Geophysical Methods in Mineral Exploration

 

Introduction

Geophysical techniques involve measuring reflectivity, magnetism, gravity, acoustic or elastic waves, radioactivity, heat flow, electricity, and electromagnetism. Most measurements are made on the surface of the land or sea, but some are taken from aircraft or satellites, and still others are made underground in boreholes or mines and at ocean depths. 

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.  

Geophysical mapping depends on the existence of a contrast in the physical properties of adjacent rockbodies - that is, between what is being mapped and what it is surrounded by.  Examples include a sequence of sedimentary layers that form a trap for oil accumulation, a drainage pattern that might affect groundwater flow, or a dike (or some other host rock) in which valuable minerals may be concentrated. Different methods depend on different physical properties. The best method that may be used depends upon what is being sought.  In most cases, however, data from a combination of methods rather than from simply one method yield a much clearer picture.

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.

GEOPHYSICAL METHODS

Although geophysical methods for oil and gas exploration and mineral prospecting include almost all the major geophysical methods ever invented, oil and gas exploration is dominated by seismic reflection method, in both land and marine settings. It’s hard to say which methods dominate mineral prospecting, but those methods can be readily airborne deployed are most efficient and wide spread methods, especially for preliminary surveys. These methods mainly include magnetic, electromagnetic (EM) and radiometric methods. Other methods which are mainly land based include direct current (DC) electrical resistivity, induced polarization (IP), gravity, magnetotelluric (MT), self potential (SP) and seismic methods. Ground penetrating radar (GPR) is not widely used or only used as a complementary method due to its limited investigation depth.

 Geophysical Methods Used in Oil and Gas Exploration

 Seismic reflection, operated on density and elastic moduli of subsurface materials, is the primary geophysical method used in oil and gas exploration. 2D and 3D seismic surveys are conducted around the world everyday, in both land and marine settings. Seismic surveys utilize artificial energy sources, such as explosives and water guns, to generate seismic/acoustic waves. The waves are bounded back at interfaces of subsurface layers, and their timings and amplitudes upon arrival back to the surface were recorded by seismographs utilizing geophones/hydrophones. Seismic data generally can provide more detailed information about subsurface materials than other methods. In the other hand, the data is also harder to analyze and interpret. Geophysicists with years of experiences on seismic data processing and interpretation are in great demand.

Other geophysical methods, including gravity, magnetic and sometimes EM, etc., are also used as subsidiary methods in oil and gas exploration for delineating geological settings in large scales.

 Geophysical Methods Used in Mineral Prospecting (Non Oil and Gas)

 Besides seismic methods, varieties of geophysical methods are used in mineral prospecting and they are chosen based on the targeted minerals and their deposit settings. For example, magnetic method is a must for delineating iron ores, resistivity methods including self potential (SP) and direct current (DC) electrical resistivity are often used for searching base metals, and induced polarization (IP) are widely used for sulfite deposits, etc.

 Magnetic and electromagnetic (EM) methods

 The magnetic field around the Earth, geomagnetic field, is believed to be mainly originated from the liquid outer core of the Earth containing high concentration of iron. Ferrous mineral deposits can be detected by measuring local variations of the geomagnetic field.

 During an EM survey, there are a primary EM field and a secondary EM field. The primary field, or the source of the EM energy, induces electrical current in the earth media, which in turn, causes the secondary EM field. The secondary EM field is generally measured through the changing rate of the magnetic flux within a circuit loop without direct contact with the earth media. The primary EM field can be natural as in the very low frequency (VLF) method, or artificial as in many EM surveys with EM transmitters.

 Both methods are widely used in large scale airborne surveys due to their high efficiencies.

 Electrical methods

 Electrical methods are operated on the electrical properties subsurface materials. These methods include SP, DC Resistivity, and IP methods. SP is essentially a 2D method and good for quick reconnaissance surveys; while DC resistivity and IP methods can provide both 2D and 3D mapping of subsurface materials.

 SP, or self potential, also called spontaneous potential, refers to the natural occurring electric potential. The origin of SP is not completed understood, but it may be related the liquid flow, chemical process or temperature gradient in the subsurface materials.

 In a DC resistivity survey, DC electrical current is injected into ground through direct contact. By measuring voltage potentials at locations in the survey area, the electrical resistivity of the earth materials can be estimated.

 IP method is similar to DC resistivity method except that the voltage potentials are measured after the electrical current is switched off. Almost identical equipments can be used for DC resistivity and IP surveys except that non-polarizable electrodes have to be used in IP surveys for potential measurements. IP method is a good method to delineate electrically conductive targets surrounded by electrically resistive host materials.

Compared to airborne magnetic and EM methods, ground-based DC resistivity and IP methods can provide subsurface images with higher resolutions.

 Magnetotelluric (MT) and Controlled Source Magnetotelluric (CSMT)

 While the EM waves originated from solar winds or lighting traveling downward from the sky, the earth materials are modeled as horizontal layers of resistors transporting EM plane waves traveling downward. Within a range of frequencies, the horizontal electrical field and the horizontal magnetic field orthogonal to each other are measured simultaneously on the ground surface, and the resistivities of the layers are calculated. The investigation depth could be from several meters to several kilometers depending on the frequencies available. In CSMT, an artificial source, typically with an electrical dipole connecting directly to the ground at a certain distance from the receivers, is used. This creates a situation similar to an EM plane wave traveling downward from the sky, in which the measurements at a certain frequency are related to the skin depth of the EM field.

 Compared to electrical methods, MT method can usually achieve greater investigation depths, up to several kilometers or more.

 Gravity method

 By measuring the gravitational forces with gravimeters, the masses and densities of the earth materials can be obtained. Gravity methods can be used for base structure mapping. For example, it can be used to search for intrusive bodies in porphyry copper prospecting.

 Radiometric method

Radiometric method measures natural gamma radiation from uranium, thorium and potassium in the rocks and soils. Radiometric surveys can be used for directly detecting radioactive minerals, but it’s often used for geological mapping, because the radioactive elements occur in greater abundance in granitic rocks.

 Nuclear Magnetic Resonance (NMR)

 NMR is mainly used for groundwater exploration. An alternating magnetic pulse is generated through a horizontal circuit loop on the ground surface at a certain frequency (resonance frequency of the hydrogen nucleus). The resonance magnetic signals from the groundwater are measured after the pulse is switched off. The uniqueness about this method is that the measured signal strength is directly related the volume of groundwater. That is, it detects water only, not any other minerals or host materials. 

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals

University

   


This website is hosted by

S. Farooq

Department of Geology

Aligarh Muslim University, Aligarh - 202 002 (India)

Phone: 91-571-2721150

email: farooq.amu@gmail.com