EXPLORATION
Are wireless seismic systems the wave of the future? Or are they a niche system?
Someday, all seismic acquisition might be done with wireless systems, but until the batteries and electronics weigh a lot less, the equipment is best applied in areas where the logistics of cabled systems are too difficult.
Perry Fischer, Editor
There’s been a not-so-quiet “revolution” going on in land-based seismic acquisition equipment over the past few years. It’s wireless acquisition. Actually, it’s been ongoing for about a decade in various ways. Fairfield’s “The Box” was one of the first to have some wireless aspects. Its primary use is for transition zones and it is still in use today.
The term “wireless” can mean many things. As it implies, of course it means a reduction in wires. But power-supply methods and requirements, and data-transfer methods, speeds and protocols all vary considerably across the various technologies.
A typical wireless unit has a battery pack, which may be of several types (e.g., alkaline, lithium ion, metal hydride), a non-volatile flash memory, a micro-processor, a GPS unit and a radio. A short wire connects the geophone sensor to the unit.
Data transmission can occur at radio frequencies but often remains in storage within the field unit, making it more like an autonomous node, except with QA/QC data that are transmitted in the field. Complete data download occurs during retrieval and battery replacement/charging. High channel counts are sometimes offered as a benefit of cableless systems. But recent improvements allow 30k to 50k and higher channel counts with either cable-based or wireless telemetry.
The actual sensors used-whether single or multi-component-are generally the same sensors that are used with cabled systems. However, except when operating in a purely autonomous manner, QC data can be wirelessly transmitted from each sensor package and reveal problems such as noise, coupling or an individual sensor malfunction early in the acquisition process.
Cableless systems allow for recording of individual sensors, but the advantages (and some disadvantages) of array grouping will be sacrificed. However, the individual data records can be combined and stacked in unique ways in the processing lab, albeit at some additional cost and time. In general, the recording of signal (P, PS and S) should be enhanced, but random noise could increase.
Whenever surveys get into the range of tens of thousands of channels, operational logistics, cost and HSE considerations become magnified. Since wireless acquisition in this channel range needs to be deployed and operational for up to a month, perhaps more, the speed at which stations can be laid out becomes very important, as does the type, longevity and weight of the batteries. The ability to reduce or eliminate traditional ground surveys with wireless GPS, as well as continuous field QC, can add to the attractiveness of the system relative to conventional systems.
CONVENTIONAL VS. STATE OF THE ART
Single sensors can remove statics that are sometimes found within arrays. A recent SEG paper1 discussed the noise and array-spacing issue: “Single sensors eliminate intra-array statics … To deliver the same final data quality, surveys with single sensors must have smaller station intervals.” The authors found that single sensor intervals must be spaced at not more than half of the group intervals to achieve the same data quality.
The authors compared the various systems generically.1 Figure 1 shows the approximate weight relationship for three types of recording equipment.
1. Wireless, individual 3C sensors
2. Cabled, state-of-the-art, digital, 3C point receivers
3. Cabled, six-geophone array systems.
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Fig. 1. Comparison of three systems in terms of equipment weight vs. receiver spacing (adapted from Lansley, et al., 2007)1.
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As common sense would dictate, as the receiver spacing gets closer, equipment weight increases. As Figure 1 shows, for receiver spacings over 30 m, the single-point digital systems have a substantial weight advantage over the six-geophone array systems, while cableless systems have a modest weight advantage over cabled single-point 3C digital systems. However, for receiver spacings 30 m and closer, the cabled system with the single-point 3C digital sensors begins to be the “lightweight” winner.
The authors concluded, “High quality, high density, multi-component 3D surveys are being acquired very cost-effectively using single sensors. As the spatial sampling becomes smaller and the trace density greater, significant operational and recording efficiency benefits are gained using cables.” This assumes, of course, that the terrain/environment will allow the use of cabled systems.
TECHNOLOGY OFFERINGS
Ascend Geo. This company has been offering its wireless acquisition system, Ultra, for the past few years. It is fully commercial, with field experience in many regions, continents and environments, including dense rain forest, desert, highly urbanized areas, farms, plains and forests. The system offers “high channel point array with digital group forming ... and analog 3C.” The system weighs in the range of 1.5-2.0 kg/channel, including internal battery, and uses only one VHF 12.5 kHz channel, although each box is programmable in the range 148-174 MHz, with other options. The system does not transmit to a central tower, but uses up to about 60 transmitters/repeaters in a field, to cover the largest area with the least power. Typically, data downloading and battery recharging occur when the field units are retrieved and plugged into a rack, which is purpose-built and is typically housed in a field trailer.
Input/Output. This company, now called Ion, is testing its recent (2006) entry into the cableless market. BP wanted to reduce environmental impact at Wamsutter field in Wyoming, and I/O needed a place to test its new FireFly cableless acquisition system. BP is the biggest player in the Wamsutter field, with 950 operated wells producing 135 MMcfgd and an interest in 352,000 leased acres.2 FireFly’s first field trial was acquired by Global Geophysical and comprised about 7,200 shot points of 3C surface seismic data over 28 sq mi-a dense design meant to acquire a multi-azimuth dataset. The sensor is I/O’s well-established MEMS-based Vectorseis unit. It lies at the end of a few-foot-long cable that connects and transmits data to the transmitter/recorder unit, Fig. 2.
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Fig. 2. I/O’s (now Ion) Firefly wireless unit.
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At Wamsutter, BP flew a LiDAR survey over the survey area. This, combined with GPS, provided enough control for survey planning that conventional survey crews were not needed. Position data is transmitted from the sensor to the recording unit via Bluetooth technology; it’s written onto the headers of the seismic data files. According to Craig Cooper, land seismic project coordinator for the North America Gas Business Unit at BP, “This will not only save time during surveying but, because the positional information is written directly into the trace headers, will mean we won’t have to worry about correcting human-generated errors in the merge step during processing.”3
Deployment for the BP trial consisted of a helicopter flying along a pre-determined route, dropping off 55-60 backpacks. Each backpack holds six complete units, including batteries and sensors. As expected, during the new technology’s first field trial, a number of “bugs” were discovered. These have mostly been resolved.
Building on the lessons learned at Wansutter, Apache Corporation tested the system in North Texas this summer. Apache, PGS and I/O teamed up on the shoot, which comprised 15 million traces in a 77-sq-mi survey area over a 30-day deploy/retrieve cycle.4 A one-week delay occurred due to a firmware bug that was discovered just after deployment of 3,000 stations, but that has been fixed. Ultimately, 7,000-8,000 stations were deployed. Some of the data from these surveys is in the process of being released.
Sercel. This company bought Vibtech last fall, a company that was established in wireless acquisition with its Infinite Telemetry (IT) 3D recording system, which was introduced in 2002. Then, in 2006, the company introduced its UnITe Cellular Seismic system. IT is configured with a four-channel remote acquisition unit and hybrid radio/cable telemetry. The company compares it to a cell phone tower system, with field repeaters to transmit to a Central Control Unit (CCU) within the shot cycle.
John F. Smith, co-founder of Vibtech, described the system:5 “The seismic spread is divided into a number of cells, each of which reports to a cell access node, which is connected by fiber optic cable to the CCU.” UnITe uses a single-channel base unit that is GPS-enabled and real time radio telemetry. “Data are sent back using wireless telemetry and simultaneously stored in the memory within the acquisition unit. Each unit can be remotely programmed to record autonomously, send reduced data sets for quality control, or send back the entire data file within the shot cycle,” Smith said.
CONCLUSIONS
Despite the press and marketing efforts on wireless technologies, their benefits range from non-existent to extraordinary. Simply transmitting data through the air does nothing beneficial, per se; in fact, it can reduce data transmission speeds and potentially introduce noise relative to cabled geophones. Most advantages hinge on equipment weight reduction and efficiency of deployment. However, in terms of point sensor density vs. weight, very high channel counts at dense sensor spacing can be more efficiently acquired using cabled, state-of-the-art, 3C point receivers.
But eliminating the connecting cables can often create cost savings through more efficient deployment, which in turn can enable much greater coverage. Perhaps the most obvious, primary advantage of wireless systems lies in HSE benefits, particularly in accessing difficult terrain, whether in terms of a rugose landscape, urban settings or mitigation of environmental disturbance. In such cases, cableless sensors may be the only way to get the job done. 
LITERATURE CITED
1 Lansley, M., Laurin, M. and S. Ronen, “Land 3D: Groups or single sensors? Cables or radio? Geophysical and operational considerations,” 77th Annual International Meeting, SEG, Expanded Abstracts, Sept. 23-28, 2007.
2 Williams, M., “Seismic without cables,” Nickle’s New Technology, April/May 2007.
3 Friedemann, C., “Unlocking the potential, of challenging gas reservoirs,” OnPoint, July 2007.
4 Williams, M., “The evolution of FireFly from Wamsutter to the Apache project,” OnPoint, June 2007.
5 Beims, T., “Array of new technologies, improved business conditions transforming land seismic,” The American Oil and Gas Reporter, July 2006.
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