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Unit Five - Petroleum Exploration Objectives - The student will be able to:
Explanation The seismic methods are the most widely used of all geophysical methods used in petroleum exploration. The main advantage is that it provides the most accurate rendition of the geometry of subsurface layers. Unfortunately the cost of seismic surveys are much greater than the cost of other types of geophysical surveys. Seismic methods measure seismic velocity of rock layers to detect both lateral and depth variations. The objective is to determine the lithology and geometry of the layers. A seismic wave can be thought of as shock wave (elastic wave) or vibration traveling through the ground. The rate of travel, or velocity, of the wave is related to the density of the rock. There are two types of elastic waves produced: 1) P-waves, which are primary or “compressional” waves, and 2) S-waves, or shear waves (Figure F24). 
The procedure used is to lay out a survey line with geophones set at equal spacings along the line. A shock wave is produced at one of the stations by dropping a heavy weight or detonating an explosion at a “shot point”. The shot point is the point on the surface directly above the zone of interest. Ground motions caused by the explosion or impact are transmitted in the form of P-waves and S-waves. A “seismic timer” is used to measure the travel time of the wave from the instant it is generated until the time the wave reflection is detected back at the surface. Times are measured for each of the successive stations along the line. Either semi-graphical or computer methods are used to determine the velocities.
The reflections are plotted on a profile, called a seismic section, which shows depths to the features of interest (Figure 25). Depths up to 20,000 feet can be measured routinely, with an accuracy level of 10 to 20 feet. Average velocities cannot be determined for intervals less than a few hundred feet. Seismic reflection has been used to map out the most common features associated with oil accumulations, such as anticlines, salt domes, reefs and faults. Since different types of rocks transmit the shock waves at different velocities, the average velocity can provide important information about the rock composition (Table T10).
Table T10:
Seismic wave velocities in selected
examples of different lithologies.
The two main
categories of seismic methods are “seismic reflection” and
seismic refraction”. The seismic reflection method seeks to
measure the travel times of seismic waves or pulses after they have been
reflected off of subsurface formations or structural features. Information
derived include the velocity (density) of the material and the depth to
the reflective surface. Seismic refraction is used primarily to
determine seismic velocity.
Seismic
Reflection
The Law of
Reflection simply states that the angle of reflection equals the angle
of incidence (Figure F26). The technique is based on analyzing the
arrival times of seismic waves (ground motion or energy waves).
After the sensor measures the precise arrival time of the wave, the velocity
of the wave can be calculated using the hyperbolic equation (Figure F26).
The wave velocity, which is related to the rock density, can then be used
to determine the lithology of Rock Unit 1.
Seismic RefractionThe seismic
refraction method is similar to the reflection method in that the same
instruments and shock wave sources are used. However, as the name
implies, the objective is to measure refraction of shock waves as they
pass across formation or structural boundaries (Figure F27). Refraction
is governed by Snell’s Law, which relates velocity to the angle
of incidence and to the angle of refraction.
Measuring
refraction requires that the detecting instruments be placed a large horizontal
distance away from the shot point, or in other words, this distance must
be much greater than the vertical depth to the horizon intended for measurement.
The results are far less accurate than those obtained with the reflection
method, but velocity information (and hence lithologic information) is
more reliable than that obtained by reflection techniques.
Field Methods | Geochemical Methods | Geophyscial Methods | Drilling Methods | Petroleum Exploration |
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