History of Geophysical Prospecting Methods
Geophysical techniques have been used in mineral prospecting for the past 300 years, beginning in Sweden around 1640 with the use of magnetic compasses in exploring for iron ore. Resistivity measurements followed in the 1800’s in the search for base metals, and by the early 1900’s the Schlumberger brothers were successfully using self potential (SP) and resistivity for this purpose. By 1912 Conrad Schlumberger had patented the induced polarization (IP) method, and had used the technique for finding economic sulfide deposits.
The use of applied geophysics for mineral and hydrocarbon exploration as we know it today began in the 1950’s, with the advent of sensitive magnetometers, gravity meters, battery-powered electronic equipment, and the application of information theory and computer processing to seismic data acquisition.
Since that time, several different frequency and time-domain electromagnetic (FEM and TEM) systems have been developed to map out low-resistivity anomalies for massive sulfide exploration. Many of these systems came from Canada, although the first TEM system in the United States was imported from Russia in the 1950’s.
During a period of extensive worldwide porphyry copper exploration in the 1960’s and 70’s, a number of different geophysical exploration methods were used with varying degrees of success: gravity, magnetics, induced polarization (IP) and self potential (SP). Gravity was used to map basement topography and to search for altered intrusive bodies; magnetics were used to search for altered rocks; and IP and SP were used to locate disseminated sulfides, mainly pyrite and chalcopyrite.
These same methods are applied today but with greater accuracy and sensitivity due to technological advances, especially in the fields of electrical geophysics and seismics. For example, IP has evolved from the traditional time-domain approach to multi-frequency IP, now called complex resistivity (CR) or spectral IP, which can be used to differentiate between anomalous responses from alteration, sulfide type, and electromagnetic coupling (an unwanted artifact of the measuring process).
Vertical sounding methods, such as controlled source audiofrequency magnetotellurics (CSAMT) and time-domain or transient electromagnetics (TDEM or TEM), are used for mapping structure and massive sulfide bodies.
CR and TEM are used in both surface and downhole survey configurations. Downhole techniques have been developed for in-hole assaying. Advances in cross-hole tomography has enabled this method to be used to assess mineralization and alteration features between drill-holes.
Airborne radiometric techniques have been developed which aid in large-scale alteration mapping. And seismic equipment development and data processing have greatly increased the resolution and interpretation capabilities for both deep and shallow applications in the last 20 years.