Geological influences on acoustic impedance

TABLE 1 - Geological influences on acoustic impedance

Geological influences on acoustic impedance

Geologic Condition

Impedance Effect

Lithological interfaces

A common cause of impedance contrasts. Generally p and V change whenever there is a change in lithology.

Porosity variations

p and V are porosity dependent. Gradual changes in porosity generate modest impedance contrasts. Abrupt changes in porosity can create large reflection coefficients.

Changes in pore fluid

Pore-fluid density affects bulk density p and velocity V. A change of pore fluid from water to oil creates a small impedance contrast that can be detected seismically only in ideal signal-to-noise conditions. A change from liquid (either water or oil) to gas can produce large impedance contrasts and robust seismic reflections.

Overpressure

p and V decrease in overpressure zones. If the onset of overpressure is gradual, the impedance contrasts may be too small to create detectable seismic reflections. Abrupt onsets of overpressure can produce strong reflections.

Cementation

Cementation affects the mechanical strength of rocks. V generally increases as mechanical strength increases (assuming a constant rock type). In some instances, variations in cementation can create impedance contrasts sufficient to result in seismic reflections.

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See also

Seismic interpretation