Applications of Geophysics

Introduction

Airborne geophysical surveying is a process of measuring the variation of several key physical or geochemical parameters of the earth. The most important parameters measured are conductivity (which is the inverse of resistivity), magnetic susceptibility, density, radioactive element concentration, and reflectance spectra. Any change in the earth's near surface that causes a measurable change in these parameters presents a potential application for airborne geophysics. The systems used to measure these parameters are electromagnetics (EM), magnetic, gravity, gamma-ray spectrometry (AGS), and (light) reflectance spectrometry (hyperspectral).

EM surveys map the three dimensional variation in conductivity, caused by changes in mineralogy, intensity of alteration, water content or salinity. Airborne magnetic surveys map the variation of the magnetic susceptibility, most often due to changes in the percentage of magnetite in the rock. Gamma-ray spectrometric surveys measure the radiation of one or more natural radioelements: potassium, uranium, or thorium, or a specific man-made radioelement. Hyperspectral reflectance spectroscopy measures the changes in the way the surface reflects light, across a broad spectrum from the visible light through infrared and short-wave infrared to the thermal infrared. Airborne gravity surveys map the variations of density within the earth.

Anomaly Detection with Geophysics

The most obvious use of airborne geophysics is to directly detect mineralisation or directly measure the changes in the ground conditions of interest, by measuring the changes of one of the geophysical parameters relative to the surrounding "host" rock. It may be more conductive, have a stronger magnetic susceptibility, etc. However, there are many indirect ways geophysics can be applied to geological problems. The absence or reduction of a response can also be a direct indicator of geologicaltargets. For example, the hydrothermal alteration processes associated with gold deposition often alter the magnetite in rock to sulphide, thereby reducing the magnetic anomaly. Such magnetic lows can be attractive targets, particularly when associated with weak conductors.

Mapping with Geophysics

Regional variations in density, resistivity, susceptibility, radioelement composition or reflectance spectra measured by geophysics can all be used to map geology or geomorphology. Large areas can be flown at a relatively low cost compared to ground surveys.The multiple sets of geophysical data can be combined with known geology to create regional geology maps and to develop priorities for follow-up on the most highly prospective ground. Interpreted geological structures such as shear zones, unconformities, or contacts may be strong indicators of economic mineralisation.

Mapping geology with geophysics has the power to extend geological knowledge into areas where the geology is covered, by transported sediment, for example. If the geophysical interpretation is constrained by regional geology, or spot observations of local geology, an effective geological map can be created for the entire survey area.

Combining geophysical methods enhances the accuracy of the map(s) generated - they are more than the sum of the parts. Many rocks may be similar in one geophysical parameter, but vastly different in another.

Case History, Ethiopia- Adola Area Mapping.