|SIMULTANEOUS SWEEPS: ARE WE STILL PICKING UP GOOD VIBRATIONS?|
When an exploration industry professional buys a new car it is quite normal for research to be carried as to the difference between the newest models. Yet, when seismic surveys are being planned, it is not uncommon for some involved in the process to devote rather little time to making themselves fully aware of the differences between the latest subsystems which make up recording hardware, despite the significant HSE and cost risk that this ignorance can bring.
For oil industry end-users, it sometimes appears to be little more than a leap of faith that what the contractor has to offer must not only be the best hardware on the market but also capable of performing whatever exploration task maybe required of it. Nowadays, this is certainly a very risky assumption and it is also often just plain erroneous.
Admittedly, until recently, there were relatively few ways to design a land seismic survey. The reason for this is that the number of acquisition techniques was not huge, and the choice in seismic hardware type was close to non-existent. Some geophysicists may not have been aware of these limitations, perhaps having for much of their careers only been involved in simple operations for which simple hardware would be enough. A few even admit that they only ever considered operations where simplicity could suffice, automatically believing that thinking "out the box" would be a waste of time given the lack variety in hardware. Fortunately, instrumentation has very recently started to enjoy a revolution, and it is gathering pace. The biggest problem today is to make sure those who plan surveys know about variety of tools now available to solve their problems.
Historically, almost all seismic recording has been thought of as a duo of sources and receivers - the two operating as a single harmoniously connected unit. Equipment was designed this way as it was seen as an advantage to have everything working as one "holistic system". The sensors are part of the ground digitizing units, which are hardwired to a central system, connected to a source controller communicating with sources. Almost every part of this varied collection of mechanical and electrical subsystems required all other parts to work in some connected and limiting way or the whole show was off. Then we realised that this entire serially-dependent set-up was not only perilously vulnerable to the failure of just one minor subsystem, it provided little adaptability when new exploration ideas had to be tried. It was up to users to make new techniques fit in with how the existing technology worked.
But in the last few years, there has been an explosion in the number of recording systems, in the types of acquisition techniques, and most recently in source controllers too. There is no longer any need for any operation to be hemmed in by the designs of the past. The issue now is to bring to the attention of a sometimes conservative industry information relevant to all that is new and geophysically game changing.
Starting with land recording instruments, until a few years ago the choice boiled down to a few digital telemetry cable-based systems. All were excellent in their own way and each did some things slightly better than the others. But in terms of overall operational variety, the minor technical differences which such systems exhibited provided choices which were little more than different flavours of vanilla. Then cablefree systems started to emerge, and now the industry has about one dozen such recorders to consider, ranging from those which are incredibly simple in their design to those which can do more than the best cabled systems.
All cablefree systems overcome the limitations of cabled hardware simply by not relying on digital spread cables. This means they are all serially independent systems which in geophysical terms allows almost any channel count, deployment regime and spread geometry to be envisaged, none of which are possible with cable. This means that the instrument imposes no change on the plan you want to carry out or on the data you want to record. So now at last the industry can not only pick up good vibrations but unaltered ones too.
In acknowledgement of the freedom that cableless hardware provides, such instruments are referred to as "independent systems". This industry has spoken about "uncommitted data" for decades - where sensors and their digitising channels are deployed so as to impose no essential change to the recorded signal. However, to achieve this we also need uncommitted channels, where ground digitizing units are not forced to be located in space and time according to how far the cable will stretch, or even if it works, and this is where "independent systems" fit in.
However, cabled equipment is still very common in this industry and is not going away overnight. For this reason, some of the newest cablefree systems have been developed so that they can help their cabled brothers to be more flexible, with the two recorders working side by side. So the cabled kit (which many contractors are forced to offer as this is all they have in their inventory) takes on the simpler part of the survey, while something cableless and more flexible takes on the more challenging part. But this is not a easy as it sounds as it almost certainly requires special interfacing before the older technology can work with the new, and of course manufacturers of the traditional equipment are unhappy that "new kids on the block" are muscling in on their technical territory. Without this interface development work which is now taking place by the smaller independent manufacturers , the operators would be forced to stick with the older hardware.
Next to consider is the issue of multiple source types on one operation. This has been no more common in the past than the mixing recorder types. However, as surveys encroach on areas which were off limits to cable-only operations, then it is inevitable that the use of multiple source types on one operation will now come into the equation. The problem is that most source controllers, just like the most common recording systems, were designed at time when it was inconceivable that anyone would want to operate in this way, and for this reason such older equipment has great difficulty coping with modern demands for increased source flexibility. In fact, this explains why new source controllers hold the key for future operations: they make no assumptions about how many different recording systems need to be driven and how many different source types will be used. Such flexible controllers are now appearing on the market.
There are also great changes underway in terms of vibroseis. In recent years we have seen the arrival of several new techniques all designed to increase dramatically the number of source points that can be made in any given period. In some cases this has allowed up to 45,000 VPs in a 24 hour period, and fold of 1,000-2,000 is not uncommon.
This productivity is achieved essentially by breaking the rules we all adhered to until fairly recently - by now allowing sweeps to overlap in some way in the time frequency or space domains, by giving sources more independence. For example, when Slipsweep was invented by Shell, hardware needed one of these new types of independence - it required the freedom of being able to control the start time in a way which had not been necessary previously, to allow sweeps to overlap within certain frequency limits. In fact, not only had such freedom been unnecessary before, it was assumed no one would ever want sweeps to overlap. This feat did not come naturally to all seismic systems or source controllers.
However, source independence in terms of time is not the only requirement. The latest vibroseis techniques require much more flexibility in controlling hardware than we thought necessary until very recently, including ConocoPhillip’s ZenSeis technique, BP’s ISS and variations on Shell’s Slip Sweep. We would like sources to be able to act independently in terms of location too so that we can control the acquisition starts based on the physical locations of the shot points, for example according to the spacing between certain objects - such as other sources. The Distance Separated Simultaneous Sweeping (DSSS) acquisition technique developed by BP is an example of such a requirement.
But allowing sources to be totally independent is not the whole story. The industry still wants to be able to monitor all source activity for QC and HSE purposes. This creates a new problem for communications systems. This is because, for example, if a crew used two fleets of five vibrators before as two separate source groups, it can now use those vibrators as ten different and independent sources.
The problem here is in terms of radio communications. It is a problem in many countries just to use ever more radio channels. Licenses simply may not be made available. Some attempts have been made to overcome this by use of off-the-shelf digital radios. But this can suffer a variety of issues, such as range and cost, and even then still limit the number of sources which can communicate with the central system. So far they also have restricted the type of QC testing and reporting that can take place, including limitations on radio sims, no way to verify timing accuracy of GPS units and so on. But recent TDMA encoder systems make it possible to solve such problems, making use of existing crew radios to provide much more communication flexibility without need for more radios licences. Now it is possible to monitor all sources, just as the latest cablefree systems let all receiver be monitored too.
Finally, active seismic recording is not the only place where instrumentation is used today. Passive and permanent monitoring is growing and this is what is now driving functionality on acquisition systems, given that passive recording imposes much greater requirements on hardware than active.
New instrumentation is the key to all this next-generation crew operational flexibility. The industry must understand that the number and types of pressure it faces will not decrease, hydrocarbons will not get easier to locate, HSE restrictions will not be lifted and such problems are only solved by better seismic systems. Additionally, it is more than likely that the industry will develop yet more acquisition techniques, challenging still further the capabilities of much of the equipment we commonly employ. And service providers, usually based in North Amerce or Europe, will face much more international and local competition from foreign companies providers who generally do take up the latest equipment more readily.
Therefore, the future of land seismic - a future which is already upon us - is very different to what we have seen even recently and equipment must be able to cope such that the geophysicist can achieve what he needs to. The best way to think about this is that instrumentation developers will no longer be able to assumptions about how a crew uses and mixes sources and recorders. From now on, anything goes. To some, this looks like a free for all (indeed, the most advanced source controller talks about supporting "free for all" vibroseis") but in reality this is just providing a long overdue level of flexibility to this industry.
Even though almost all equipment in use today can be described as electronically sophisticated, many of the ideas which lead to the development of these systems are firmly embedded in the eighties. New techniques are showing the deficiencies of these systems and limiting what crews can do. New generations of "independent" recording systems and source controllers are set to open up new areas on land seismic, which will also be able to scale up to whatever is geophysically necessary.
I want to thank Peter Pecholcs of Saudi Aramco, also John Giles and Mike Morriss of Seismic Source Company for their help in putting this paper together.