Seismic shift

I am not referring to any particular technical developments, even though there have surely been those.

I refer instead to the almost sudden and now definitely accelerating acceptance of modern cable-free land acquisition systems, of which there are already about a half a dozen on the market.

Another thing that is amazing is that some of these products are not really that new anymore, at least not in terms of the philosophy of how they function.

One such operational “new era” cable-free system has an odyssey of development going back to 2001 and is actually in its third generation of hardware to have worked in the field, quietly beavering away, finding out how to enhance and perfect cable-free systems and operations.

A different cable-free system was announced almost two and half years ago, and has hardly been out of the limelight since. So how come it has only been only relatively recently that red carpet has more readily been rolled out for this technology?

The reasons are not too difficult to understand. Seismic contractors are some of the most conservative organisations on the planet.

They find hardware that works for them along with a method of operating, and stick with it.

Additionally, aided and abetted by attitudes of a few oil companies that can also occasionally veer towards the uninventive, there may be little top-down pressure on contractors to make radical changes, so the status tends very much to stay quo-ed.

The status that prevailed for so long prior to the acceptance of new era cable-free was the use of digital cable-based systems for land acquisition.

For those familiar with the way some occasions are celebrated in the west, the introduction of such technology will enjoy its “Pearl Anniversary” (thirty years) in 2009 according to my calculations.

Over the course of these three cable-dominating decades, one manufacturer alone has brought out five generations of cable-connected hardware, some of which enjoyed backwards compatibility with earlier products, and some that did not. But in either case – not a bad run for its money.

The chances are that almost no one under the age of 50 who has ever worked in land seismic has seen, or maybe even thought about, any other way to shoot land surveys but the digital cable way.

But there’s the rub – few technologies enjoy an unbroken run lasting close to one third of a century. While products often change on the outside, basic theory was always going to show that inside, digital telemetry hardware was one day going to hit various technological buffers when pushed out of its comfort zone.

And it was probably the sudden sight of these on-coming buffers that has made so many realise that land exploration is not going to make any more major leaps without a switch of technology.

Also, the fact that almost every major instrument developer now claims to have some sort of cable-free offering is making even some die-hards sit up and take notice that cable really is approaching its “best before date”.

Indeed, at a recent convention, one manufacturer said that within ten years, there would be virtually no cable-based acquisition at all.

There’s a growing list of basic problems for cable as demands increase on land seismic crews. But it seems that many cable-system detractors have jumped on the bandwagon of weight as the main, or even the only difficulty for cable-based hardware.

Therefore, it is interesting to see the reaction to this from some manufacturers who have special and vested interest in cable. They appear to be mounting a fight-back by trying to show that weight problems may not be as bad as portrayed, and that the product has many other redeeming qualities so the industry should stick with it.

This seems like the response of how some fast food vendors now offer healthy salads – in the hope that you will still find their whole package more acceptable.

But this is to misunderstand the problem; if we want to “superseis” a land crew, then we must not only concentrate on cables potential weight-increasing characteristics but also consider the other important cable consequences when a crew gets bigger.

Perhaps a way to think about this is to consider that you use a cordless phone around the house rather than stick with the old one wired to the wall not simply because cordless is lighter.

Obviously, as with all sort of other products that have managed to do away with wires, cable freedom brings many other conveniences.

Having said that, it is possible to come up with a set of operating conditions where the lightest cable systems can be made to approach the heaviest cable-free systems on a weight/channel basis but these conditions seem to be rather extreme.

You can also cheat, as I have seen, by comparing weights of cable systems using point receiver accelerometers with cable-free products using arrays of geophones but that really does smack of desperation.

For most surveys today, the cable itself may account for three quarters of the system-specific weight while the batteries and power distribution system usually come in second place.

Of course, the weight per channel attributed to the cable reduces with trace interval but, all other things being equal, the best that a cabled system can approach is only when the trace interval is so small that the telemetry cable used per channel is extremely minimal (ultra high trace densities) and thus proclaim that the cable system can be considered for weight purposes as cablefree.

The problem here is that, when a cabled crew gets to use trace intervals this close, weight of the technology is the least of your worries.

There is also the matter of wishing to use new huge channel capacity to provide much greater ranges of offsets and azimuths as a first step, rather than just higher spatial densities.

In this case, cable systems cannot claim to approach anything like the same weight as cable-free and may present the wholly new problem of how to cope with 1 000 kms of cable on the ground!

So let us return to the real world of trace intervals measured in some tens of metres. For currently common shooting parameters, this means the cable contribution to channel weight will be in the 2 – 5 kg bracket, and that’s a lot of weight, which means more people, trucks or helicopters than if you had a crew without cables.

And, if you manufacture or work in the European Union and want to import hardware into that domain, cables may also mean worry about carbon dioxide emissions from their production, transport and disposal.

Despite advances in electronics, the weight of seismic cables has hardly changed since the mid-nineties, and this is because some of them are already as anorexic as they dare be.

Any lighter and they may not function at all since they would probably fail too easily in some field conditions. Some cables are already not much more than some low weight wires, as little strengthening as you can get away with, and a jacket – so no room there for slimming down.

But as stated, this is not the only problem, and it’s not even the main one, as crews head towards higher channel counts. There’s the suitability of a technology first employed in the seventies to take us as quickly forward as we now need to go.

Most seismic land cables today make use of twisted pairs of conductors for transmission of data rates of high speed, and that introduces us to Mother Nature’s speed limit in the shape of basic transmission theory.

Twisted pairs are the business end of a cable. Imagine two wires wrapped around one another, each wire covered in an insulator whose dielectric properties have to be chosen rather carefully to make the cable work at high frequencies.

Then imagine one or two more such pairs of conductors snuggling up closely and jacketed in some protective outer layer.

One or more of these pairs may have to carry tens of millions of bits of data per second, which is not much of a trick if the cable is handled nicely and carefully in an office environment.

But subject it to the average seismic field scenario and understand that what would have been a minor injury for that cable if it just had to carry a few kilobits per second looks like a life threatening condition to the flow of hundreds of megabits.

The reason for this is that the cable’s impedance, a complex mixture of its series resistance, inductance and capacitance (the latter two having frequency dependent terms, and thus related to transmission rate) cannot afford to vary too much without looking like a major obstacle to the transfer of high date rate bits.

A cable subjected too often to the wheel of a Land Cruiser or stretched too much by over-zealous labour may give rise to such a digital obstacle.

You might think that the answer is to share the data load by adding more twisted pairs.

Whereas this may work technically, commercially is would be suicide. You’d end up with even greater power consumption, and heavier/costlier cables making contractors, oil companies and the EU even more upset.

And not last, nor least, is the issue of reliability. Any complex system, whether it’s an aircraft, a person, cabled seismic hardware or a cable-free one, is only as good as the weakest link of its serially connected chain of parts.

This is why aircraft manufacturers design in multiple-redundant systems for critical functionality.

Autonomous cable-free hardware is not generally serially dependent since, as the name suggests, it has ground units which do not need, or even know about each other for their operation, meaning that there is no difference between the reliability of a 1 000 channel cable-free operation and one with 100 000 channels.

However, cable systems do not easily enjoy these luxuries.

From the point of view of reliability, let us first consider a cable system as a series of connected identical elements each with an equal probability of failure, which we can call Pef and having a value between 0 and 1. These identical elements could be, for example, the individual cables of the system.

Because all these elements are connected and generally rely on each other for their function, then the failure of one element will bring about the demise of the whole shooting match. In this case, the probability of failure of the entire system Psf may be expressed as:

Psf = 1 – (1- Pef)n

The important thing to note here is that this is a power law. As it gets bigger, i.e the channel count increases, the likelihood of failure increases rapidly and expensively.

Of course, a cable seismic system is a bit more complex than a collection of simple subsystems thrown together, relying instead on a number of different parts for its function but we see that this doesn’t help things very much.

These different items may include some the crucial pieces of transmission electronics, the line connectors, sections of cable and so on. If such elements of the system have different probabilities Pefi of failure then the overall probability of failure is:

Psf = 1 – (1- Pef1) (1- Pef2)… (1- Pefi)…(1- Pefn)

But note that the reliability of the system is going to be dominated by the weakest link in the chain, and the more there are of them, or the more any one piece of that chain is pushed beyond what it is technically easily able to do, the greater the chance of failure.

In other words, a modern land seismic cable system may offer acceptable downtime in certain environments with x thousand channels, but with 5 x thousand, it may never spend enough time up and running to be viable.

This is why it is crucial for cable-free systems to be as autonomous as possible – so that affordable channel count is not limited by system reliability, or the entire crew operation brought to a halt by one small item seismicly seizing up.

This is not the first time we have seen this reliability lesson. Some may remember that we discussed wanting to find alternative ways to increase crew channel count significantly by using hardware which, would need to be much lighter and more reliable.

We felt that, once we discovered such a geophysically-unchallenged gismo it would open up new areas of land exploration. Eventually we found that hardware and a transition did happen – to digital cable telemetry, and despite some early scepticism of the technology and reticence to change ways of working, the result was viable and affordable 3D on land.

The industry now sees again that it’s time to review that old lesson and take the next step in instrumentation. As before, it may mean the discarding of some habits learned with out-dated technology so that we can make the most of what cable-free has to offer.

But one thing’s for sure, we cannot just go on the way we have before. The good news is that, as much of the industry is now understanding, great things are just round the corner for those who leave the cable-comfort zone.

Acknowledgments

Oliver King, Cambridge University, UK.

Larry Denver, Ascend Geo, USA.

Institute of Polymer Technology, University of Loughborough, UK.

Bill Ayres, Ayres Group, Dubai, UAE.