Several papers have been written over the last few years heralding the introduction of land seismic systems that are minimum cable or cable-free systems and that may or may not depend on radio links for executing various tasks. In this article, we report on the actual experiences of three operators in four different geographic areas. In all cases, the GSR (Geospace Seismic Recorder) system from OYO Geospace was used. Each station consists solely of the recorder, a ~2-ft (0.6-m) power cable, and the battery. All inter-station communication is wireless. The anticipation was that these systems would produce several advantages over conventional cable systems.

Dual-targets surveyed in Turkey

Viking Geophysical Service is currently shooting a 3-D survey in Edirne, Turkey, comprising 78.1 sq miles (200 sq km). It is being shot high-density to image a shallow gas target, with long offsets to image a deeper target. Viking typically shoots a 960-channel patch but deploys up to 2,700 channels at any given time for operational efficiency. Receiver station spacing, receiver line spacing, source spacing, and source line spacing are all 98.4 ft (30 m). Viking is using a 3,000-channel (single-channel units) GSR system with five vibrators, 14 pickup trucks, and 38 people, including surveyors (less than 60% of a traditional crew). The goal was to offer an operation that was nimble and lightweight and would operate with fewer people, fewer and smaller vehicles, less fuel consumption, and lower operating cost than competitors. That goal has been achieved through the efforts of a talented crew and the cable-less technology chosen.

A Toyota Hilux carries 40 complete stations in each half of the pickup truck. (Images courtesy of OYO Geospace)

Each day, Viking planned to devote an hour or two to equipment check-out. After a very short time, surveyors realized this plan was unnecessary as they found only two or three bad channels in every 2,000 checked. The latest statistics show only 31 failures in 22,260 channels checked, a failure rate of 0.14%.

The cable-less system comes with a Field Management Console, which allows the observer to be more hands-off dealing with the spread and more hands-on dealing with the sources. According to Robert Dunn, technical operations manager for Viking, managing the cables is a thing of the past with this system. “The observer is looking for vibe signatures — is the vibe putting out what we say it’s going to put out on the ground? What’s the phase of the vibe doing? What’s the distortion?” he said. “In a cable or radio-based system, you are very concerned with how the spread is looking. Now we’re able to start identifying problems with the vibrator before problems compound to something serious.”

Viking has found the cable-less system easily learned by field crews. It is easily containerized, therefore easily shipped from country to country, and easily scaled up to any channel count. The experience to date has confirmed all of the perceived advantages of a cable-less system, and, so far, has not supported any of the perceived disadvantages. Viking has placed an order for another 1,100 GSR stations to outfit another crew.

BP tests the system in Libya

At the 2009 SEG, Dave Howe and others presented a paper describing the full-scale implementation of BP’s ISS method in Libya to acquire a 3-D survey of more than 5,078 sq miles (13,000 sq km).

This photo shows the setup for transporting 40 GSR stations and 40 batteries in each half of the pickup truck.

Newer recording systems no longer require real-time synchronization of sources with the recording systems, which allow for recording spreads to be continually active. This leads to the ability to operate a large number of sources simultaneously, thereby vastly increasing acquisition productivity. BP conducted several field trials of the GSR cable-less node recording system, which the authors feel has the potential to change the way land seismic crews are configured and operated when combined with the ISS approach. The combination of these two technologies may enable land 3-D to be acquired for exploration purposes at costs comparable to marine exploration 3-D’s.

The 3-D survey is being conducted in the Ghadames area of Libya with a WesternGeco crew using a 10,000-channel Sercel cable-based system with 14 vibrators operating in ISS mode. BP purchased 1,000 GSR units for the first field trial, designed to assess the reliability of the recording, battery life, and positioning capabilities. The cable-less system was able to record data continuously, due primarily to its built-in GPS capability, its relatively long-lived batteries, and its 4 GB flash memory. The system also proved its flexibility to lay receivers at any spacing. A total of 1,225 receiver stations were occupied using the cable-less system during the field trial.

After success in this test, BP purchased another 1,000 channels, and 2,000 GS-One geophones, for more extensive tests with different geometries, including single-sensor node designs. The BP Libya project demonstrated the cost-effectiveness of the cable-less system compared to cable-based systems by reducing the manpower needed by a factor of three, simplifying the layout around obstacles, dramatically increasing production due to the reliability of the system, and requiring much less maintenance time.

A cable-less seismic recorder station being connected to its battery pack is small, rugged, and easily portable. It uses an integral GPS to pinpoint its location.

The data quality was improved due to better subsurface coverage given the ability to handle any layout geometry required. According to Howe, seismic acquisition specialist for BP, “Permitting issues and omissions are one of the major factors affecting the quality of land data. Reducing the environmental impact of operations through the use of cable-less technology may help to gain access to areas that are currently off-limits for cable operations.” It was easier to operate more safely because fewer people were required for shorter periods of time with equipment that weighed less and occupied a smaller volume. The Libya test showed that land crews which combine cable-less system technology with the ISS method may change fundamentally the way in which land 3-D surveys are acquired.

Apache straddles the border

Apache Corp. needed to map seamlessly across the border between Chile and Argentina on the island of Tierra del Fuego. The company had previously acquired a 3-D survey on the Argentine (east) side of the border using a cable system, desired to acquire a 3-D survey on the Chilean (west) side, and needed to achieve continuous sampling to avoid unwanted migration edge-effects over the area underlying the border. The company used a Sercel cable system on the Chilean side and used the cable-less node system on the Argentina side. (For more information about this survey, see p. __.)

BP performs a 2-D jungle survey

The test conducted by BP in Papua in June, 2009, was an important although small-scale test, incorporating only 12 GSR stations. It was designed to address two critical issues: could wireless units receive GPS satellite positioning and time information when deployed on the jungle floor (under very heavy canopy), and could these units, deployed using marine floats, be held in one position despite currents and waves in shallow water on the coast?

The waterproof GSR station easily fits in a floatation device for shallow-water or transition-zone applications.

This test addressed multiple facets of the flexibility of the system and whether or not receiving the required GPS signal was a problem in this environment. (Previous tests had shown that the GPS signal was readily received under 3 to 4 ft (.9 to 1.2 m) of snow, beneath 6 to 8 in. of soil, inside buildings, and sheltered by heavy brush. It is also established that the GPS signal is not receivable when the unit is under water.)

In the canopy test, BP determined that all six GSR units achieved lock quickly at deployment with numerous satellites, that deployment was straightforward, and that the equipment was easy to assemble and disassemble. BP concluded that the deployment of this system is feasible in jungle conditions. The flexibility afforded by the GPS continuous positioning would be a significant advantage in the jungle. Cutting and clearing could be minimized, and new station locations could be chosen on the fly if obstacles were encountered.

In the shallow-water test (Figure 4), five GSR units were deployed and achieved GPS lock quickly (one unit did not get a GPS lock and was not deployed). These five units maintained GPS lock during the test except for one unit, which became submerged for approximately 1.7 hours and lost lock. When it resurfaced, it regained lock, demonstrating that the unit and battery operations were robust in wet field conditions. The robust float and anchor system maintained the original unit locations in 2 to 3 knot currents. The seas were considered mild, with waves less than or equal to 3.3 ft (1 m), and the water depth where the test was conducted was 6.5 to 13 ft (2 to 4 m). The ability to place units in floats provides a means to continue measurements across shallow lakes, rivers, ponds, and other water obstacles as well as addressing the transition zone.

Pros clearly outweigh cons

When comparing pre-test conceptions with actual observations for the four projects, it is a bit surprising is that the perceived disadvantages have not been well-supported. The limited life of batteries and the inability to view data in near-real time are shown to be a non-issue by these examples. The four projects described here have provided fact-based insight into what cable-less systems are actually capable of delivering. It is understood that cable-based systems will not be replaced by cable-less systems overnight. It is readily apparent, however, that cable-less systems are proving to be more cost-effective than cable-based systems and will eventually dominate the industry when it becomes comfortable with not reviewing the seismic data in near-real time.