There are four general types of geophysical techniques which consist of seismic, gravity, electricity, and electromagnetism measurements. Although the main focus in measurements are on the land or sea surface but there are activities to measure properties underground in boreholes or mines, at ocean depths and of course the others are from aircraft or satellites.
Since mid-1600s in Sweden that a magnetic compass was used in iron ore deposits finding, the early activities have been started in order to apply surface geophysical surveys to mineral and petroleum exploration. According to different physical properties, various methods are applied so the best one is a method which is exactly related to what is being sought. However, most of the time the combination of different methods has a better result.
Various geophysical techniques are directly dependent on contrasts detection in different physical properties of layers. So these methods won’t work when contrasts do not exist.
The main geophysical studies goal is determination of subsurface materials characteristics without direct observation. Each one of these surveys has their own capabilities and limitations which are necessary to be considered during policy and decision making of a geophysical investigations program.
As it is mentioned above, various geophysical methods are used for oil and gas exploration and mineral prospecting and among them, seismic reflection method in both land and marine settings is dominated for oil and gas exploration which is the most accurate one in comparison with other methods.
Geophysical Methods in Oil and Gas Exploration
The primary geophysical method is Seismic reflection, operated on density and elastic moduli of subsurface materials. Nowadays, 2D and 3D seismic surveys in both land and marine settings are performed all around the world. In this method artificial energy sources like explosives and water guns are utilized to generate seismic or acoustic waves. Comparing other methods analysis and interpretation seismic data is harder but it can provide more detailed information about subsurface materials. Other geophysical techniques, such as gravity, magnetic and sometimes EM, etc., are usually used as subsidiary approaches in oil and gas exploration in large scale geological settings determination.
Geophysical Methods in Mineral Prospecting
According to the target minerals and their deposit settings, all various types of geophysical methods are used in mineral prospecting. Here are different methods which are available in this field:
Magnetic and electromagnetic (EM) methods
It is believed that the geomagnetic field, the magnetic field around the Earth, is originated from the liquid outer core of the Earth which is containing high concentration of iron. Thus, by measuring local variations of the geomagnetic field, ferrous mineral deposits can be detected. So minerals are detected based on their own magnetic field. Due to their high efficiencies these methods are widely used in large scale airborne surveys.
EM is method with a low frequency time-varying magnetic field source in order to excite electrical currents in the ground based on the Faraday’s law of electromagnetic induction principle and Ampère’s law (which indicates that electric currents generate magnetic fields). This scalable method which can gain data from centimeter to meters and kilometers in mine tunnels, structures, ground, airborne and etc., is sensitive subsurface electrical conductivity. The physical properties consist of magnetic permeability, electrical permitivity and conductivity and the data can be both electric and magnetic field values. Using this method it is possible to answer questions around depth, thickness, material distribution, existence and extent of mineral and hydrocarbons in analyzed layers and etc. Although the GPR, DC resistivity and IP methods work In accordance with electromagnetism laws because of existed difference in ground excitation, they are in other categories.
Electrical methods which are applied to the electrical properties subsurface materials consist of SP (self-potential which is called spontaneous potential too and refers to natural occurring electric potential due to the liquid flow, chemical process or temperature gradient in the subsurface materials or any other unknown reason), DC Resistivity (which the electrical resistivity of the earth materials can be estimated by injecting DC electrical current into ground directly and measuring voltage potentials), and IP methods (which is the same as DC method except that the voltage potentials are measured after the electrical current is switched off. This is a suitable approach for determining electrically conductive targets surrounded by electrically resistive host materials). SP is a 2D method and suitable for quick reconnaissance surveys; while DC resistivity and IP methods can provide both 2D and 3D mapping of subsurface materials. Ground-based DC resistivity and IP methods have a better result in high resolution images comparing to airborne magnetic and EM methods.
Magnetotelluric (MT) and Controlled Source Magnetotelluric (CSMT)
The magnetotelluric (MT) method as a passive electromagnetic (EM) exploration method can measure orthogonal components of the electric and magnetic fields on the earth’s surface. Depending on the available frequencies, it is possible to study depth in a range from several meters to several kilometers. In CSMT, there is an artificial source, which typically has an electrical dipole that is connected to the ground at a certain distance from the receivers directly. In MT method greater depths up to several kilometers or more can be studied which is not achievable in electrical methods.
According to this method, in order to find earth materials masses and densities, it is possible to measure the gravitational forces with gravimeters.
Measuring natural gamma radiation from uranium, thorium and potassium in the rocks and soils is useful in radioactive minerals detection and geological mapping (radioactive elements are usually located in granitic rocks).
Nuclear Magnetic Resonance (NMR)
In NMR which is mainly used for groundwater exploration, an alternating magnetic pulse is generated through a horizontal circuit loop on the ground surface at the resonance frequency of the hydrogen nucleus. When the pulse is switched off, the resonance magnetic signals from the groundwater are measured. In this method because of measuring signal characteristic strength which is directly related the volume of groundwater, except water none of the minerals or host materials are detected.
BEM as a new broadband electromagnetic frequency domain geophysical VLF (Very Low Frequency) method is a unique approach for subsurface structures analysis which studies complicate elements electromagnetic reactions to the electromagnetic wave in VLF band. It has a high sensitive receiver (-140db) which can work in frequency range from 100 Hz to 40 KHz. By using this technology, both shallow and deep ground water, metallic and nonmetallic exploration, fault detection and any existed subsurface anomaly is achievable.
There are brilliant points that attract researchers to work with this technique. BEM method is capable in all metallic, non-metallic and even playa regions and it is able to match with other EM devices. This comprehensive method is a good solution for finding better understanding in all parts of investigation in an area which is helpful in achieving a more successful exploration. Using BEM technique makes it possible to record data in area with rough topography and determine zones with fractions, joints and gaps.
It is able to separate subsurface layers based on resistivity and also is able to increase power of intensity to perform investigation in deeper layers. Furthermore, performing the analytical solution of the equations accurately for wave processing differs BEM from other methods which don’t work on this part. Depending on the preferable type of study region and target, BEM method is able to use wave packages that is impossible in most of EM techniques because they work with frequencies one by one.
The BEM core competency is providing data about the dielectric coefficient and in some cases estimating material type besides other characteristics that can be obtained from common EM methods. It is achieved by performing analytical solution of the equations in wave processing accurately which is not observed in other methods. This method can use wave packages according to the preferable type of study region and target.