make creative use of this channel capacity through 2D and 3D design for little or no extra cost. Now the recording system can be tailored to the geometry of the program rather than designing the program around the recording system.

The impact of all of these channels on 3D is fairly obvious. Large, high fold, long offset 3D's can be planned and executed efficiently. However, industry has underutilized this channel availability for traditional 2D seismic projects. 120 and 240 channel crews still appear to be the norm.

Some examples of creative use of this channel capability exist. An additional Swath¹ line, for example, can be recorded with perhaps a 10% increase in cost, especially if one can use existing access. Not only is this an inexpensive way to increase seismic coverage, it provides a useful technique for undershooting physical obstacles and reluctant landowners. As well, many swath lines produce excellent data as they lie outside the noise cone of ground roll and air blast. For the same reasons, swath lines are interesting experiments in longer offset recording. The next few years should reveal many more creative uses for this channel capability.

Software Driven Recording Systems

The software driven recording system has been with us for some time now, but few earth scientists realize the profound impact that it has had on field recording techniques. It has increased recording productivity, and thereby reduced cost, by allowing for random shooting and providing real time quality control. It has also reduced the number of shooting sequence errors that occur in the field.

In the software driven recording system, mechanical roll switches are replaced by PC's, using script files built before the observer moves to the field. The script files contain all the information regarding which geophone groups are associated with certain shots. This is especially relevant to 3D shooting, where each shot needs to be associated with a patch of geophones. In the 3D case, the script files are usually built directly from a 3D modelling package. The explorationist builds a model, including all missed shots, skids and offsets and sends a diskette to the field. Not only does this tell the observer exactly how the program should be shot, it gives the program planner the ability to visualize the impact of skidded and missed shots. In this fashion, deficiencies in fold, azimuth or offset can be remedied BEFORE the program is shot. While this may seem like a somewhat trivial point, software driven recording systems have had an enormous impact in two areas; productivity and quality control. First, as the observer is working from script files, he/she can move around the program randomly and very quickly. (This has wreaked havoc on a few processing shops, since the data may now come in essentially random order and require extensive sorting.) The practical spin-off is a significant increase in shooting speed. As there is no requirement to shoot a line in sequence, a restriction imposed by roll switches and channel limitations, multiple shooters can be deployed along the line. It is not uncommon for a 3D project to employ five or even six shooters. Under perfect conditions, with little roll-a-long or down time, a crew could conceivably fire 600 shots in a good day's work. We recently shot a small 3D (11 square km) in 6 hours. Naturally, safety must be incorporated into any seismic operation, and highly productive operations are no exception. However, as the multiple shooters are often kilometres apart, there is no conflict between safety and productivity in this case.

Second, the software is capable of monitoring and testing all of the seismic recording equipment and vibrators while recording. This means that the observer can check the line and, look ahead and monitor vibrator quality without impacting production. No more stopping to check things.

There is one interesting side to this leap in recording productivity. A fully modern seismic recording crew, working at peak productivity, can keep approximately three front end crews occupied. It is not possible for one team of cats, slashers, surveyors and drills to keep pace with this type of efficiency.

High Instantaneous Dynamic Range of Modern Recording Instruments

The recent release of 24 bit converters and Delta Sigma technology have vastly increased the system dynamic range and instantaneous dynamic range of several instruments. As a result, saturating the dynamic range of modem instruments is next to impossible. In the past, exceeding an instrument's dynamic range was always a concern. With a state-of-the-art instrument, nothing short of a nuclear blast will overload the system. It's just not much of an issue anymore.

Consequently, seismic programs may now be designed with signal enhancement in mind, rather than noise rejection. Previously, the fear was that high amplitude coherent noise, in the form of ground roll, would exceed the dynamic range of the recording system and distort valuable, low level signals. As dynamic range limitations are no longer a concern, long, multi-phone geophone arrays and harsh, low-cut filters are no longer absolutely necessary for noise rejection reasons. Geophysicists continue to use arrays and filters for other reasons, many of them very good, but they now have far more freedom since ground roll rejection is not the overriding design criteria.

At least one instrument manufacturer has implied that geophone arrays will become obsolete. They suggest that a single geophone is all that's necessary. It's my opinion that this extreme will not become the norm, as arrays also enhance SIN by random noise rejection. However, the number of elements in the array and the array length will likely become smaller, increasing resolution, reducing smearing and eliminating elevation artifacts. Several exploration companies are currently using 4-phone arrays. This will have a fairly large impact on portable seismic projects.

It's quite important to note that the ground roll problem hasn't disappeared. If we don't deal with ground roll in the field, we'll have to deal with it elsewhere. However, as long as the signal is preserved by the large dynamic range of the recording system, we have the luxury of handling the ground roll in the processing shop.

Preponderance of 3D Acquisition

It's no secret that 3D seismic has been sweeping North America for several years. A rarity in the mid eighties, 3D seismic now makes up over 50% of many contractors' workload. Its growth in popularity will likely continue.

3D surveys have been used primarily as a development and exploitation tool. The American basins, well into maturity, are prime candidates for 3D seismic. Here the method is used to guide infill and step-out wells. The Western Canadian basin, though not as mature, is also strongly focussed on exploitation and development. Here again, 3D is the method of choice.

In terms of statistics, 3D seismic currently accounts for 17% of the programs submitted to the Alberta government.² However, it also accounts for 52% of the kilometres recorded. The implication is that 3D's tend to cover large areas of land and use fairly tight line spacing. The trend appears to be toward more 3D acquisition and larger 3D programs.

There is a strong perception that 3D is a very expensive tool. Most of the "sticker shock" stories depict enormous, high resolution 3D's, covering hundreds of square kilometres. While it's true that 3D's can carry multi-million dollar price tags, it's important to note their objective, resolution, and areal extent. Smaller, well-focussed, well-planned 3D's can often achieve their objectives for under $200,000. In order to hammer the point home, examine typical "ballpark" 20 and 3D costs. 20 development infill projects can cost $32,000/km² (high resolution 20 grid, lines on the quarter section). By contrast, typical 3D costs are in the $20,000/km² range.³ Even disregarding the technical advantages and resolving power of 3D, it's apparent that if detailed work is necessary, 3D is cost effective.

Low Impact Seismic

One of the dominant trends in seismic data acquisition involves the method by which lines are opened, accessed and travelled. Concern for the environment is determining the method by which seismic program is laid out and approved by government agencies. Exploration tactics have changed to adopting Low Impact Seismic (LIS) to reflect environmental concerns.

While the definition of Low Impact Seismic is controversial and currently under discussion, the concept is quite simple. The idea is to have as little impact on the ecosystem as possible and still achieve the geologic and economic objectives.

One definition states that, "LIS may have: lines up to seven metres in width; straight line sections; timber avoidance sections; handcut sections; and use wheeled or tracked vehicles, or any method that minimizes the disturbance to the surface, and may be a one-pass operation".⁴

This definition is very broad, but in its more extreme forms LIS uses timber avoidance and hand cuts. Avoidance cutting leaves the large stands of timber intact, while the access line follows a sinuous path through the forest. Smaller lighter cats are employed, because in a one-pass operation, drills are the only traffic that will follow. The cats wind their way through the forest attempting to cross the seismic line at each source point. This allows the drills to place source points on line. Other than this simple requirement, the cats follow the path of least environmental resistance.

A hand cut line of sight, perhaps a metre and a half wide and directly on line, gives surveyors and jughounds foot access to the area. In the 3D case, receiver Iines are often foot accessed, 0.5 to 1.5 metre hand cuts. Geophone layout and pick up is commonly supported by helicopter.

Although LIS appears to be inevitable, not everyone agrees on the definition or operating methods. Proponents of LIS cite the following advantages:

1. Environmental impact is minimized. Large trees are left standing. Source lines are only travelled on once by the cats and once by the drills. Receiver lines may not be travelled upon by vehicle traffic. Minimal ground disturbance occurs.
2. Visual impact is minimized. Seismic lines are much less visible from the air and from the ground. Note: As this will result in an interesting spin off, in a few years "old" seismic lines will be quite hard to find.
3. Increased access. Forestly will often allow Low Impact Seismic in areas and time periods previously restricted.
4. Gaps are minimized. Data quality IS improved.
5. Salvage, timber damage and reclamation costs are reduced.

Opponents of LIS level the following criticisms:

1. LIS may be more dangerous than conventional seismic.
2. It may be more expensive to operate in an US mode.

It is important to note that LIS is only economically viable because of the advances that have taken place in instrument technology. Mile for mile, LIS could be more expensive than any conventional method. But because there are a multitude of low cost, light telemetry channels, because instruments are software driven and can accommodate multiple shooters, and because the number of geophone array elements is decreasing, LIS can be economically competitive and environmentally friendly.

3D techniques may also help reduce environmental impact. With 20 seismic, line location is usually critical. The line must generally cross directly over a specific feature in order to image it. In 3D acquisition, line location is just one of many variables that go into the program design. During the acquisition process a large area, the patch, is essentially undershot. As the entire region will eventually be undershot, exact surface line location requirements are usually less stringent. This is not to say that source and receiver line configurations can be altered radically. Fold, azimuth, offset, cost and channel requirements still have to be met.

However, unlike 2D, there are usually several acceptable alternatives available to the program planner. If a source line has to be moved out of a river valley or off Main street, it can often be done without impacting the technical quality of the program.

Crew Activity

Seismic activity is well down from its peak during the eighties 5 Activity levels for the early nineties were fairly stable at 30-40% of their mid eighties peak, but will likely show an increase for '93 and '94. Although the total workload is down, there are fewer contractors available to complete the work. Those that are left are being kept quite busy. The winter of 93/94 is shaping up to continue this trend. As I write this in early October, several crews are already booked through to spring breakup.

Winter is still the prime season for data acquisition, but many crews are busy through the summer and a few even managed to work straight through spring breakup. Seismic is gradually becoming less of a one season activity. The "mini-boom" in the Alberta energy industry is fueling a great part of this year round activity, but enhanced operating procedures (LIS) are allowing it to happen.

While program cost is still often the main driver in seismic acquisition, program planners now have degrees of freedom that were not available even a few years ago. Advances in instrument technology, operating methods and channel availability allow for programs that achieve unparalleled technical objectives while keeping costs in line.