Dead Sea Research
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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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U.S. Department of the Interior |
U.S. Geological Survey
