Borehole Geophysics

geophysical research to study the rock mass near and between boreholes, for distances ranging from fractions of a meter to hundreds of meters. Borehole geophysics differs from well logging, which studies the geologic column along the walls of boreholes, chiefly in that it is broader in scope.

The basic tasks of borehole geophysics are to discover bodies of useful minerals and determine the bodies’ position, dimensions, shape, and bedding; to evaluate the bodies’ physical parameters and mineral composition; to calculate reserves of useful minerals; and to correlate and construct geologic columns by means of logging data. Borehole geophysics is the only way to study the area around boreholes and the spaces between boreholes at depths of greater than 200–300 m.

Borehole geophysics makes use of DC and low-frequency electrical techniques, including the charge, natural-field, induced-polarization, and induction techniques; radio-wave techniques; and magnetic techniques. Other techniques use transitional processes, including seismoacoustic and piezoelectric processes.

Electrical techniques are used to study the distribution of electric or magnetic fields created by man-made and natural sources of direct and low-frequency current (less than 10 kHz). Charge techniques study the electric fields of point and dipolar sources. The natural-field technique studies the electric fields created by oxidation-reduction reactions that take place on the boundaries of ore bodies. The polarization of rocks that results when an electric current passes through the rocks produces the fields studied by the techniques of induced polarization. Induction methods measure magnetic fields created by dipolar boreholes and looped ground sources. The transitional-processes technique is used to study the magnetic fields that arise when the current in a source is turned off.

Radio-wave techniques are used to study the distribution of high-frequency electric and magnetic fields (0.1–40 MHz), the absorption coefficient of the rocks, and the screening coefficient of the bodies being sought. Magnetic techniques measure the components of the permanent magnetic field created by ore bodies within increased magnetic permeability.

Seismoacoustic techniques mainly make use of the attenuation of seismic oscillations. Piezoelectric techniques are used to study the electric fields created when piezoelectric minerals are acted upon by elastic oscillations. Sometimes techniques are used that depend on measuring borehole temperatures, the relationships between the potentials of electrochemical reactions at mineral contacts and the strength of the current passing through the minerals, or the mu-meson component of cosmic radiation. Most of the techniques used in borehole geophysics have three variations: two-well (interwell), one-well, and well-surface.

Borehole geophysics originated with the use of the DC charge technique proposed by the French scientist C. Schlumberger in 1932. It developed into an independent branch of exploratory geophysics in the 1960’s and 1970’s through research done in the USSR. Work in mining geophysics, pioneered in the USSR by A. A. Petrovskii in 1923–25, played a major role in developing the theory and methods of individual techniques used in borehole geophysics. Similar work has been done abroad.