Crustal scattering at the KTB from a combined microearthquake and receiver analysis

Abstract

In December of 1994 a fluid injection experiment which triggered several hundreds of microearthquakes was conducted at the KTB main borehole (Oberpfalz, Germany). These events were recorded with a temporal seismic network at the surface. Out of the complete data set, a cluster of five events recorded at four mini-arrays consisting of eight or nine stations was used to investigate the crustal scattering properties in the vicinity of the KTB. For this purpose, the 'Double Beam Method' (DBM; Kruger et al. 1993, 1995, 1996) and the 'Double Beam Imaging Method' (Scherbaum, Kruger and Weber 1997) were extended to curved wave fronts to drop the restriction of planewave propagation. This technique is used for imaging the crustal scattering strength using earthquake clusters recorded at close-by mini-arrays. The results of the array analysis show that the composition of the P coda is mainly affected by the site location of the arrays. Near-surface and deeper crustal scattering contribute in a very complicated pattern. Furthermore, with the present data set it was possible to identify reflections from the top of the Erbendorf Body. This is a very pronounced arrival in most of the recorded traces. In one of the arrays its amplitudes are even greater than the direct P phases. Five to eight coherent phases could be identified by the mini-arrays. Using only these phases, synthetic P-coda traces were constructed, which only contain the coherent part of the observed wavefield. By subtracting the synthetic coherent wavefield from the original traces we achieve a variance reduction in the P coda of up to 37 per cent. This leads to the conclusion that a large amount of the P coda at the KTB can be modelled by a simple deterministic single-scattering model using a small number of individual scatterers.