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; Kr&\#252;ger et al. 1993 , 1995, 1996) and the `Double Beam
Imaging Method' (Scherbaum, Kr&\#252;ger &\#38; Weber 1997) were
extended to curved wave fronts to drop the restriction of plane-wave
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.
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