seismic reflection surveying

seismic reflection surveying In seismic reflection surveying, seismic waves are propagated through the subsurface and the travel times are measured of those waves that return to the surface after reflection at geological surfaces. Knowledge of the velocities with which the subsurface transmits the seismic waves makes it possible to convert the travel times into the depths to the surfaces. The geological structure can then be determined.

Seismic reflection is the most widely used of the geophysical techniques because it is the only method that provides a representation of the subsurface structure that is sufficiently detailed to identify potential hydrocarbon traps.

The method is analogous to that used by echo-sounders to determine water depth from the arrival times of sonic pulses reflected from the sea bed, but it uses waves of much lower frequency. The technique relies on the fact that the behaviour of seismic waves is similar to that of light in that they are reflected (and refracted) when they meet a boundary between media with different seismic velocities. In general, the seismic velocities of rocks increase as the porosity decreases and the density increases.

Seismic waves are detected by instruments known as geophones when used on land or hydrophones in water. They essentially convert the seismic wave motion into an electrical signal.

Seismic reflection may be used at many different scales. Thus, the top few tens of metres of the subsurface might be studied using seismic waves generated by blows from a sledge-hammer, while penetration through the continental crust can be achieved using more potent sources. Most surveys are undertaken using non-explosive sources such as the air gun, which rapidly discharges high-pressure air into the water column and produces a bubble that oscillates in the same way as an explosion. Sources of this type are more convenient, can be fired again rapidly, and provide an exactly repeatable source ‘signature’ that can be utilized by certain modern data-processing techniques. Indeed, computer-based data processing is commonly used to enhance the reflected waves at the expense of various types of noise.

The interpretation of seismic reflections is often confused by the presence of multiple reflections: events that have been reflected more than once before reaching the detector. These ‘multiples’ can be removed by some processing techniques.

Two-dimensional seismic profiles are built up by moving the source and an array of detectors along a line. More complex arrangements enable three-dimensional coverage of an area to be obtained. Traces for each shot-detector set-up are reproduced closely side by side so that the seismic reflections appear to blend into a continuous band and the section so produced mimics a geological section.

P. Kearey