Geophysical exploration methods depend on differences in physical properties: a gravity anomaly indicates that density is distributed anomalously, a magnetic anomaly points to anomalous magnetisation. These properties are known as the diagnostic properties of the corresponding methods. The most important method of exploration geophysics is reflection seismics, and the corresponding diagnostic property is the seismic impedance: a seismic wave is reflected at the interface between two strata only if they differ in their seismic impedance. Nothing else will do: they can differ in colour, age, density, velocity, porosity, fluid content -all to no avail. If they do not also differ in seismic impedance, the seismic waves will take no notice of the interface. Seismic impedance is thus the most important property rocks possess-at least if we take the (admittedly somewhat special) standpoint of the reflection seismologist. What then hides behind this scientifically sounding name?
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K. Helbig (1983) studied this question.