A REVIEW OF MODERN AND FUTURE ELECTROMAGNETIC

 

 

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Introduction

Geophysical methods plays a very crucial role in the earth research and exploration applications hence over the recent past there has been a drastic increase in their applications in the earth exploration  ventures. However, among the most often used area of geophysics is usually the electromagnetic field whose source may be natural or controlled (artificial). Thus electromagnetic methods usually constitute one key principle technologies that are utilized in the applied geophysics (Roy, 2008).  Therefore this has tremendously increased  the applications of the most modern and advanced geophysical methods which may include: direct current, magnetotelluric, induced polarization and the controlled source electromagnetic (Hossain and Mustafa, 2007).

However, numerous electromagnetic methods are been utilized today in the geophysics which usually deals with the earth as well as it surrounding space. Thus electromagnetic methods in this field have been in the studying the earth shape, magnetic and gravitational fields, earth’s dynamics as a whole together all other factors that are associated with the earth and its components (Telford, Geldart and Sheriff, 1990). Moreover, the application of electromagnetic is also a key player in the geophysics that are involved in  the societal needs which includes natural hazards mitigation, mineral resources and environmental protection (Hossain and Mustafa, 2007). Thus geophysical survey data obtained a result of the applications of the electromagnetic are key to the analysis of reservoirs of potential petroleum and mineral deposits, locating groundwater, archaeological finds, as well as in finding the soils and glaciers thicknesses together with the remediation of the environment (Al-Bassam and Hussein, 2008).

Moreover, the electromagnetic have been used in the exploration geophysics involved with utilizing surface methods which facilitates the measuring of the earth surface physical properties in the process of detecting or inferring the presence as well as the  position of ore minerals, groundwater reservoirs,  geothermal reservoirs, hydrocarbons, and other structures of geology. Thus it is involved with practical application of electromagnetic methods which aid in the measuring of the rocks’ physical properties, and particularly the detection of  the existing measurable physical differences among the rocks within a certain ore (Roy, 2008).

Therefore the electromagnetic exploration geophysics is capable of directly detecting the mineralization target style through the aspect of directly measuring its physical properties. However, there are various geophysical methods used where the main ones are the electromagnetic methods which include ground penetrating radar, magnetotellurics and time-domain/transient electromagnetic (Hossain and Mustafa, 2007).

The use of electromagnetic in the geophysical exploration also has the potential of mapping a region’s  the subsurface structure, as well as elucidating the underlying structures, and also determining rock units spatial distribution which helps in the detection of structures such as folds,  faults, as well as intrusive rocks (Roy, 2008). This therefore could be termed as an indirect way which can be used in the assessment of chances of the accumulations of hydrocarbon or ore deposits (Al-Bassam and Hussein, 2008).

However, these electromagnetic geophysical methods usually posses a huge potential in the attempts of finding deposits of minerals as well as hydrocarbons. Moreover, there has also been the proliferation of the uses of these methods in the monitoring of the environment, investigations of ground water,  mapping of subsurface salinity, subsurface archeological sites imaging, investigations of civil engineering sites as well as interplanetary imaging.

A review of modern applications of electromagnetic in geophysics

The electromagnetic methods are frequently used in the geophysical techniques that are frequently applied in the geotechnical and environmental studies. Currently the electromagnetic methods used in geophysics usually apply active sensing technology whereby the generated electromagnetic field induces secondary electromagnetic response within the investigated mediums. Therefore this is mainly the principle which is used in the electromagnetic methods which are currently on use (Hossain and Mustafa, 2007).

Another  electromagnetic geophysical method commonly used currently is the magnetotellurics which is involved in the earth’s subsurface imaging  by taking  measures of magnetic and electrical fields  natural variations at the surface of the earth. The depth of investigation may ranges from 300 m up to approximately 10,000 m or more. This technique has nowadays become a key method in most exploration surveys throughout the world. Moreover, this method has gained commercial applications such as exploration of hydrocarbon which include oil and gas, mining exploration, geothermal exploration, as well as groundwater and hydrocarbon and monitoring (Roy, 2008). This method also has research applications such as experimentation aimed at furthering the technique, deep and long-period exploration of the crustal as well as acting as precursors of earthquake prediction (Telford, Geldart and Sheriff, 1990).

Transient electromagnetic which is also referred to as the time-domain electromagnetic is also another technique of geophysical exploration whereby magnetic and electric fields are usually induced by the electric current transient pulses which is then followed by measuring the subsequent decay response. These methods have been generally used in the process of determining electrical properties of the subsurface; however they are still sensitive to the magnetic properties of the subsurface (McNeill, 1994).  The transient electromagnetic surveys area often surface electromagnetic technique commonly used in the mineral exploration, environmental mapping, and for groundwater exploration, which are used globally both for offshore and onshore applications (Nabighian, 1991).

The near future applications of electromagnetic in geophysics

Most of the applications of the electromagnetic geophysical techniques in the near future will tend to drastically as a result of advancements in both technical and information technology. Among the areas destined to change include the mineral exploration, civil engineering and archeology (Al-Bassam and Hussein, 2008). Electromagnetic surveys will be frequently used in the process of detecting a wide range sulphide metal deposits by detecting the anomalies in conductivity which are capable to be generated around the subsurface sulphide bodies (Hossain and Mustafa, 2007). Electromagnetic surveys will be probably used in the process of detecting deposits of the palaeochannel-hosted uranium mostly found around the shallow aquifers, capable of responding to the electromagnetic surveys in conductive overburden (Roy, 2008). Therefore this will be one of the indirect methods of inferential mineralization detection.  There is also an expected advancement in the regional electromagnetic surveys which are mostly conducted by use airborne methods such as aircrafts. Moreover, the surface electromagnetic methods are also expected to advance so as to be able to map out the sulphide bodies in three dimensions within the earth which will greatly help the geologists (Telford, Geldart and Sheriff, 1990).

Controlled source electromagnetic and magnetotellurics will also be able to provide hydrocarbons pseudo-direct detection via detection of their resistivity changes. However, this will also go a long way in complementing seismic data in the processes of  below salt imaging. Moreover, the ground penetrating radar which is another electromagnetic method has the potential to be utilized in the civil engineering sector in varied ways such as detection of utilities including buried sewerage, water, telecommunication and electrical cables as well as  mapping of soft soils. Magnetotellurics also has a high potential of  groundwater reservoirs delineation in the future as well as mapping faults within areas in which there is storage of  hazardous substances such as nuclear waste storage facilities and nuclear power stations (Hossain and Mustafa, 2007).

Moreover, these methods will also find there use in archaeology. For instance, it will be possible to use the ground penetrating radar in mapping buried artifacts which include graves, wreck sites, as well as other archaeological sites that are shallowly buried. All these potential applications will be of great significance in the geophysical field and more preferably to the geophysicists, archeologists and geologists (Roy, 2008).

An overview of future applications of electromagnetic in geophysical techniques

Frequency Domain Electromagnetic

This techniques which are also often known as the ground conductivity has the potential of being utilized in the future for the purposes of measuring the rocks and soils electrical conductivity through determination of their magnitude combined with the phase of electromagnetic current induced. However, there will also the possibility of generation of secondary electromagnetic fields which will be able to detect and be used in locating nonferrous and ferrous metal objects.

Therefore the frequency domain electromagnetic will be able to effectively delineate: structures that are near-surface which includes the faults, metallic objects such as drums, tanks and pipelines as well as the rocks and soils lithology lateral variations.

The instruments that will be used in the process of ground conductivity will be responsible for  inducing currents, that will be generated by a variety of electromagnetic fields, to the earth subsurface in a way such that their amplitude will be in a linear proportion to ground conductivity (Zhadnor, 2009). However, there will be also other factors which will be liable to affect the ground conductivity technique such as the structure, constituents as well as the moisture content of the rocks and soil (Roy, 2008). This is mainly because a large number of the rock and soil constituents such as the quartz, aluminum oxide coatings, mica, feldspar, and iron are very high resistivity electrical insulators.

Time Domain Electromagnetic Geophysics

The Time domain electromagnetic shortly referred to as TDEM is another area geophysical technique which will be used in the process of monitoring wells. The TDEM surveys are expected to often used in the investigations of the extent to which the saltwater intrusion has occurred within the freshwater aquifers that are found beneath the coastal plain (Stewart and Gay, 1986). Nowadays the TDEM utilizes a transmitter which is responsible for driving an alternating current via an insulated electrical cable that is square loop and usually laid on the ground. However, in the future there are high chances that as the technology advances this technique will also advance leading to an increased precision of  determining the extent of intrusion (Roy, 2008). Moreover, the current and frequencies used will also have to be adjusted in order to ensure that there is efficiency in the electromagnetic fields produced.

Currently the investigation depth is mainly dependent on the time interval that follows the current  shutoff, mainly because at times the eddy currents  are sensed by the receiver at greater depths progressively. However, the eddy currents intensity at specific depths and times stands to be determined by the subsurface rock units and the fluids contained in them bulk conductivity (McNeill, 1994).  Moreover, the future trend is expected to be of high quality since there will be improvement in the sensitivity of the eddy currents receiver which will then increase (Nabighian, 1991).

However, due to the fact that electrical conductivity is always the resistivity inverse, therefore the raw TDEM data processing will usually be resulting to a vertical profile which usually resembling the formation resistivity vs. depth which is nowadays used in the borehole resistivity profiles in order to monitor wells using the conventional logging tools (Roy, 2008). However, since the level of resistivity is the one which indicates the freshness or saltiness of water then the future of this process is expected to tremendously improve on the precision of resistivity measurements which will then be used to accurately determine groundwater resistivity values (Hossain and Mustafa, 2007).

Thus in comparison to the conventional methods which involves the cost of siting as well as drilling another monitoring well, this geophysical technique of TDEM soundings will therefore be easily and cheaply carried out thus saving time and money in future. As indicated in the figure below, the advanced TDEM resistivity data collected is relatively comparable to the geophysical logs of boreholes (Al-Bassam and Hussein, 2008).

 

However, the issue of encroachment of the saltwater into freshwater aquifers most frequently occurring beneath the coastal plains has for a long period of time proven to be a serious challenge to many parts of the world (Roy, 2008). Moreover, the  future expected combination of borehole information with the advanced processes of TDEM soundings will have the potential to be used in the precise and accurate identification of the interface between saltwater and fresh zones, as well as monitoring the extent to which there has been the occurrence of  saltwater intrusion as indicated in the diagram below (Stewart and Gay, 1986).

 

 

 

Moreover, the electromagnetic geophysical surveys will also be of great importance in measuring ground conductivity whereby the electromagnetic induction process is utilized. However, the conventional electromagnetic system used in this method mainly consists of a transmitter as well as a receiver coil both of which are usually spaced at a configuration that is standard (Roy, 2008). However, the conventional and the future methods are expected to be working on same principle but their operating frequencies will be totally different, thus the future one is expected be providing a variety of depth penetrations as well as different applications resolutions (Zhadnor, 2009).

Moreover, the low frequency systems are often used in the process of subsurface ground conditions investigations, or to find large cavities that are underground including caves as well as mine workings (McNeill, 1994). However, the systems that operate at intermediate frequencies can be used in the locating of discrete objects including abandoned mineshafts or sinkhole compared to the systems of high frequency electromagnetic capable of locating smaller targets. However, as a result in the advancement in the frequencies this technique will also be expected to locate very minute objects within the earth’s subsurface (Zhadnor, 2009).

Moreover, the geophysical electromagnetic techniques will also be expected to have greater applicability in the sea explorations (Nabighian, 1991). This will only be possible through continued concentration on developing of novel and advanced marine electromagnetic methods as a result of  invention, through theory which is then accompanied by experimental design and constructing suitable equipment responsible for the collection as well as interpreting the obtained marine data (Roy, 2008). Thus the advancement in these marine exploration procedures will in future make explorations very easy and affordable no matter the environment in which they will be carried out.

Conclusion

The geophysical electromagnetic techniques have over the recent past contributed immensely into the facilitation of the work of geophysicists mainly because the continuous advancements have allowed them to  conduct procedures that were not possible there before easily (McNeill, 1994). Therefore despite the presence of several other geophysical techniques the electromagnetic technique is most commonly used and also which posses the potential of growing in the future (Nabighian, 1991).

 

Currently there are numerous applications of geophysical electromagnetic techniques in a wide variety of sectors such as mineral exploration, civil engineering, underground water characterization as well as archeology. Moreover, these techniques will tend to tremendously improve in the future mainly as a result of technical and information technology advancements. These therefore will ensure that all the geophysical electromagnetic techniques have drastically improved (Hossain and Mustafa, 2007).

 

References

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