Site hosted by Angelfire.com: Build your free website today!

Geophysical Methods in Mineral Exploration

 

Introduction

Surface geophysical surveys have been applied to mineral and petroleum exploration for many years. A magnetic compass was used in Sweden in the mid-1600s to find iron ore deposits. The lateral extent of the Comstock ore body was mapped using self-potential methods in the 1880s. A very crude type of seismic survey measured the energy resulting from blasting operations in Ireland in the late 1800s. The idea that energy travels through a material with a certain velocity came from this survey. During World War I, geophysical techniques were used to locate artillery pieces. Anti-submarine warfare in World War II led to magnetic and sonar surveys.

The main emphasis of geophysical surveys in the formative years was petroleum exploration. Technology developed for oil and gas surveys led to the use of geophysical surveys in many important facets of geotechnical investigations. Geophysical surveys have been applied to civil engineering investigations since the late 1920s, when seismic and electrical resistivity surveys were used for dam siting studies. A seismic survey was performed in the 1950s in St. Peter’s Basilica to locate buried catacombs prior to a renovation project. From the late 1950s until the present time, geophysical techniques have had an increasing role in both groundwater exploration and in geotechnical investigations. Geophysical surveys are now used routinely as part of geological investigations and to provide information on site parameters (i.e., in place dynamic properties, cathodic protection, depth to bedrock) that in some instances are not obtainable by other methods. Values derived from seismic geophysical surveys are obtained at strain levels different from some site parameters obtained by other means.

 

Various Methods

All geophysical techniques are based on the detection of contrasts in different physical properties of materials. If contrasts do not exist, geophysical methods will not work. Reflection and refraction seismic methods contrast compressional or shear wave velocities of different materials.  Electrical methods depend on the contrasts in electrical resistivities. Contrasts in the densities of different materials permit gravity surveys to be used in certain types of investigations. Contrasts in magnetic susceptibilities of materials permit magnetic surveying to be used in some investigations. Contrasts in the magnitude of the naturally existing electric current within the earth can be detected by self-potential (SP) surveys.

Seismic refraction surveys are used to map the depth to bedrock and to provide information on the compressional and shear wave velocities of the various units overlying bedrock. Velocity information also can be used to calculate in place small-strain dynamic properties of these units. Electrical resistivity surveys are used to provide information on the depth to bedrock and information on the electrical properties of bedrock and the overlying units. Resistivity surveys have proven very useful in delineating areas of contamination within soils and rock and also in aquifer delineation. Gravity and magnetic surveys are not used to the extent of seismic and resistivity surveys in geotechnical investigations, but these surveys have been used to locate buried utilities.  Self-potential surveys have been used to map leakage from dams and reservoirs.

Geophysical surveys provide indirect information. The objective of these surveys is to determine characteristics of subsurface materials without seeing them directly.  Each type of geophysical survey has capabilities and limitations and these must be understood and considered when designing a geophysical investigations program.

Geophysical interpretations should be correlated with real “ground-truth”data such as drill hole logs.  It is very important that the results of geophysical surveys be integrated with the results of other geologic investigations so that accurate interpretation of the geophysical surveys can be made.

 

Airborne versus Ground Surveys

In general, airborne geophysical methods are used in reconnaissance and ground geophysical methods are used in more detailed investigations. There are, however, many instances in which either airborne or ground methods could be used. In an extended exploration program, combinations and sequences of methods may be appropriate, and there is often a need to weigh their individual advantages.

  • Airborne surveys have some impressive characteristics. They are fast, they are relatively inexpensive per unit area, they can obtain several kinds of surveys at once, and they can provide a more objective coverage than ground surveys in many kinds of terrain. For example, several hundred line­kilometers of airborne electromagnetic surveying can be done in a day compared with three to five line-kilometers per crew in a ground electromag­netic survey.

  • The cost of an airborne electromagnetic survey, with magnetic and radiometric data included, is likely to be one-fourth to one-fifth the cost of an equivalent ground electromagnetic survey. Airborne survey patterns are reasonably uniform and complete because they do not have the access and traverse problems of ground surveys in swamps, dense brush, and rugged topography.

  • Airborne methods may sometimes be advantageous because competing exploration groups and mineral land speculators may be lurking in the area. It I is easy to locate someone's field camp, trace their newly cut ground survey lines, and "join the crowd." Airborne surveys, on the other hand, can operate in a less conspicuous pattern from supply bases outside the target area.

  • The airborne advantage in time, cost, and security applies to work in relatively large areas where the cost of aircraft operation can be spread over quite a few line-kilometers of work. Most airborne methods are neither economical nor appropriate in target areas of only a few square kilometers.

  • Airborne surveys have considerable flexibility, but they have some specific weather and terrain limitations as well. Since many surveys must be flown with a terrain clearance of less than 150 m in order to obtain a suitable signal, days or weeks may be lost because of low clouds.

  • Flight-track recovery - the relating of the finished survey to ground features - is often done by selecting points in a narrow strip of ground photographed during the survey; for this, too, weather must permit some recognizable features to be visible.

  • An airborne survey will give more accuracy than a ground survey in some areas, but it will seldom provide such detail or such sharp signals as a ground survey. A ground survey can be made with more closely spaced lines, and it can be done with a wider choice of methods and equipment.

  • Less preliminary work is needed on the actual exploration site for airborne surveys than for ground surveys, but more accurate base maps and photo­graphic coverage may be needed.

  • Ground geophysical surveys have the advantage of being able to tie in to occasional control points and stations, but airborne geophysical surveys are flown so fast and so low that the ground control features must be numerous, accurately plotted, and readily visible.

  • In monotonous terrain where recognizable features are sparse, it may be necessary to follow flight lines by an inertial navigation system or by a doppler (radar) navigation system.

 

Using Helicopters or Fixed-wing Aircraft:

  • A choice must sometimes be made between helicopter and fixed-wing aircraft for an airborne electromagnetic or radiometric survey. Helicopters have an advantage in being able to maintain a more constant ground clearance above rugged terrain. Also, helicopters have a slow-flying capability, which allows for greater accuracy and they can land for a ground check in critical areas.

  • Helicopter geophysical surveys can therefore be used in detailed work as well as in reconnaissance. Still, there are disadvantages. Helicopters are much more expensive to operate than are fixed-wing aircraft, they can cover only a third as many line-kilometers per day at best, they have a relatively short range of operation, and they require more maintenance work per flying hour.

  • The decision to use a helicopter in a geophysical survey is generally based on the assumption that the helicopter will permit an essential level of accuracy or detail that could not be matched in a fixed-wing survey.

 


This website is hosted by

S. Farooq

Department of Geology

Aligarh Muslim University, Aligarh - 202 002 (India)

Phone: 91-571-2721150

email: farooqs@sancharnet.in