HR: 17:45h
AN: GP14A-07    [Abstracts]
TI: LOTEM and SHOTEM measurements at the Dead-Sea-Transform within the DESERT-project
AU: * Koch, O
EM: koch@geo.uni-koeln.de
AU: Scholl, C
EM: scholl@geo.uni-koeln.de
AU: Helwig, S L
EM: helwig@geo.uni-koeln.de
AU: Martin, R
EM: martin@geo.uni-koeln.de
AU: Meqbel, N
EM: meqbel@geo.uni-koeln.de
AU: group, D
EM: mhw@gfz-potsdam.de
AB: Within the Dead-Sea-Rift-Transec-(DESERT)-project seismic, seismological, electromagnetic, gravity, magnetic, geodynamic and geological studies were done at the Dead Sea Rift/Dead Sea Transform (DST) to give answers to the questions: "How do shear zones work and what controls them?". Among large scale insights of the DST structure which were derived e.g. from magnetotelluric(MT)- and wide angle seismic-measurements many interesting results on mid and small scales were obtained within the project. Most of the mid and small scale experiments were located on the Araba-fault, which is the main fault of the DST-system between the Dead- and Red-Sea. In this region, we carried out transient-electromagnetic-measurements (TEM) in the years 2002 and 2004. We used two different methods with different penetration depths. With short-offset-TEM (SHOTEM) measurements we explored the conductivity structure of the fault in near surface regions from several meters up to 100 m depth. Using Long-Offset-TEM (LOTEM) measurements we resolved conductivity structures from 100 m up to three kilometers. The field setups of both methods were designed to allow 2-D interpretation. All TEM profiles cross the fault approximately in the middle. The LOTEM setup consisted of a 10 km long profile with four grounded dipole transmitters (two on each side of the fault) and 64 receiver stations. With the SHOTEM method we realized four parallel profiles of 1 km length with station spacing of 50 m station in central-loop configuration. To further enhance the spatial resolution in the central part of each profile, we carried out in-loop measurements with different positions of the receiver. Most of the SHOTEM measurements were realized with a new 3-component-TEM antenna attached to a three channel Nanotem receiver. Using this setup, we were able to record horizontal and vertical signals simultaneously. The design and construction of this new 3-component TEM antenna was done at our institute in Cologne. With 1D inversions of the LOTEM data we obtained models showing similar conductivity features to the MT results. On the west side of the Araba fault our data can confirm the existence of a conductive feature with resistivities below 10 Ohmm in a depth of about 1 km. This structure seems to disappear on the east side of the fault. 1-D results from the SHOTEM measurements across the fault indicate conductivity structures which correlate with velocity structures derived by seismics. Furthermore, our in-loop measurements seem to confirm the existence of a conductive vertical near-surface anomaly at the fault trace. On the southernmost profile, this anomaly is located at the same position where a seismic experiment found guided waves. The existence of guided waves is correlated to the damage zone of the fault. The damage zone itself was expected to be a conductive feature. The final interpretation of the recorded TEM data will be done using 2-D models. For this purpose we develope a new 2.5-D time-domain inversion code. The inversion code was tested with synthetic data and the results are promising. Due to the huge memory demand we currently are running inversions using subsets of the measured data. With a 2.5 inversion of the full data we expect a more detailed insight in the conductivity structure of the DST.
DE: 0624 Guided waves
DE: 0634 Measurement and standards
DE: 0639 Nonlinear electromagnetics
DE: 0684 Transient and time domain
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting