Exploration by seismology

In the early days of petroleum exploration, one had to be content to drill the traps visible from the surface, generally the anticlines (bulges of the geological layers in the substratum). But it was very quickly realised that this was no longer sufficient: many structures are masked by deposits of sediments that were laid down on top by the subterranean movement that created the trap.

In addition, traps situated under the sea are completely invisible to the eye. From the 1930’s on, the miracle method was developed: seismic reflection. Its principle is simple: waves, caused by an explosion or by a heavy mass falling onto the ground, are sent through the ground. These vibrations move outwards in all directions. When they meet a geological layer, some of the waves are reflected (as with a mirror) and return towards the surface, whilst the others are refracted, continuing their way deeper. And so on. By placing very sensitive receptors (geophones) at a distance from the transmitter, a complex series of waves can be recovered and recorded. The first to arrive are those which moved along the surface, then come those which were reflected by the first geographical layer, then those reflected by the second, and so on. In this way the time can be measured that a wave reflected by a geological layer takes to move from transmitter to receiver. By changing the position of transmitter and receiver numerous times, an image can be constructed of the substratum and the geological layers in time and in 2 dimensions (2D).

Hypotheses are then constructed concerning the speed of wave propagation in the different layers. This allows construction of an in-depth image that is extremely interesting for the geologists and drillers. Based on this image, a more detailed section can be developed.
Using the whole series of these 2D images in time and in depth, maps of the substratum are drawn to evaluate the hydrocarbon traps.

At sea, the seismic recording is made from a boat trawling behind it a series of floating receivers, the hydrophones. It is easier than on land because there is no natural obstacle to the displacement of the transmitter and the receivers of the waves.

Computer analysis of the seismic waves that are recorded is extremely complex and necessitates very significant calculating capacity. Only progress in computing has made this type of operation possible. The computing power used is indeed similar to that used in weather forecasting.
The result of the computer analysis is an imagery of the substratum in 2 or 3 dimensions. The integration of this data by the geophysicists is done with the aid of extremely sophisticated programmes that help them to reconstitute the forms and physical properties of the geological layers.Using suitable glasses, the virtual 3D vision of the substratum allows the interpreters to better understand the geometry of the substratum.

Seismic imagery is unfortunately not perfect and never 100% reliable. Problems can occur at the recording stage. There are difficulties of access in mountainous zones or in tropical forests (which slows down the work!). In addition, land with a soft and heterogeneous surface generates alterations in the waves that are often difficult to correct at the time of analysis. In the depth sections, it is also possible to have images that do not correspond to reality (artefacts), just like mirages. These are not always easy to differentiate from the genuine signal. Moreover, the signal weakens with depth (the farther it has to travel, the more energy it loses!). Finally seismic speeds are not known with precision, above all in little-drilled zones where the speeds have not been able to be measured. This can result in errors in the seismic sections and the maps in depth.