ALP2002: Abstracts AGU 2003

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AGU FALL MEETING 2003

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AGU FALL MEETING in San Francisco, 08-12 December 2003

ABSTRACTS

BRÜCKL, E., BEHM, M., CHWATAL, W., 2003. The application of signal detection and stacking techniques to refraction seismic data.
From 1997 to 2003 large refraction seismic experiments were carried out in order to study the lithosphere of Central Europe and the Alpine area (Guterch et al., 2003). These experiments are characterized by a net of intersecting lines with a total about 1000 seismic recorders. Shots are recorded on all lines deployed contemporaneously. Generally, the seismic records show a high signal to noise ratio and picking the Pn-phase or crustal arrivals can be done accurately. However, in some mountainous areas like the Alps and the Carpathians the quality of the seismic records is severely decreased by poor seismic energy transmission. The application of signal detection and stacking methods can enhance the seismic signal and provide a reliable interpretation even in these areas. Stacking of seismic waveforms makes only sense if constructive interference can be achieved. In refraction studies of the lithosphere we usually do not have the information to establish coherency by appropriate traveltime corrections. Therefore, we apply the STA/LTA (short time average / long time average) signal detection algorithm to the records. The output of the STA/LTA algorithm displays only positive amplitudes and stacking of these traces is very robust (Astiz et al., 1996). The next step is sorting the processed traces to common cell gathers, which cover the investigation area. We use two sorting keys, CMP and SRC\&RCV. The CMP sorting generates trace gathers with midpoints in common cells, the SRC\&RCV generates sorting gathers with the source or the receiver locations in the common cells. Finally, traces within a common cell are stacked in offset bins building one offset stack for each cell. CMP-sorted offset stacks enhance either diving waves like Pg- or Sg-phases, or refracted waves like the Pn-phase in order to extract refractor velocity. SRC\&RCV-sorted offset stacks concentrate on delay times of refractors like the crystalline basement of sedimentary basins or the Moho. The inversion of the data extracted from the stacks is based on tomography and delay time decomposition. The capability of the techniques will be demonstrated by a synthetic data set and real data from the Alpine area. Guterch, A., M. Grad, A. Spicak, E. Brueckl, E. Hegedues, G. R. Keller, H. Thybo and CELEBRATION 2000, ALP2002, SUDETES 2003 Working Groups (2003). An overview of recent seismic refraction experiments in Central Europe. Stud.Geophys.Geod., in press Astiz, L., P. Earle and P. Shearer (1996). Global stacking of broadband seismograms. Seismological Research Letters, 67, 4, 8-18.
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BEHM, M., BRUECKL, E., CHWATAL, W., RUMPFHUBER, E., CELEBRATION 2000 and ALP2002 Working Groups, 2003. A 3-D seismic model of the Eastern Alps derived from CELEBRATION 2000 and ALP2002 data.
CELEBRATION 2000 and ALP2002 are two large 3-D-refraction campaigns, which target the crustal and upper mantle structure in Central Europe. This study is based on these seismic data sets and concentrates on the area of the Eastern Alps region and the surrounding forelands and basins. The tectonic setting of the investigation area is characterized by a continent-continent-collision of the Adriatic and European plate and subsequently by a lateral extrusion eastwards to the Pannonian basin. As a consequence of these geodynamic processes we have to expect a complex structure of the lithosphere demanding a 3-D interpretation. The model parameters describing the crust by continuous velocity-depth functions and the depths and velocities of refractors (basis of sediments and Moho) are given in a 31x34-horizontal grid with 20 km spacing. The depth nodes of the velocity-depth functions have a spacing of approximately 1 km, starting with smaller intervals near the surface. Signal detection and stacking techniques were applied to the data in order to guarantee a reliable interpretation even in areas of degraded seismic energy transmission. The essential elements of these techniques are the STA/LTA algorithm, the sorting to common cell gathers with the common midpoint (CMP) or the source and receiver locations (SRC\&RCV) as sorting keys and the stacking to offset bins. The signal detection and stacking techniques and the related inversion methods will also be presented at this meeting (Brueckl et al., 2003). The offset stacks of CMP-sorted traces enhance significantly Pg-phases at large offsets, thus gaining greater penetration depths. A 3-D velocity depth model covering an area of about 300000 km2.was generated inverting these data by a 1-D tomographic approach at all grid points with sufficient coverage. The penetration depth of the method is 15 - 20 km in the Alps up to 40 km. As a test, the same method was applied to Sg-phases along one line. An S-wave velocity-depth section was created similar in penetration depth to the corresponding P-wave section and showing similar structures. The quality of Pn-arrivals in the Alpine area is generally low. By the generation of CMP-sorted and SRC\&RCV-sorted offset stacks and an inversion technique based on the delay time decomposition Pn-velocities and a reliable image of the delay times was generated. The significance of the model is checked by 3-D ray tracing. Brueckl, E., M. Behm and W. Chwatal, (2003). The application of signal detection and stacking techniques to refraction seismic data.
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update 2006-08-23