Published:January 01, 2004
The porosity ϕ of a rock is the pore volume per unit volume. The pore space is generally filled with salt water except near the surface, where air may be present, and in petroleum deposits, where the pore spaces contain oil and/or gas.
Pore spaces are usually sufficiently intercon-nected so that the fluid pressure approximates that of a fluid column extending to the surface; this is normal pressure. The weight of the rock column exerts an overburden pressure. The differential, effective, or net pressure on the rock ma-trix is the overburden pressure less the interstitial fluid pressure. However, if there is no communi-cation between the pore spaces and the surface, the interstitial fluid pressure may be greater than the normal pressure causing the differential pressure (and velocity) to be lower than usual for a given depth—a situation known as overpressure.
Figures & Tables
Problems in Exploration Seismology and their Solutions
Geophysicists are often turned off by equations. This is unfortunate because equations are simply compact, quantitative expressions of relationships, and one should make an effort to understand the information that they convey. They tell us what factors are important in a relationship and their relative importance. They also suggest what factors are not relevant, except perhaps through indirect effects on the relevant factors. Graphs often help us visualize equations more clearly. We may think of derivatives as simply measures of the slopes of curves, maxima and minima being merely the places where the slopes are zero, and integration as simply summing up the area under a curve. An imaginary exponential indicates a periodic function. Limitations imposed by initial assumptions or by approximations in their derivations apply to most equations, and these should be appreciated in order to avoid drawing erroneous conclusions from the equations.