Open AccessSeismic wave attenuation from borehole and surface records in the top 2.5 km beneath the city of Basel, Switzerland
Author(s) -
Bethmann Falko,
Deichmann Nicholas,
Mai P. Martin
Publication year - 2012
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.2012.05555.x
Subject(s) - geology , attenuation , borehole , seismology , seismic wave , surface wave , mineralogy , optics , geotechnical engineering , physics
SUMMARY We investigate attenuation ( Q −1 ) of sediments of 2.5–3.5 km thickness underneath the city of Basel, Switzerland. We use recordings of 195 induced events that were obtained during and after the stimulation of a reservoir for a Deep Heat Mining Project in 2006 and 2007. The data set is ideally suited to estimate Q as all events are confined to a small source volume and were recorded by a dense surface network as well as six borehole sensors at various depths. The deepest borehole sensor is positioned at a depth of 2.7 km inside the crystalline basement at a mean hypocentral distance of 1.8 km. This allows us to measure Q for frequencies between 10 and 130 Hz. We apply two different methods to estimate Q . First, we use a standard spectral ratio technique to obtain Q , and as a second measure we estimate Q in the time domain, by convolving signals recorded by the deepest sensor with a Q operator and then comparing the convolved signals to recordings at the shallower stations. Both methods deliver comparable values for Q . We also observe similar attenuation for P‐ and S‐ waves (Q P ∼ Q S ). As expected, Q increases with depth, but with values around 30–50, it is low even for the consolidated Permian and Mesozoic sediments between 500 and 2700 m.