The seismic conversion log and its application to vertical seismic profiling at the German Continental Deep Drilling Site (KTB)
The conversion of compressional to shear waves in a vertical seismic profile (VSP), recorded in 1999 between 3-8.5 km depth at the German Continental Deep Drilling Site (KTB) was quantified with the newly developed conversion log method and analyzed in detail. The experiment yielded high quality seismic data in terms of signal bandwidth, signal-to-noise ratio, amplitude reference, and stability of the source signal. Important facts were obtained: The source signal proved to be simple. Its multi-phase signature is a result of multi-pathing within the crust. The determination of attenuation values in a complicated crustal environment is hampered by the structural features. In a scattering environment the wave conversion log method is not only capable of determining the conversion amplitude from the direct vicinity of the borehole but also from distant conversion points. The conversion in the field data as well as in model data is frequency dependent in an unusual manner; intermediate frequency signals (30-60 Hz) convert less than low (15-30 Hz) and high frequency signals (60-240 Hz). In the crust at the KTB site only about 10% - 50% of the conversion originates from the variation of the mineral composition of the rocks. Wave conversion depends on the fracture density. However, wave conversion is also highly dependent on chlorite content that seems to 'heal' the influence of cracks and fissures. This is important for crustal surface seismic experiments aiming at the reflection analysis of fault zones.
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