Seismic refraction profile, Kingdom of Saudi Arabia: field operations, instrumentation, and initial results

Abstract

In February 1978 a seismic deep-refraction profile was recorded by the USGS along a 1000-km line across the Arabian Shield in western Saudi Arabia. The line begins in Paleozoic and Mesozoic cover rocks near Riyadh on the Arabian Platform, leads southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan (Tihamat Asir), and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, including 19 in the Farasan Islands. Six shot points were used--five on land, with charges placed mostly below water table in drill holes, and one at sea, with charges placed on the sea floor and fired from a ship. The total charge consumed was slightly in excess of 61 metric tons in 21 discrete firings. Seismic energy was recorded by means of a set of 100 newly developed portable seismic stations. Each station consists of a standard 2-Hz vertical geophone coupled to a self-contained analog recording instrument equipped with a magnetic-tape cassette. The stations were deployed in groups of 20 by five observer teams, each generally consisting of two scientist-technicians and a surveyor-guide. On the day prior to deployment, the instruments were calibrated and programmed for automatic operation by means of a specially designed device called a hand-held tester. At each of ten pre-selected recording time windows on a designated firing day, the instruments were programmed to turn on, stabilize, record internal calibration signals, record the seismic signals at three levels of amplification, and then deactivate. After the final window in the firing sequence, all instruments were retrieved and their data tapes removed for processing. A specially designed, field tape- dubbing system was utilized at shot point camps to organize and edit data recorded on the cassette tapes. The main functions of this system are to concatenate all data from each shot on any given day onto a single shot tape, and to provide hard copy for monitoring recorder performance so that any problems can be corrected prior to the next deployment. Composite digital record sections were produced from the dubbed tapes for each shot point by a portable processing and plotting system. The heart of this system is a DEC PDP 11VO3 computer, which controls a cassette playback unit identical to those used in the recorders and dubbers, a set of discriminators, a time-code translator, a digitizer, and a digital plotter. The system was used to maintain various informational data sets and to produce tabulations and listings of various sorts during the field operations, in addition to its main task of producing digital record sections. Two master clocks, both set to time signals broadcast by the British Broadcasting Corporation, provided absolute time for the recording operations. One was located on the ship and the other was stationed at a base camp on the mainland. The land-based master clock was used to set three additional master clocks located at the other active shot points a few days in advance of each firing, and these clocks were then used to set the internal clocks in the portable seismic stations via the hand-held tester. A master clock signal was also linked to the firing system at each shot point for determination of the absolute shot instant. It is possible to construct a generalized crustal model from examination of the six shot point composite record sections obtained in the field. Such a model rests upon a number of simplifying assumptions and will almost certainly be modified at a later stage of interpretation. The main assumptions are that the crust consists of two homogeneous isotropic layers having no velocity inversion,, that the Mohorovicic discontinuity is sharp, and that effects of surface inhomogeneities and elevation changes can be ignored. The main characteristics of the tentative model are the following: (1) The thickness of th