The desire to conduct surveys without cables has been an elusive, perhaps unattainable goal since the dawn of seismic exploration. A fundamental problem is that channel counts – the number of geophones on the ground – have been growing faster than wireless technology grew to support them. Has technology finally caught up to make cableless seismic surveys practical?
Strings of Geophones
In the late 1970s, seismic surveys were done with cabled, multi-channel acquisition systems, conducted in what we now call 2D: a string of geophones in a line connected by multi-conductor cable, typically hundreds of wires in a heavy polyurethane jacket. The cables consisted of short sections connected by hermaphroditic connectors, the length of individual segments set to what a person could carry. As the number of channels grew, the lines got longer and the segments got shorter. Each segment had what are called ‘takeouts’ where geophones were attached.
With the introduction of 3D surveys, ‘distributed systems’ were developed to support the larger arrays of parallel lines of geophones. Modules called remote boxes digitized the signals from a handful of geophone groups. The digital data was sent down a coaxial or fiber-optic cable to a central recording system, eliminating electrical noise after the remote box, allowing for very long transmission paths and wide areal coverage.
However, because the geophones were separated by as much as 50m between stations, th ere were still analog cables connected to the boxes, plus the longer digital cables. Systems expanded to thousands of channels. As they grew, new problems appeared. A typical 3D seismic crew might have 150 km of a ssorted cables on the ground, so road, river and railway crossings were a problem. In many areas, animals chewed up the cables overnight, so that a few hours were required every morning for repairs. Once a system grows to 10,000 channels in West Texas, it becomes difficult to manage repairs, although much larger systems could be deployed in places like North Africa, where there is a lot of open ground and few animals.
Early Cableless Systems
Meanwhile, geophysicists considered the advantages of eliminating cables. As is the usual pattern in geophysics, patents were filed and granted long before the technology became available to make these ideas practical. People worked with what was available: there were no portable hard disk drives, cheap A/D converters, GPS timing signals, or low-power solutions suitable for easily portable batteries.
The earliest cableless seismic systems used cartridge tape drives to store the data, but because of their complexity, power consumption, weight and cost they never achieved wide acceptance. They were used only when cables were impossible, generally for environmental concerns. The OpSeis Eagle and the Fairfield BOX® offered wireless recording for modest numbers of channels, limited by the bandwidth available in the RF band.
The first cableless system to become broadly successful was the RSR™, built by Input/Output (now ION Geophysical®). This was a six-channel acquisition unit that stored its data on a disk drive. The units operated autonomously, and could be located almost anywhere. Data was collected by visiting the unit periodically and transferring it to a second device used to transport it to a central computer. The RSR features VHF radio communication that can send quality control information to a central recorder, and if you were patient, you could even display a screen image from a shot. Bandwidth was too limited to provide more than the occasional snapshot of the data.
The RSR was introduced in 1996, and despite its age, is still in use today for surveys with access problems. Dawson Geophysical®, for example, used an RSR system to survey the grounds of the Dallas-Ft. Worth airport for Chesapeake Energy Corporation® – a project that would have been virtually impossible with a cabled system because of the runways, taxiways, and large buildings.
Similarly, environmental and access problems restrict the deployment of cable systems in many areas – you cannot run a bulldozer through a national park or lay cables across roads in cities and some rugged terrain.
First Real-Time System
The first serious attempt at a full-record, real-time wireless seismic system was by Vibtech Ltd, founded in 1996 in Scotland. The system was based on cellular technology, where a group of individual units in a cell communicated with specially erected towers spread around the survey site, and (initially) connected by a fiber optic cable to a central computer. This required quite a bit of infrastructure, and still suffered from bandwidth problems. The company was sold to Sercel® in 2006, who developed the system further under the name UNITE.”
Also in 2006, Input/Output introduced a cableless system called “FireFly®” at the annual meeting of the Society of Exploration Geophysicists. This operated similarly to the RSR, with VHF radio communication to a central recorder with QC status and samples of data. The initial Firefly system was only available with VectorSeis™, Input/Output’s 3-component MEMS sensor, although an adapter to allow use with conventional geophones became available later.
OYO Geospace introduced the GSR™ in September 2007. This is a blind acquisition system, with a relatively efficient transcription method, and has become the most successful of the autonomous recording systems. As of June 2011, OYO had sold about 100,000 channels; a small percentage of the number of channels of cable systems sold in any typical year – there is much room for growth.
Unite from Sercel is a cable-free system which offers a lot of flexibility. Within a 1000m range area, remote acquisition units are able to communicate directly to the dog house, so an operator will receive data and QC information automatically and can adjust the parameters as necessary. The system can also be integrated with cabled systems such as Sercel 428XL, recording data in a single SEGD File and sending data and conducting QC in real time. It has a useful capability – drive-by data retrieval, whereby a technician carries a data retrieval device past each of the units at regular intervals. This collects data wirelessly from the units and also affords an opportunity for QC checks on battery level, sensor quality and memory status can be at the same time.
Fairfield Nodal has an autonomous system similar in concept to their successful ocean-bottom recording packages. They combine the acquisition circuitry, geophone, and battery into a single, self-contained plastic cylinder that is placed on the ground or buried flush with the surface. The units are collected after use and placed in a rack that extracts the data and charges the battery.
Wireless Seismic Inc. has also introduced a real-time cableless seismic system, eliminating much of the usual wireless infrastructure by using the acquisition units as radio relays. The seismic data is collected from the local geophone group and relayed down the line from station to station. Because the relay distance is short, the radios can be low power and still achieve a reasonable bandwidth. After the data from all the units in the line is collected at the base station, it is passed on wirelessly to a central recorder. Instructions and acquisition parameters can be sent back up the line using the same approach. Because the system operates in real time, it provides a noise monitor and a complete suite of interactive self-test functions. The central recording system resembles that of a cable-based system in form and function.
Cableless More Efficient?
Much of the work being done with cableless systems has been for “in-fill,” in combination with a cable-based survey. This is typically an area with particularly difficult access, or where extra channels are required to improve resolution over an interesting target. At present, very few crews are operating exclusively with high-channel-count cableless acquisition units, but the number is expected to grow. There have been some anecdotal reports of significant improvements in efficiency, resulting in faster surveys with smaller crews.
So, are seismic systems without cables finally within reach? Contractors are working today with cableless systems or with partially cableless systems filling in. Cables are still the norm, and probably will represent a significant percentage of seismic surveys for the immediate future, but sometimes they can’t be used because of logistic, regulatory, or environmental reasons. If the anticipated economies are substantiated, crews will have to go cableless to remain competitive. Seismic surveys are continually evolving, driven by the demand for more channels on the ground, larger arrays, and tighter resolution, and of course by the fruits of technological innovation.