New acquisition techniques minimize the intrusiveness of seismic surveys without compromising data quality.

Ask a dozen different explorationists how they define "low-impact seismic" (LIS) and you'll get at least 12 different answers. This signifies the variety of methods used and experiences gained in different world regions to achieve LIS. However, it also is an indication of the growing importance of the LIS factor when planning seismic acquisition operations.

Generally, we can say that LIS is a generic term, covering how surveys should be carried out so as to have the smallest effect on the local environment. The LIS issue on one crew may be able to minimize the damage when working in an area of outstanding natural beauty. In another, it's about having the ability to operate so that the crew can be in and out before the local population even knows it's there. With the growing security problems of crews from the West operating in regions where they simply aren't welcome, it is important to cope with the security side of LIS.

In some areas, the requirements from landowners or environmental authorities to fulfill LIS conditions can radically change how a survey is carried out. Reducing impact while trying to achieve some worthwhile goal almost always costs money in the short term. But in land seismic, the cost of getting the impact low enough can make the whole operation unviable.

Most people in the industry feel that LIS pressures are likely to become tougher with every passing year. The problem is that there are already a number of areas around the world where seismic reflection surveys have stopped, as the cost of acquiring such data is higher than the profit to be made from mineral exploitation. There are even more areas in the world where the cost of complying with LIS requirements has become so high that the total cost of the survey versus potential "knowledge reward" is now rather marginal.

Most explorationists have little or no issue with LIS. They no more want to see a wrecked local environment than the people who live there. With pressures from green groups tending to increase, no oil exploration-related blot on the landscape ever goes unreported (or, quite often, unexaggerated). However, the current financial problems (some call it "crisis") in land seismic have exacerbated the situation; it is said that a major international bank has described some sectors of exploration as "an unsustainable business model." Few in the oil industry think we need a bank to tell us that, but how much more unsustainable it is due to meeting increasing LIS requirements is still an unknown.

Geophysical data needed by oil companies nowadays is being used more and more for reservoir delineation on known fields, for monitoring production effects and so on, and thus needs to be of very high resolution. To get the ultra-detailed geophysical information required to accurately estimate reservoir characteristics requires huge seismic effort; that effort is more likely to leave its mark on the environment, making LIS compliance even more problematic.

Seismic land acquisition is characterized by various teams of people covering some section of the area under investigation. This may start with surveyors staking out where seismic shots and receivers are to be placed. If the seismic source is explosive, drilling crews come along and make relatively shallow holes and plant dynamite charges. Then one or more crews who lay acquisition instrumentation come along to deposit and connect their hardware. Shooting crews come along and activate the charges when the recording system is ready. Once the seismic shots are taken, the whole process is reversed; crews pick everything up, clear it and move on.

If we can minimize the number of people or vehicles needed to visit each part of the survey, or reduce the need to return to a particular spot on the survey area, then we not only save money, but we lessen the chance of disturbing the environment. One solution is employing "heavy effort crews," where more equipment than normally expected is used, but a lot more can be laid out at one time, reducing the need to pick up and replant equipment as the survey progresses.

However, the problem is that many crews carry around literally hundreds of kilometers (more than a thousand in some cases) of seismic cables. It is not unusual for some large desert crews to cart 125 miles (200 km) or more of cable from the back of the survey prospect to the front on a daily basis. The total weight of crew instrumentation can weigh hundreds of tonnes. With this much weight, the preferred form of transport is some sort of land vehicle or fleet of them. But the greatest cause of death and injury on most seismic crews is vehicle-related. A fatality on a crew can shut it down entirely, and in some parts of the world, the large number of crew vehicles is causing excess emissions, raising LIS concerns.

Even after we lay down all this equipment and start recording, creatures great and small can nibble away at these highways of cables, and operations must stop while chewed cables are changed, as they are no longer transmitting the needed data. Often, the sheer volume of cables and connectors that must remain in operation in tough environments increases the odds of failure more often than we can afford from an economic and an LIS point of view. Also, we need to energize all this equipment. Even the way a system is powered can impact both the LIS factor and the potential profitability of an operation. If the battery dies before we finish recording, we have to traipse back to the battery to swap it out just to continue recording. Therefore, systems that use a low number of batteries to power a large number of seismic channels require larger batteries (a potential weight hazard for starters) but also are more susceptible to running out of energy before the survey is completed.

In really tough environments, helicopters can carry equipment around, but at around US $1,000/hour, their use must be kept to a minimum, or costs spiral even further out of control. In areas where crew security is an issue, helicopters are also a bit of a give-away about the presence of "foreigners."

The alternative ways to carry this equipment around are to use man or beast. Large "man-portable" crews can look like something out of a Cecile B. de Mille film, with casts of thousands, LIS impact and costs to match. Using vehicles, or even large man-portable crews, requires wide tracks to be cut into forest or jungle, which is not conducive to undisturbed wildlife (flora or fauna) and the LIS requirement that, after a crew has gone, it should not be obvious it was ever there at all.

The single most obvious way to achieve all of the above is to reduce weight and volume of seismic hardware. Modern telemetry systems have come a long way in this respect in recent years, and a number of manufacturers are keen to tell us just how light they have managed to get their equipment. But the fact is that the total weight of any crew using cables is really rather massive. The main component of almost every land cable system is the cable itself, accounting for between 60% and 85% of the system weight. Ongoing cable maintenance and cable replacement costs can run into millions of dollars per year for a medium-sized operator. Recording time lost due to cable (and the thousands of connectors that link them together) in some cases is the biggest single cause of lost recording time.

Acquisition systems using VHF radio channels instead of cables to broadcast seismic channel information also have progressed; fewer VHF channels are required for a given amount of seismic data to be passed to the central recorder. But VHF-based systems have a variety of their own problems, not the least of which are cost, licensing and the need to take up significant sections of the crowded VHF bandwidth to get seismic data back from the sensor to the central recorder in something approaching real time. These are some of the reasons we still tend to use cable systems even in logistically difficult areas.

So the problem for land seismic is that cost and weight/volume of traditional VHF and cable systems is still nowhere near where it needs to be to change the face of the industry to the extent we require. It is clear that land exploration needs to change. We need to find a way to radically reduce the weight per channel of equipment and the cable/connector-dependence. This helps the health, safety and environment (HSE) and LIS issues and the profitability of the crew. Ideally we would like to do away with as much cable and as many connectors as possible. This requires the use of some form of radio, though not the type described above. How about a system that could take advantage of some form of advanced cellular radio transmission?

If such hardware existed, we seriously could alter the weight, labor and vehicle requirements on a crew, allowing crews some profitability while more easily meeting LIS requirements. The lower the system's weight, the easier it is to deploy, and the better it is for acquiring the large numbers of channels needed for these ultra high-resolution surveys. A hybrid system would not force operators to think in terms of just scraping in to met LIS needs or laying out just enough channels in the hope of getting reasonable data. Operators would not need to think in terms of "minimal" any more.

If the total system weight could be low enough, we could replace geophone strings with single sensors, saving more cost and weight since the geophone array becomes heavy and expensive compared to the additional electronics. A move toward single sensor or smaller seismic group interval helps remove the "acquisition footprint," which can mask the true nature of the subsurface, allowing the operator to make better drilling decisions.

This "best of all worlds" system would have the logistical flexibility of cable-free RF hardware but have no radio licensing problems, a sort of hybrid system. Ideally it also would have the capacity for an unlimited number of channels and no LIS impact during deployment, allowing operators to deploy extra channels "at will and at random."

Fortunately, such a system does now exist. It is a start-from-scratch product called the Infinite Telemetry System - the "it" system - and has been 6 years in the making. The system is a new breed of 24-bit acquisition hardware, although using a concept familiar to us in other areas of our hi-tech lives; along with "cable seismic" and "RF seismic," we now have "cellular seismic." Mobile phones freed users to communicate easily compared to a fixed land line, and this system releases crews from the drawbacks of traditional RF and the strait-jacket of cables.

The ground equipment weighs only fractions of the next lightest product, so it's very low-cost and "environmentally-friendly" to deploy. A typical heavy effort crew using the system would be perhaps 20 tons lighter than its nearest neighbor. Using figures supplied by a major oil company, it has been estimated that some crews could see their direct costs halved and greatly reduce indirect overheads such as LIS and HSE exposure.

Employing patented, transmission error-free cell radio technology, this universal system's four-channel remote acquisition units (RAUs) communicate with their nearest cell access node using the license-free 2.4 GHz ISM band. Real-time data rates within each cell are massive - around 11 Mbs - so all sensor data can be transmitted to the recording unit within the shot cycle, making it suitable for impulsive and vibroseis crews.

The RAUs can acquire data using conventional arrays, hydrophones and digital sensors as well as single or three-component (3-C) sensors. Each RAU has: expandable onboard shot memory to allow huge system flexibility; and a powerful DSP permitting a variety of "number crunching" functions, including correlation, stacking, noise editing, compression and onboard testing, making it ideal for high density or multi-component surveys.

The last major breakthrough in land seismic acquisition was at a time when LIS had not been invented - the 1960s, when we witnessed the change from analog to digital recording. It enabled land exploration to advance with more channels and less environmental impact than would otherwise have been possible. The latest system is the long overdue leap to bring viability once more to land operations.
For more information, visit www.vibtech.co.uk.