Saeed S. Alshahrani

This chapter, “Overview of geophysical techniques,” is taken from the book, Exploration Geophysics, which was written in 2009 by Mamdouh Gadallah and Ray Fisher. The chapter describes the three main active methods of the geophysical approach: gravity, magnetic, and seismic, each of which has its own advantages and disadvantages and purposes of using. The first two methods are cheaper and faster, but they do not give the much subsurface information as seismic does. The subsurface materials differ from each other in some ways such as density, magnetic susceptibility, velocity, resistivity, and electromagnetic property.
Gadallah and Fisher showed that the gravimeter is an instrument used to measure the acceleration of gravity. The gravity’s concept is how the mass of the earth accelerates the mass of the gravimeter in one position while the important corrections are earth’s elevation, latitude, and topography. As we draw a contour map, the gravity anomalies will appear when we have closed contours. The gravity value at sea level is 980 gal; the unit is referred to Galileo who explored this unit in the first time. On the other hand, the magnetic field comes from the convection currents in the outer core of the earth. The magnetic field has been changing its direction each year because of a slight moving of the earth poles. Gadallah and Fisher reported that the magnetic field is drawn by magnetic lines of force, and they utilize them to map and locate the basement ferromagnetic ore deposits. The instrument for measuring the magnetic field is the magnetometer, and the unit is Tesla.
In addition, Gadallah and Fisher described the seismic method that is considered recently as the most popular method. Its concept is that energy, as a source, moves into the earth and comes again at different times to the receivers, the geophones, which are located on the surface. Reflection and refraction are the two paths in seismic method, and they differ from each other because the velocities of the waves are different in each layer of the earth. The authors showed that there are different waves that can be recorded by the receivers, surface waves, reflection waves, refraction waves, and noise waves. Finally, the authors explained how geologists process the data and obtain a seismic section for interpretation.
Reference
Gadallah. M. R. & Fisher. R. (2009). “Overview of geophysical technique”. Geophysical Exploration (pp. 7-16). New York, NY. Springer.

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