The physical properties of rocks have been used to devise geophysical methods that are essential in the search for minerals, oil and gas and other geological and environmental problems.

These methods are:

• Gravity method
• Seismic method
• Electromagnetic method
• Geothermal method
• Magnetic method
• Electrical method
• Radiometric method

Geophysical methods respond to the physical properties of the subsurface media (rocks, sediments, water, voids, etc.. ) and can be used Successfully when one region differs sufficiently from another in some physical property.

Passive methods

Which detect variations within the natural fields associated with the earth, like the gravitational and magnetic fields, such as gravit, magnetic, some electric and some electromagnetic methods, radioactive and geothermal methods

Active motheds

These artificially generated signals transmitted into the ground and then modify the received signals in ways that are characteristic of the materials through which they travel. Examples of these methods are seismic and some electrical methods.

Generally, natural field methods (passive methods) can provide information on earth properties to greater depths and are simpler to carry out than artificial source methods (active methods). Moreover, the artificial source methods are capable of producing a more detailed and better resolved picture of the subsurface geology.

Geophysical methods may from part of a larger survey and thus geophysicists must be in contact with the whole survey team and particularly to the client.

Few, if any geophysical methods provide a unique solution to a particular geological situation. It is possible to obtain a very large number of geophysical solutions to some problems, some of which may be geologically non-sensical. It is necessary, therefore, always to ask the question: “Is the geophysical model geologically plausible?. If it is not, then the geophysical model has to be rejected and a new one developed which does provide a reasonable geological solution.

Applications

1. Hydrocarbon exploration (coal, gas, oil)
2. Regional geological studies (over areas of 100s of km2 )
3. Exploration of mineral deposits.
4. Engineering site investigation.
5. Hydrogeological investigation .
6. Detection of subsurface cavities .
7. Mapping of leachate and contaminant plumes.
8. Location and definition of buried metallic objects.
9. Archaeo-geophysics.
10. Forensic geophysics.

Several geophysical surveying methods can be used at sea ( marine geophysics ) or in the air (aerogeophysics )

Reconnaissance surveys are often carried out from the air because of the high speed of operation.

Airborne versus ground geophysical methods:

• Airborne geophysical methods are used in reconnaissance work, but the ground methods are used in more detailed investigations.
• They are fast and are relatively inexpensive per unit area.
• Several kinds of surveys can be done at once.
• They can provide a more objective coverage than ground surveys in many kinds of terrains.
• For example: several hundred line kilometers of airborne electromagnetic surveying can be done in a day compared with three to five line kilometers per crew in a ground EM survey .
• The cost of an airborne electromagnetic survey, with magnetic and radiometric data included is likely to be 1/4 to 1/5 the cost of an equivalent ground EM survey
• Airborne survey patterns are reasonably uniform and complete because they do not have the access and traverse problems of ground survey in swamps, dense brush and rugged topography.
• 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 .

Gravity method:

• It is mainly used for oil exploration. Sometimes in mineral and ground water prospecting.
• Gravity prospecting involves the measurement of variations in the gravitational field of the earth (i.e. minute variations in the pull of gravity from rock within the first few miles of the earth’s surface).
• Different types of rock have different densities and the denser rocks have the greater gravitational attraction.
• If the higher–density formations are arched upward in a structural high, such as an anticline, the earth’s gravitational field will be greater over the axis of the structure than along its flanks.

A salt dome which is generally less dense than the rock into which it is intruded, can be detected from the low value of gravity recorded gravity recorded above it compared with that measured on either side.

Anomalies in gravity which are sought in oil exploration may represent only one – millionth or even one – ten – millionth of the earth’s total field.

For this reason, gravity instruments are designed to measure variations in the force of gravity from one place to another than the absolute force itself.

The gravity method is useful wherever the formations of interest have densities which are appreciably different from those of surrounding formations.

Gravity is an effective means of mapping sedimentary basins where the basement rocks have a higher density than the sediments.

Gravity is also suitable for locating and mapping salt bodies because of the low density of salt compared with that of surrounding formations.

Gravity can be used for direct detection of heavy minerals such as chromite

Magnetic method:

Magnetic method deals with variations in the magnetic field of the earth which are related to changes of structures or magnetic susceptibility in certain near surface rocks.

Magnetic surveys are designed to map structure on or inside the basement rocks or to detect magnetic mineral directly.

In mining exploration, magnetic methods are employed for direct location of ores containing magnetic minerals such as magnetite.

Intrusive bodies such as dikes can often be distinguished on the basis of magnetic observations alone.

Electrical methods:

Electrical prospecting uses many techniques, based on different electrical properties of the earth’s materials such as:

• The resistively method is designed to give information about the electrical conductivity of the earth’s rocks.
• In resistivity method the current is driven through the ground using a pair of electrodes and the resulting distribution of the potential in the ground is mapped by using another pair of electrodes connected to a sensitive voltmeter.
• The resistivity method has been used to map boundaries between layers having different conductivities.
• It is employed in engineering geophysics to map bedrock.
• It is used in groundwater studies to determine salinity. – The induced polarization (IP) makes use ionic exchanges on the surfaces of metallic grains (disseminated sulphides).
• Telluric current and magneto-telluric methods use natural earth currents and anomalies are sought in the passage of such currents through earth materials.
• The self potential method is used to detect the presence of certain minerals which react with electrolytes in the earth to generate electrochemical potentials.
• Electromagnetic methods detect anomalies in the inductive properties of the earth’s subsurface rocks.
• The method involves the propagation of time varying, low frequency electromagnetic fields in and over the earth.
• An alternating voltage is introduced into the earth by induction from transmitting coils and the amplitude and phase shift of the induced potential generated in the subsurface are measured by detecting coils and recorded.
• Electromagnetic methods are used to detect metallic ore bodies.

Seismic methods:

There are two main seismic methods, reflection and refraction:

Seismic reflection method

This method is used to map the structure of subsurface formations by measuring the times required for a seismic wave, generated in the earth by a near surface exploration of dynamite, mechanical impact or vibration, to return to the surface after reflection from interface between formations having different physical properties.

The reflections are recorded by detecting interments which are called geophones responsive to ground motion.

Variations in the reflection times from place to place on the surface indicate structural features in the strata below.

Depths to reflecting can be determined from the times using seismic velocity information.

Reflections from depths as great as 20,000 feet can be observed from a single explosion, so that in most areas, geologic structures can be determined throughout the sedimentary section. S.P. G Reflected Ray Layer 1, V1 Layer 2, V2 Reflector

With reflection method one can locate and map such features as anticlines, faults, salt domes and reefs. Many of these are associated with the accumulation of oil and gas.

Seismic refraction method:

In refraction method, the detecting instruments recorded the arrival times of the seismic waves when refracted from the surface of discontinuity.

These times give information on the velocities and depths of the subsurface formations along which they propagate. Refracted Ray Refractor Layer 1, V1 Layer 2, V2 S.P. G

Refraction method makes it possible to cover a given area in a shorter time and more economically than with the reflection method.

Radioactive Method

This method is used to detect radioactive minerals such as uranium and thorium.

Well logging method

• This involves probing the earth with instruments which give continues readings recorded at the surface as they are lowered into boreholes.
• The rock properties which are covered by well logging techniques are electrical resistivity, self potential, gamma ray generation density, magnetic susceptibility and acoustic velocity.
• Well logging is one of the most widely used of all geophysical techniques

Reference

Dr. El-Arabi H. Shendi ( 2007 ) Introduction of Geophysics, Suez Canal University Faculty of Science, Professor of Applied & Environmental Geophysics