- Copyright © 2001 Society of Exploration Geophysicists
Seismic velocities are one of our most important geophysical parameters and tools. Velocity and density contrasts permit us to image reservoirs. Velocities can be used directly in such applications as overpressure prediction. They can be used through their influence on reflection coefficients and amplitudes for direct hydrocarbon indication. Attempts are commonly made to predict velocity changes during recovery processes for reservoir monitoring. Obviously, there are many reasons why we need a complete understanding of how velocity, behaves and relates to our objectives.
Considerable effort is expended to reconcile velocity values made through surface seismic, crosshole, well log, and laboratory techniques. Even in a completely homogeneous rock, frequency dependent velocities, or dispersion, yield nonconstant values between different measurement bands. This dispersion is a complex function of pore fluid properties and mobility. As a result, with sufficient information, dispersion could itself be used as a fluid indicator or as a remote measurement of permeability.
Fluid mobility determines pore pressure distribution as a rock is deformed slightly due to the passage of a seismic wave. Thus, seismic properties are not only influenced by the kind of pore fluid, but also by its ability to move within the rock. We define fluid mobility (M) here as M = k/η where k is permeability and η is viscosity. For any frequency, if mobility is low, pore pressure remains out of equilibrium and we are necessarily in the high-frequency regime. Note: Since most rocks in the sedimentary column have very low intrinsic permeabilities (i.e., shales, siltstones, tight limestones, etc.) for most rocks, even seismic frequencies will be in the high-frequency regime.
Our research quantifies the possible levels of dispersion and relates them to rock and fluid properties. We have conducted velocity measurements over a broad frequency band ranging from below seismic to ultrasonic. Commonly used …