Implications of salt-related propagation and mode conversion effects on the analysis of induced seismicity

A number of natural gas fields in the North of the
Netherlands show moderate seismicity induced by gas extraction.
The gas reservoirs are located underneath a thick
layer of Zechstein evaporites (salt). The presence of the salt
has two important effects on the wave motions of induced
events at the surface. The first effect is defocusing of seismic
energy in upward direction with effects on amplitudes
and radiation patterns. The second effect is relatively efficient
conversion of P- to S-energy at the bottom of the salt
leading to the presence of S-wave precursors. Failure to recognize
these effects may lead to misinterpretation of source
location and mechanism. Moreover, the S-wave precursors
provide a handle to reduce uncertainty in depth estimation.