Seismic wave attenuation from borehole and surface records in the top 2.5 km beneath the city of Basel, Switzerland

by Falko Bethmann, Nicholas Deichmann, P. Martin Mai
Year: 2012

Extra Information

Geophysical Journal International. August 2012, Vol. 190, Issue 2, Pages 1257-1270. 


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 (QPQS). 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.