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Supplementary material to “Helicopter Gravity Survey in the Dead Sea Area”
23 March 2010
H.-J. Götze, Christian-Albrechts-Universität zu Kiel, Kiel, Germany
Uwe Meyer, Bundesanstalt für Geowissenschaften und Rohstoffe, Hanover, Germany
Sungchan Choi, Christian-Albrechts-Universität zu Kiel, Kiel, Germany
Citation:
Götze, H.-J., U. Meyer, and S. Choi (2010), Helicopter gravity survey in the Dead Sea area, Eos Trans. AGU, 91(12), 109–110. [Full Article (pdf)]
Studies of historical earthquakes that have occurred during the past few thousand years, paleoseismic studies and instrumentally-recorded earthquakes demonstrate that the Dead Sea Transform is a seismically active and hazardous plate boundary. This transform is described as a long shear zone that extends from the Gulf of Aqaba/Eilat through the Dead Sea into the Lake of Galilee [e.g. Garfunkel, 1981; Garfunkel and Ben Avraham, 1996; Mechie et al., 2009].
With steep escarpments on either side, the Dead Sea area is very difficult to access to establish a dense network of ground-based gravity measurements. Therefore, airborne gravity is the method of choice to overcome the difficulties associated with terrestrial gravity measurements [ten Brink et al., 1993; Götze et al., 2007] in the Dead Sea region.
Survey set-up
For political and pragmatic reasons, the Dead Sea helicopter gravity survey was divided into two parts in Israel and Jordan. On the Israelian side, the En Yahav air field was chosen as the logistical base for the airborne work. A reference point in the Israel gravity network close to En Yahav was used for relative base readings and to tie helicopter gravity measurements to the existing surface gravity network. When the survey in Israel was finished, the helicopter flew to Jordan. The southern half was flown from Aqaba International airport. A map of the survey flights is shown in Fig. S1 and a summary of flight statistics is given in Table S1.
Table S1: Flight statistics for the Dead Sea helicopter gravity survey.
Helicopter installations
In addition to standard instruments for navigation, the BGR Sikorsky S-76B helicopter was equipped with several Novatel and Ashtech GPS-antennae and receivers, a Novatel inertial navigation unit (SPAN), the GT-1A gravity meter and a radar altimeter for height control (Table S2).
Table S2: Short list of Installations
Gravity meter
The gravity meter used was a GT-1A, an airborne, single vertical sensor, GPS-INS scalar gravity meter with a Schuler-tuned three-axis inertial platform. The meter itself weighs about 50 kg. The GT-1A platform with the gravity sensing element (GSE) can tilt up to a maximum of 45 degree for both pitch and roll. The GT-1A uses an intelligent platform control to maintain the meter vertically during flights. The vertical accelerometer or GSE has an axial design with a reference mass on a spring suspension, a photoelectric position pick-up and a moving-coil force feedback transducer. Two dynamic ranges are measured and recorded simultaneously: +/- 2.50 m/s2 (0.25 g) and +/- 5 m/s2 (0.50 g). For 95% of the survey the meter operated in the fine scale mode. The GT-1A gravity meter mounted in the helicopter is shown in figure S3.
Gravity data processing
The airborne data set acquired during the survey was mainly processed by Canadian MicroGravity (CMG) in Perth, Australia. The time domain filter used for the initial data processing was 100 s, resulting in a spatial resolution of approximately 1.6 km geological half-wavelength. The resulting free-air anomaly data were combined in a regular grid of 1000 m cell size. The gravity data processing was done by the BGR and CMG. CMG utilized GPS data from an Ashtech Z-Xtreme airborne receiver combined with a Thales ground station. To achieve higher resolution gravity data, enhanced GPS data processing was performed using a combination of additional NovAtel OEM4 receivers on the ground and on board the helicopter, inertial navigation system (SPAN) data and stations of the International GPS Service (IGS). The filter used for this second solution is 60 s, resulting in a spatial resolution of approximately 1 km geological half wavelength along a single profile. This resulted in better spatial resolution but at the cost of a significantly higher noise level.
Acknowledgement
We thank Dirk Henrik Stockhausen and Vera Stercken of the German Embassies in Amman and Tel Aviv for their political aid and support. We are grateful for the essential contact with the Israeli military authorities provided by Aharon Shahar. We owe many thanks to Maher Hijazin, director of NRA, for organizing all necessary contacts to the Jordanian military and civil authorities involved in the project and smoothing our path for the survey. We owe special thanks to Boaz Peleg who is not only an excellent helicopter pilot but proved to be a successful logistician and field manager. Many friendly thanks to Col. Nabil Abadneh who safely guarded us through the Jordanian air space and managed our flight logistics in Jordan. We thank H. Khaldoon and J. Khataybeh for their efficient help in the Jordanian field work. Thank you also to Ron Hackney (GA, Canberra, Australia) for his language editing. This project was funded by the Deutsche Forschungsgemeinschaft (DFG, GO-380 24/ 1+2).
References
Garfunkel Z. (1981), Internal structure of the Dead Sea leaky transform (rift) in relation to plate kinematics. Tectonophysics 80: 81–108.
Garfunkel, Z., and Z. Ben Avraham (1996), The structure of the Dead Sea basin, Tectonophysics, 266, 155 176.
Götze, H.-J., R. El-Kelani, S. Schmidt, M. Rybakov, M. Hassouneh, H.-J. Förster, J. Ebbing, and DESERT Group (2007), Integrated 3D density modelling and segmentation of the Dead Sea Transform (DST), Int., J., Earth Sci., doi:10.1007/s00531-006-0095-5
Mechie, J., K. Abu-Ayyash, Z. Ben-Avraham, R. El-Kelani, I. Qabbani, M. Weber, and DESIRE Group (2009), Crustal structure of the southern Dead Sea basin derived from project DESIRE wide-angle seismic data. Geophys. J. Int., 178: 457–478, doi:10.1111/j.1365-246X.2009.04161.x.
ten Brink, U., Z. Ben-Avraham, R. Bell, M. Hassouneh, D. Coleman, G. Andreasen, G. Tibor, and B. Coakley (1993), Structure of the Dead Sea Pull-Apart Basin From Gravity Analyses, J. Geophys. Res., 98 (B12), 21877–21894.
Figure S1:
Topographic map shows the Dead Sea helicopter gravity survey flight lines in Israel and Jordan (red lines). The stars indicate the operational bases used during the survey. From north to south these were Amman airport, Alajoun NRA field camp, En Yahav air field and Aqaba airport. Profile 314 is the flight line from Aquaba to Amman.
Figure S2:
Helicopter equipment and installations (see main text for details).
Figure S3:
Photo showing the GT-1A gravity meter installed in the helicopter. The photo shows the stabilized platform with the single vertical sensor on top and the processing unit (left of platform).