Sediment structure at GRF from polarization analysis of P waves of nuclear explosions

Abstract

Near-vertical incidence angles and quasi-elliptical particle motions characterize broadband P waves from nuclear explosions recorded at the three 3-component stations of the Grafenberg array (GRF) in southern Germany, which is situated on the Jurassic sediments of the Franconian Jura. Comparison with data from the broadband stations KHC (Kaˇperske Hory, Czech Republic) and KSP (Ksiaz, Poland), located on gneiss in the Bohemian Massif and Precambrian sediments, respectively, show that the observed effect is caused by local receiver conditions. Polarization analysis shows that P - to SV -wave conversions at the basement/sediment boundary and multiples in the sedimentary layers are mainly responsible for these observations at the GRF array. Borehole and refraction data confirm low P -wave velocities in the sediments under the GRF stations. By modeling the broadband particle motions of the P waves of nuclear explosions, average models for the sedimentary layers below the GRF stations are derived. The main modifications to the model of Aichele (1976) are one (GRF-C1) and two low-velocity layers (GRF-B1 and GRF-A1) between the out-cropping limestone and the basement. The first low-velocity layer has an S -wave velocity of about 1.0 km/sec and a thickness of about 200 m below the three 3-component stations and a P -wave velocity about 2.8 km/sec, which is not well constrained. The second low-velocity layer underlying the first low-velocity layer at the stations GRF-B1 and GRF-A1 has a lower P -wave velocity of about 2.4 km/sec but a higher S -wave velocity of about 1.6 km/sec. Together with the surficial limestone layer (about 200 m thick), the total thickness of the sediment column increases from about 400 m below GRF-C1 in the south to about 750 m below GRF-B1 and to about 1150 m below GRF-A1 in the north. Slow sediment layers near the surface are important for the polarization of teleseismic P waves. In turn, polarization analysis of broadband recordings can be used to analyze the structure of shallow sediment layers.