During
seismic exploration,
P-waves (also known as primary or compressive waves) penetrate down into the earth. Due to
mode conversion
Mode conversion is the transformation of a wave at an interface into other wave types (modes).
Principle
Mode conversion occurs when a wave encounters an interface between materials of different impedances and the incident angle is not normal ...
, a P-wave can reflect upwards as an
S-wave (also known as a secondary, shear or transverse wave) when it hits an interface (e.g., solid-liquid). Other P-wave to S-wave (P-S) conversions can occur, but the down-up conversion is the primary focus. Unlike P-waves, converted shear waves are largely unaffected by fluids.
By analyzing the original and converted waves, seismologists obtain additional subsurface information, especially due to (1) differential velocity (V
P/V
S), (2) asymmetry in the waves' angles of
incidence and
reflection and (3)
amplitude variations.
As opposed to analysis of P-wave to P-wave (P-P) reflection, c-wave (P-S) analysis is more complex. C-wave analysis requires at least three times as many measurement channels per station. Variations in reflection depths can cause significant analytic problems. Gathering, mapping, and
binning c-wave data is also more difficult than P-P data. However, c-wave analysis can provide additional information needed to create a three-dimensional depth image of rock type, structure, and saturant. For example, changes in V
S with respect to V
P suggest changing
lithology
The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lit ...
and
pore geometry.
References
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Geophysics