Multipathing and spectral modulation of the downgoing wavefield in a complex crust: an example from the KTB (Germany) borehole.

Pujol, J., Bohlen, T., Beilecke, T. and Rabbel, W. (2007) Multipathing and spectral modulation of the downgoing wavefield in a complex crust: an example from the KTB (Germany) borehole. Geophysical Journal International, 169 (2). pp. 563-578. DOI 10.1111/j.1365-246X.2007.03335.x.

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VSP data collected in the KTB (Germany) borehole to a depth of 8.5 km in 1999 show a surprising spectral modulation of the downgoing wavefield. After filtering the data with the singular value decomposition technique it was found that below about 6.2 km there are two depth intervals where the modulation can be explained in terms of a basic wavelet plus two weighted and delayed copies of that wavelet, with the delay for each wavelet remaining almost constant in each interval. The boundary between the two intervals is at about 7.25 km depth and above and below this depth the delay for the second wavelet is almost the same, while the delay for the third wavelet is significantly different. Neither the modulation nor its depth variation are source related and cannot be explained in terms of multiple reflections in a subhorizontal low-velocity layer. On the other hand, finite difference synthetic data show that subvertical layering (which is prevalent in the borehole area) provides a mechanism that can explain the observations. This mechanism has analogies with the generation of the standard refracted (i.e. head) waves. When a plane wave front propagates perpendicular to the boundaries of a vertical low-velocity layer surrounded by two vertical high-velocity layers, refracted wave fronts are generated in the low-velocity layer, which in turn generate secondary wave fronts in the high-velocity layers. These wave fronts trail the primary wave fronts by a constant delay whose magnitude has a simple dependence on the thickness of the low-velocity layer and the velocities involved. This process creates multipath arrivals that in geological settings with steeply inclined and faulted layers may appear and disappear rather abruptly, which may contribute to a scattered appearance of the wavefield.

Document Type: Article
Keywords: borehole geophysics continental crust KTB seismic wave propagation lower continental-crust drilling program ktb dry lower crust reflections explosions yardley,bruce,w.d. valley,john,w velocities fluids water
Research affiliation: Kiel University
DOI etc.: 10.1111/j.1365-246X.2007.03335.x
ISSN: 0956-540X
Date Deposited: 12 Jan 2012 06:03
Last Modified: 23 Sep 2019 23:49

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