ALBERT

Description: As part of Project International Deep Profiling of Tibet and the Himalaya III, a 400-km-long, densely spaced array of 57 broadband and short-period seismic stations was deployed in central Tibet from August 1998 through May 1999. Although originally designed to image the lithosphere with teleseismic events, the array also recorded numerous local and regional seismic events. More than 900 local and regional events were detected on at least 10 stations during the 1-year deployment, and we were able to locate 267 local earthquakes. A substantial number of the events were found to cluster in or near large grabens and along known strike-slip faults, while other events show no obvious correlation with known structures. In addition to spatial clustering, at least one of the large clusters also exhibits temporal clustering that may be associated with magmatic or geothermal activity in the upper crust. The average Vp and Vs are estimated to be 5.85 and 3.35 km/sec for the upper crust and 7.0 and 3.9 km/sec for the lower crust, respectively. The 50 focal mechanisms computed from this set of events are consistent with north–south shortening and east–west extension; there are no clear indications of significant local perturbations in the regional stress field induced by the collision between India and Eurasia. The majority of the focal mechanisms indicate normal and strike-slip faulting. At least six of the newly computed focal mechanisms, however, indicate thrust faulting, which is a phenomenon not well documented previously. Ninety-nine percent of the local earthquakes have focal depths shallower than 25 km, and the locations of the few deeper events are poorly constrained. The shallow earthquake focal depths are consistent with high temperatures and proposed ductile or aseismic behavior in the middle to lower crust of central Tibet.

Description: This letter proposes a building characterization tech-nique for L-band polarimetric interferometric synthetic apertureradar (SAR) data. This characterization consists of building iden-tification and height estimation. Initially, a polarimetric interfer-ometric segmentation is performed to isolate buildings from theirsurroundings. This classification identifies three basic categories:single bounce, double bounce, and volume diffusion. In order tocompensate for the misclassifications among the volume and thedouble-bounce classes, interferometric phases given by the high-resolution Estimation of Signal Parameters via Rotational Invari-ance Techniques (ESPRIT) method are analyzed. Once buildingsare localized, a phase-to-height procedure is applied to retrievebuilding height information. The method is validated using E-SAR,German Aerospace Center (DLR) fully polarimetric SAR data, atL-band, repeat-pass mode, over the Oberpfaffenhofen, Germany,test site, with a spatial resolution of 1.5 m in range and azimuth.More than 80% of buildings are retrieved with acceptably accu-rate height estimates

Description: The large-scale POLONAISE'97 seismic experiment investigated the velocity structure of the crust and upper mantle in the Trans-European suture zone (TESZ) region between the Precambrian east European craton (EEC) and Paleozoic platform that comprises terranes added during the Caledonian and Variscan orogenies (530–370 and 370–225 Ma, respectively). This experiment included 64 shots recorded by 613 seismic stations during two deployments. Very good quality data were recorded along five profiles, and the longest and most important one (P4) is the focus of this paper. Clear first arrivals and later phases of waves reflected/refracted in the crust and Moho were interpreted using two-dimensional (2-D) tomographic inversion and ray-tracing techniques. The crustal thickness along the profile varies from 30–35 km in the Paleozoic platform area to ∼40 km below and due northeast of the TESZ, to ∼43 km in the Polish part of the EEC, and to ∼50 km in Lithuania. The Paleozoic platform and EEC are divided by the Polish basin, so the upper crustal structure varies considerably. In the area of the Polish basin, the P wave velocity is very low (V P 〈 6.1 km/s) down to depths of 15–20 km, indicating that a very thick sedimentary sequence is present. We suggest two possible tectonic interpretations of the velocity models: (1) Baltica indented Avalonia, obducting its upper crust and underthrusting its lower crust in a tectonic flake structure and (2) a rifted margin of Baltica underlies the Polish basin. This model is similar to other interpretations of seismic profiles recorded in the Baltic Sea. The second model implies that the Paleozoic platform solely consists of Avalonian lithosphere and the EEC of Baltica lithosphere. It offers a simple explanation of the difference in crustal thickness of the two platforms. It also implies that the Caledonian and Variscan orogenies in this area were relatively “soft” collisions that left this continental margin largely intact.

Description: The method presented here is a direct transformation of magnetic transfer functions into apparent resistivities and phases . The derivation is an extension of the two-dimensional procedure developed by Gharibi and Pedersen (2000) to arbitrary conductivity structures in the subsurface. The VLF technique (very-low-frequency) is a well established electromagnetic tool for mapping near surface structures of geological targets. Here, artificial source fields generated by powerful VLF transmitters situated at several locations all over the world and radio transmitters are used in induction studies for shallow applications. By combining two or more transmitters located in different directions, one can calculate a set of transmitterindependent induction parameters for each point of measurement (Pedersen [1989), Pedersen et al. (1994], Gharibi & Pedersen [2000]), which are the same as the magnetic transfer function in geomagnetic depth sounding applications (GDS). The Geological Survey of Sweden (SGU) operates a 'tensor'-VLF device as an add-on tool in airborne surveys and acquired maps of magnetic transfer functions (tipper) for frequencies around 16 kHz. We show, that the magnetic transfer function is related to the full TE-mode impedance tensor besides a constant in the electric field, which has to be estimated by other means. Use must be made of the potential field character of the magnetic field in the air half-space, yielding an successive approximation of the anomalous magnetic fields for an arbitrary primary magnetic field. When the anomalous vertical component is explicitly known, solving Faraday's equation in the wavenumber domain by using the divergence theorem leads to an estimation of the anomalous electric field of TE-mode. The normal electric field is provided by an impedance measurement at one point in the area of investigation. We demonstrate , that structural information in terms of apparent resistivity and phase reflecting different kinds of lithologies can be extracted from magnetic measurements and related to geological investigations. In the following sections, the decomposition of electric and magnetic fields and transfer functions is introduced and the procedure of the transformation is developed.

Description: This paper describes the analysis of built-up areas using fully polarimetric interferometric SAR data at L-band. This approach uses a polarimetric interferometric segmentation to determine the number of dominant scattering mechanisms required by an interferometric phase estimation using ESPRIT method.