Question

A low velocity layer isn't likley to be detected by a gravity survey. Why?

Answer 1

Gravity surveys have been used to obtain information on bedrock depths and the top of rock configuration in areas where it has not been possible or practical to use other geophysical techniques.

Low velocity layer that cannot be detected by refraction methods/ gravity survey; typically, a low velocity layer beneath a high velocity layer. The surface layer that has low seismic velocity.An extensive, relatively high-velocity layer underlying lower velocity layers that transmits a refraction wave nearly horizontally.

low velocity layers are detected rather reliably for all of the models but some complications are apparent when the velocity contrasts are low .

Example:

an additional false, weakly expressed low-velocity layer appears in the central part of the crustal model when the velocity contrast is 0.2 km/s. Further reduction of velocity contrast to 0.1 and 0.05 km/s results in loss of information on the position of upper boundary of the deeper low-velocity layer. In this situation, only the midpoint of this low-velocity layer can be detected relative to the starting velocity – depth model . All of these complications are connected with additional noise as well as with superposition of the phases from different conversion boundaries.