Seismic Acquisition Geometry in a Class of its’ Own

I like to simplify things, especially if I can do it through classification. So, I thought I might try to do the same for the world of seismic acquisition. 

Any, and all seismic surveys, wherever and whenever they happened in the world, had two things in common; some kind of seismic source and some kind of seismic receiver. The source generated the energy which was recorded some time later by the receiver. There is, of course a vast range of both, each with their own advantages and disadvantages, complexities and appeals depending on when and where they were deployed and why. The goal here is to take that complexity of deployment and type and to simplify it into a general classification.

A seismic source can be active as in the case of a marine airgun, a land vibroseis truck, dynamite or a hammer and a metal plate. They can be used on the ground or buried, in the ocean or deployed in a wellbore and they come in many shapes and sizes. A seismic source can also be passive as in the case of a microseismic ‘event’ caused by a hydraulic stimulation. I have a prototype airgun on my bookshelf designed by my father in law. Some people have books and maybe vases or the like, I have an airgun. It’s a Geophysics thing. Isn’t it ? Don’t we all ?

A seismic receiver can take many forms; a single geophone or an array of geophones or accelerometers responding to particle velocity and acceleration respectively, a hydrophone responding to pressure, or the strain response of a fiber optic cable measured as part of a DAS experiment.

For the purposes of this classification I will first group both sources and receivers into those which are deployed on or near the surface or in wellbore. 

Airgun prototype courtesy Leonid Akentiev circa 1982

Source and receiver classification based on position relative to surface

Another, more appealing way of looking at this, is to consider the relative positions of source(s) and receiver(s) with the type of survey. This lends itself nicely to a simple cartesian classification in which all seismic survey types can be accommodated.

The upper quadrants contain those surveys which use a source at or near the surface and the lower ones (logically?) contain the downhole source surveys. Surface receiver deployments are to the right and downhole receivers to the left. Now let’s look at each quadrant individually.

In quadrant I (top right), we have surface sources and receivers. This is where all surface seismic projects reside; land, marine, transition, 2D, 3D and 4D. Whichever sources and receivers were used, this is the domain of surface seismic. In quadrant II (top left) we have surface sources and downhole receivers. This is the domain of the Vertical Seismic Profile (VSP). Zero offset, offset, walkaway, walkabove, 3DVSP, onshore, offshore, whichever type of VSP, they all belong here. Opposite to the VSP domain in quadrant III, is the home of those surveys utilizing surface receivers and a downhole source. This is probably the least common type of survey and includes, on the active source side, reverse VSPs and on the passive source side, surface microseismic monitoring. Quadrant IV is where we have both downhole sources and receivers, so, crosswell seismic using active sources and microseismic monitoring of passive sources associated with hydraulic stimulation in unconventional reservoirs.

Seismic survey type by relative position of seismic sources and receivers.

The various quadrants are colored to represent the relative frequency content and hence resolution of each domain. With both sources and receivers at the surface the ray paths are long; energy travels down through the earth and is reflected back. This results in a loss, or attenuation, of some of the high frequency energy and hence reduced resolution. There are many, many, ways this can be minimized with acquisition techniques and, replaced, with processing techniques but, nevertheless it is an inherent challenge associated with the geometry of the survey. As soon as we position either source or receiver downhole, we reduce the length of the ray path as the energy does not need to travel back to the surface, this reduces the attenuation, preserves the high frequencies and hence the resolution. When both sources and receivers are downhole the ray path is least, high frequencies are retained, and the resolution is maximized. Naturally there are some inherent assumptions in these comments; the initial frequency content of each source is assumed the same, the size of the sources and the sensitivity of the receivers etc. are all considered the same. That being said it is reasonable to expect data from a crosswell seismic data to have higher frequency content than a VSP which in turn has higher frequency content that the surface seismic in the same area.

Of course, there are many projects which utilize multiple configurations at the same time. There have been projects which combine surface and downhole microseismic and those which acquire a VSP and surface seismic simultaneously to name but a few.

Still, for now I am happy! Kind of.