Cabled versus Cableless Seismic Recording Systems
16 Sep

Cabled versus Cableless Seismic Recording Systems

Reprint from Seismic Profile, Autumn 2013, Issue 5 

People who toil in the fields to collect seismic data don't much like cables.  They didn't like them in the days of central acquisition systems, with their miles of analog cables, and they still don't like them now in the days of distributed seismic systems with even more miles of the digital cables needed for 3D surveys.

Cables are heavy and bulky [Figure 1].  Even though the fiber optic data pipe is miniscule, the polyurethane jacket that protects it and the copper wires are not.  The HSE exposure from cables is substantial.  Wrapping 20+ kg or line-tap cable around your shoulders and walking through uneven terrain safely is a difficult task.  In many environmentally sensitive areas, regulatory issues make deployment of cables problematic.  In developed areas with roads, buildings, and railroad tracks, deploying cables can be a real problem.  And, you need permission to lay a cable on someone's property.  In many areas, creatures large and small enjoy chewing them, adding to the morning trouble-shooting routine.

Cables work very well in North Africa and the Middle East, where surveys with 100,000 channels can be laid out.  There are few highways, rivers, animals or people to speak of, labor is inexpensive, and the biggest problem is finding your geophones after they've been buried in the sand.  But vast areas of the oil patch are antagonistic toward cables.

Geophysicists imagined solutions to the problems of cables well before the technology existed to eliminate them.  Interim solutions with limited capacity were developed and almost instantly made impractical by the growth in the number of channels required for a modern survey.  Then, with the advent of the personal computer, usable hardware became available that designers could exploit to build practical cableless systems.  The first effective cable-free seismic system was the RSR from Input/Output (now INOVA Geophysical).  It was a stand-alone acquisition system that collected data from six geophone groups and wrote it into memory in the box.  The RSR could communicate system status to the central recorder by VHF radio, but the seismic record was collected by physically visiting the box and transcribing the data in a portable harvester.  Introduced about 20 years ago, RSR systems are still in use, but are finally being retired in favor or more modern acquisition systems.

In the mid-1990s, a number of technologies came together that allowed clever designers to build the next generation of cableless seismic systems.  All the building blocks became available:  affordable 24-bit A/D converters, low-cost powerful microprocessors, relatively high capacity memory chips, wireless radio chips, and lithium batteries.  As cell phones, personal computers, and wireless internet devices became ubiquitous, the component parts available to build a wireless seismic system became more powerful and affordable.

Geophysicists have never been short on creativity, and several companies jumped into the emerging market for cableless seismic, designing systems with a variety of capabilities, slightly different form factors, but identical in their ability to collect high-quality seismic data, one way or another.

The most basic implementation is sometimes called "autonomous node" (or the slightly less precise "nodal").  These units are placed in the field at the desired location and left to acquire data and write it into an internal memory.  Each unit has a geophone, an acquisition unit, and a battery with enough capacity to power the unit for an extended time in the field, sometimes as much as two months on a 12-hour cycle.  As the survey rolls by and it is time to move the unit, it is picked up and transferred to a central location where the data are extracted from the memory, a freshly charged battery is attached, and the unit is re-deployed to the field.  The data are annotated with collection time from a GPS, which also confirms which station coordinate it came from.  The information from each station is merged into a seismic record with a data base program.  The record from a single unit is in a common-receiver format, which does not provide a good look at the results from a geologic perspective.  The operator only gets a good look at the partial geophysical record when it is reformatted into a common source gather, much later in the process.

Because you can't see the data as it is acquired, systems that operate in this fashion are often called "blind" systems.  As the design of a blind system is easier than the design of a real-time system, autonomous nodes were the first to penetrate the cableless market in significant quantities.  Geospace has been the market leader with the GSR and GSX model [Figure 2], with 230,000 channels sold as of June 2013.  Other equipment providers, including the FairfieldNodal Z-Land system, the Global Geophysical AustoSeis system, the Sercel Unite (used in nodal form), the iSeis Sigma, and the INOVA Hawk, have all made significant sales to the marketplace, as well.

Seismic observers, and especially the client representatives, are not too happy with blind systems.  However, because they simplify the acquisition process and provide meaningful productivity improvements, they are becoming more widely used.  Despite early concerns, reliability has not been an issue, with less than a 2% failure rate.  In addition to the lack of visualization of the geophysical data, observers worry about noise -- wind and trains, for example -- which cannot be monitored.  Vandalism can be a problem in some areas, with not just the loss of the unit, but of the data inside the unit, as well.

So, what really is desired is a system that offers the logistical convenience and flexibility of a nodal system with real-time data return and data visualization like a cable-based seismic system.

How do you bring home your data in real time without cables?  By radio!  One solution was conceived and patented by the company, Vibtech, which developed the UNITE system, now a product from Sercel.  This system used a number of radio towers scattered over the survey area, connected by radio or fiber-optic cables.  The individual units communicated with nearby towers that passed the information to the central recorder.  It worked, but the tower infrastructure is so burdensome that what users normally do is put up just one or two towers to monitor seismic noise and then operate the bulk of the array blind.  Sercel has developed a unique "drive-by" collection process that retrieves the data on a next-day basis using a harvesting device that is transported about the survey area within sight of the individual units.

There are two immediate problems in designing a fully functional real-time wireless system:  radio bandwidth and radio range.  There may be tens of thousands of stations in a survey, all wanting to communicate with the central recorder, which might be 15 Km away, perhaps behind a mountain.  While the data rate of an individual seismic channel is modest (upwards of 12,000 bits/second), collectively it amounts to a lot of data.  And distance equates to power, which means large batteries.  A seismic system isn't very portable if every geophone station requires an automotive battery for power.

Wireless Seismic, Inc. has resolved these issues with RT System 2, a cleverly conceived, radio-relay acquisition system that passes data in a "bucket brigade" fashion (like a line of volunteers passing water from bucket to bucket to fight a fire).  Consider Figure 3, which illustrates how a single line of geophone stations passes data towards the central recorder.  Each station digitizes data from the local geophone.  The farthest unit transmits the data to the next adjacent station.  That station receives the data, add its own data, and forwards all the data down the line.  As the data gets closer and closer to the central recorder, the amount of data being sent during each "hop" increases until it reaches the maximum bandwidth of the 2.4 GHz radio.  (Note that this is an alternating continuous process:  half the units are receiving and half are transmitting at any one time, using several different channels in the band.)  At this point, a backhaul unit is deployed that converts the data into Ethernet packets and uses high-bandwidth 5.8 GHz radios to send the data to the central recorder.  Since the distance between units is only one seismic group interval (e.g., 25 to 70 meters), the radios use very little battery power.  The relay system can march up and over the terrain [Figure 4].  The 2.4 GHz band is license free in most of the world, so the users do not require radio licenses.

A 3D deployment of Wireless Seismic's RT System 2 resembles a cable system, with receiver lines of acquisition units and a main backhaul, laid out in the cross-line direction [Figure 5].  Once you get inside the central recorder, the environment is very familiar to observers used to a cable-based system [Figure 6].

RT System 2 has been deployed on a small to medium scale since early 2011, being successfully used in several locations in North America, the Arctic, and the jungles of Indonesia [Figure 7].  The system has been used for 2D and 3D surveys and for microseismic monitoring of hydrofracturing, which requires continuous recording for several 24-hour days.  The completion engineer and geophysicist can watch the microseismic events within minutes of their occurrence in time to react to fluids flowing outside the desired area, casing failures which can contaminate ground water, or even induced minor earthquakes, as reported in the news of late.

The first large-scale 3D deployment of RT System 2 has been in the Kurdistan autonomous region of Iraq [Figure 8].  Asian Oilfield Services Limited (AOSL) acquired a 7,500-channel RT System 2 for a 350 sq km 3D survey.  The active spread is 14 lines each of 336 channels, or 4,704 live channels.  Regulations require relatively inexperienced local workers to be used on the crew.  Simple errors, like neglecting to connect the geophones to the remote units, have been quickly detected by using the real-time monitoring capability of the Wireless Seismic system.  Despite these types of problems, the system has been very stable and productive.  When road construction started in one area of the line, the vibrators were put on standby for a while.  When children in the villages were playing with the ground equipment, crew personnel were dispatched to help them understand the proper use of the antennas.  High winds have shut down acquisition on occasion.  Despite the 50+ degree C daytime temperatures, the crew has worked through the start-up challenges and the recorded data quality has been excellent.

The expected reduction in crew headcount with the advent of the nodal systems has not materialized, as new crew tasks such as line checking, manual data collection, and manual data transcription have replaced the reduced headcount from the layout teams, though productivity has increased.  With RT System 2, no line checkers, data collectors or data transcribers are needed, so the crew headcount has been reduced in most cases.

So, are cable systems on their way to obsolescence?  Not quite, but cableless systems are starting to take an ever-increasing share of the market.  There are locations where cable systems work well, and other location where a cableless system is the only practical solution.  With the challenges of increasingly denser surveys and channel counts pushing well beyond the 20,000 size per crew, the future of cabless/wireless systems looks bright indeed!

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