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Peace and Science in the Middle East


Planning a seismic refraction experiment to answer the question:
What lies under the Dead Sea basin and how deep is it?

      The volatile politics of the Middle East often require scientists to change their field plans. Originally, we planned to produce acoustically reflected images of the earth to a depth of about 1 km across the Dead Sea fault system, but this plan was cancelled after months of preparation due to security concerns following the Second Intifada. An alternate plan was devised and executed within four months to image the earth crust in a lower resolution, but to a greater depth (30km). This plan replaced vibrating trucks with explosives as acoustic sources and cabled seismometers with stand-alone seismometers for acoustic recorders. The experiment took place in October 2004 and consisted of two wide-angle seismic reflection and refraction profiles: a 280-km long profile along the international border between Jordan, Israel and the Palestinian Territories at the center of the Dead Sea rift, and a 250 km long profile from Gaza strip to eastern Jordan across the Dead Sea rift. The wide-angle seismic reflection and refraction method uses explosion-generated seismic waves which are transmitted laterally for many kilometers within the earth crust. This method maps horizontal and vertical variations in the speed of the waves which are a proxy to variations in rock composition.

      The University of Texas, El Paso (UTEP) , and the Incorporated Research Institutions in Seismology (IRIS) have graciously lended the seismometers on short notice. Permits were obtained to deploy seismometers along a closed military zone by the Jordan River, and within the property of the Arab Potash Company, and to transport and detonate 16.5 metric tons of explosives. Commercial outfits in Jordan and Israel were contracted to carry out the drilling, and loading of the explosives, and the detonation of explosives

Instrumentation

      The acoustic signals were recorded by stand-alone miniature seismic recorders. Each recorder was connected by a short cable to a single vertical 4.5 Hz geophone. Both the geophones and the recorders were buried a few inches below the surface to improve the signal quality and to prevent vandalism. 755 instruments were sent to the Middle East, 169 to Israel, and 598 to Jordan, and were deployed in the field, generally at distance intervals of 650 m with occasional larger intervals. 409 instruments were deployed along a north-south line within the rift valley in Jordan, and the remaining 334 instruments were deployed along an east-west line crossing the rift at the south-central part of the Dead Sea basin.

      Instruments could not be left in the field for more than a week because of limited battery power. Eight teams worked simultaneously to speed the deployment and collection of this large number of instruments. Each team consisted of two people, and occasionally a soldier. The deployment lasted 1.5-2 days and the collection took another day. Local Bedouin guards were hired in a few places to prevent theft and vandalism. Nevertheless, five instruments were lost, four could not be found, and one instrument was carried by jackals into a minefield and could not be retrieved. Several additional instruments were pulled out of the ground by jackals, and only a handful of recorders had electronic problems.

Acoustic Sources

      All 11 explosions had to be detonated during daylight hours and within two days. The Jordanian boreholes had to be loaded and then detonated in the same day. This made for a tight schedule. Time windows for detonation were assigned to the Israeli and Jordanian detonating teams. Eight of the explosions consisted of 1-ton dynamite detonated at the bottom of a 20 meters deep hole, one 3-ton explosion was detonated at the bottom of a 35-meter hole in the Jordan Valley, and two 750 kg explosions suspended at a depth of 50 meters from floats in the Dead Sea. The marine explosions were recorded on the permanent Geophysical Instituted of Israel (GII) seismic network with equivalent earthquake local magnitudes of 3.2 (ML) and 3.0 (ML) respectively. Additional natural earthquakes and commercial mining explosions that took place during this time period were also recorded. The entire dataset of explosions, earthquakes, and mining shots were used to obtain 2-dimentional models of P-wave rock velocity along and across the Dead Sea rift.

      go to Interpreting the data

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This page last modified on Monday, 14-Jan-2013 04:50:55 EST