Publication Date:
2014-02-13
Description:
Using formulae for both tensile and shear sources, we investigate spectral characteristics of microearthquakes induced by hydraulic fracturing, with application to passive-seismic data recorded during a multistage treatment programme in western Canada. For small moment magnitudes ( M w 〈 0), reliable determination of corner frequency requires accurate knowledge of Q P and Q S , although spectral estimates of magnitude are relatively unaffected by uncertainty in seismic attenuation. Here, we estimate Q P and Q S using spectral ratios derived from perforation shots. Of the microseismic events analysed during the hydraulic-fracture treatment, 17 of 20 exhibit an S / P spectral ratio 〈5, which is consistent with tensile failure. In addition, four microseismic events are characterized by a modulating source spectrum containing quasi-periodic notches. We interpret this spectral character to reflect a complex rupture pattern that involves rapid (5–8 ms) opening and closing of tensile cracks. In general, for tensile rupture on a penny-shaped crack, our model predicts that source radius ( a ) is related to moment magnitude ( M w) and internal fluid pressure within the fracture ( P i ) by a simple empirical scaling relation: log 10 ( a ) = [9 – log 10 2]/3 + 0.5 M w – log 10 ( P i )/3.
Print ISSN:
0956-540X
Electronic ISSN:
1365-246X
Topics:
Geosciences
Published by
Oxford University Press
on behalf of
The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
