Exploration 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.
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.
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