SEDIMENT STRUCTURE AT GRF FROM POLARIZATION ANALYSIS OF P-WAVES OF NUCLEAR-EXPLOSIONS

Near-vertical incidence angles and quasi-elliptical particle motions c haracterize broadband P waves from nuclear explosions recorded at the three 3-component stations of the Grafenberg array (GRF) in southern G ermany, which is situated on the Jurassic sediments of the Franconian Jura. Comparison with data from the broadband stations KHC (Kaperske H ory, Czech Republic) and KSP (Ksiaz, Poland), located on gneiss in the Bohemian Massif and Precambrian sediments, respectively, show that th e observed effect is caused by local receiver conditions. Polarization analysis shows that P- to SV-wave conversions at the basement/sedimen t boundary and multiples in the sedimentary layers are mainly responsi ble for these observations at the GRF array. Borehole and refraction d ata confirm low P-wave velocities in the sediments under the GRF stati ons. By modeling the broadband particle motions of the P waves of nucl ear explosions, average models for the sedimentary layers below the GR F 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-A 1) between the outcropping limestone and the basement. The first low-v elocity layer has an S-wave velocity of about 1.0 km/sec and a thickne ss of about 200 m below the three 3-component stations and a P-wave ve locity about 2.8 km/sec, which is not well constrained. The second low -velocity layer underlying the first low-velocity layer at the station s GRF-B1 and GRF-A1 has a lower P-wave velocity of about 2.4 km/sec bu t a higher S-wave velocity of about 1.6 km/sec. Together with the surf icial limestone layer (about 200 m thick), the total thickness of the sediment column increases from about 400 m below GRF-C1 in the south t o about 750 m below GRF-BI and to about 1150 m below GRF-A1 in the nor th. Slow sediment layers near the surface are important for the polari zation of telescismic P waves. In turn, polarization analysis of broad band recordings can be used to analyze the structure of shallow sedime nt layers.