Very Low Frequency Electromagnetic (VLF)

Basic Concept

The very low frequency electromagnetic (VLF-EM) method is a passive electromagnetic (EM) method that exploits the EM radiation emitted by preexisting radio transmitters. Such structures are located around the globe and used by the military for navigation and communication. The VLF-EM method utilizes the principles of electromagnetic induction to image and qualitatively assess electrically conductive subsurface objects and features. It is considered one of the simplest EM methods used to efficiently identify near-surface conductors.

VLF-EM surveys provide analytical assessments of anomalous subsurface conductive bodies. Since its introduction into geophysical prospecting in the mid-twentieth century, the method has seen many improvements and adaptations, including use in airborne surveys. The VLF-EM method is ideally suited for near-vertical planar contacts and/or conductive bodies (e.g., dikes, fault mineralization). However, it has been applied to numerous projects such as regional geologic mapping, landslide/pollution monitoring, ore/mineral prospecting, groundwater investigations, and contamination studies (Sharma and others, 2014).

Theory

The VLF-EM method takes advantage of electromagnetic signals that are generated by high-powered submarine radiocommunication transmitters, of which forty are currently known to exist around the globe. The transmitters operate in the radiofrequency band of 3-30 kHz, which is higher than the typical frequency range of more standardized EM methods (i.e., 1-3 kHz). However, with respect to radio transmission, this operating band is very low, which is why the method is named as such (Gnaneshwar and others, 2011).

The radio transmitters are typically comprised of a large vertical wire through which current alternates, and plane EM waves are subsequently generated. The emitted EM radiation contains an electric and magnetic component, both of which travel considerable distances up to 20,000 km. However, the magnetic component of the transmitted radiation is of primary interest because it is more energetic and can be used in VLF-EM survey (Gnaneshwar and others, 2011).

Through electromagnetic induction processes, the transmitting antenna generates a primary magnetic field, and the magnetic field lines are oriented horizontally. When significantly far from the antenna, these field lines exhibit a negligible curvature and can be considered straight and parallel relative to the ground surface. As with other EM methods, the primary magnetic field induces eddy currents within subsurface conductors, and such current flow induces a secondary magnetic field (Paterson and Ronka, 1971).

It is impossible to counteract any influence the primary field has on the received signal, which is measured by a receiver that is energized by both magnetic fields. However, the primary field orientation (i.e., horizontal) is known, which allows for the assumption that the secondary field is approximately vertical. Thus, any observed tilt or alteration in the recorded signal is attributed to the secondary field and the influence of subsurface conductors (Paterson and Ronka, 1971).

When near an electrically conductive body, the combined magnetic fields of the transmitter and target create a polarization ellipse. The ellipse has an inclination and eccentricity that reflect the relative amplitudes and phase of the two fields. These parameters are also proportional to the in-phase (i.e. real) and out-of-phase (i.e. quadrature or imaginary) components of the secondary field. In practice, measurements of the secondary field are determined by comparing the signals collected in the horizontal and vertical directions (Paterson and Ronka, 1971).

Applications

VLF-EM surveys are typically conducted parallel to the primary magnetic field. A VLF-EM instrument is a directional radio receiver that is tuned to the selected radio transmitter. The unit contains a horizontally oriented reference coil and a signal coil that is situated along a vertical axis. Measurements of eccentricity and inclination are collected at points along the transect to produce plots of the in-phase and quadrature components of the secondary magnetic field

Variations in both signal components that occur along the survey transect are analyzed to estimate the size, position, depth, and conductivity of subsurface conductors. A vertically planar conductor produces a symmetrical anomaly with a positive and negative component, of which one indicates the conductor boundaries. Signal amplitude depends on conductivity and depth of conductor, the quadrature component can indicate conductivity, and the in-phase component inflection point reflects the center of the conductor (Mussett and Khan, 2000).

However, not all conductors are as simple, and irregular anomalies are further interpreted based on anomaly curves, monograms, and more complex analysis techniques. Qualitative subsurface information can be derived from applying simple filters to received signals to produce contour plots. Though there is not yet a standard approach, numerical modeling and inversions are needed to qualitatively assess geometry and physical properties (Gnaneshwar and others, 2011).

The VLF-EM method is generally able to measure significant anomalies even when imaging weaker conductors. However, there exist some limitations and factors that necessitate consideration. Because the frequency band is so high, the depth of penetration may suffer due to higher rates of attenuation of both the primary and secondary signals. Even so, the method has been able to successfully depict conductors up to 200 meters below the surface when encased within highly resistive country rock (Sharma and others, 2014).

If there are highly conductive subsurface and/or surface objects nearby, signal coupling can occur and potentially hinder the success of the method. Furthermore, variations in topography may affect the signal readings such that false positives are produced uphill and false negatives downhill. Regardless, the VLF-EM method is an extremely useful, user friendly, economical, and efficient EM method. It has proven advantageous, especially when used in conjunction with other geophysical methods, in the following types of applications:

Geological mapping
General and contaminated groundwater studies
Detecting mineralized fault zones/fractures
Surveying massive sulphides
Identifying disseminated ore/orebodies
Mapping coal-seams
Constraining pollution sources

Examples/Case studies

Bayrak, M., 2002, Exploration of chrome ore in Southwestern Turkey by VLF-EM: Journal of the Balkan Geophysical Society, v. 5, no. 2, p. 35-46.

Abstract: Geophysical exploration for chrome ore deposits is rather complicated, and integrated geophysical methods should be used. For an integrated data interpretation respective data sets has been collected from VLF-EM (“Very Low Frequency”-electromagnetic), induced polarization (IP), gravity, magnetic and self potential (SP) data in southwestern Turkey. This area is known for its occurrence of chrome ore. VLF-EM parameters such as the apparent resistivity, phase, real and imaginary components of the vertical magnetic field and tilt angle of the magnetic polarization ellipse were acquired using 16 kHz (GBR, Rugby, England) radio signal. Mapping of the VLF-EM resistivity, phase, real component of vertical magnetic field and filters (Fraser filter, 1969 and Karous and Hjelt filter, 1983) yields good results in distinguishing conducting ore bearing fault zones within the resistive ultrabasic rocks. The dominant feature of the reconnaissance mapping is that the low values of resistivities (<100 Ohm.m) extend in about N 25o E direction. This characteristic direction correlated with the extension of known chrome occurrences in the field. Displaying of the VLF-EM real 2-D current density pseudosections along three profiles, secondary currents were followed with depth in the ground.

Benson, A.K. and Stubben, M.A., 1995, Interval Resistivities and Very Low Frequency Electromagnetic Induction — An Aid to Detecting Groundwater Contamination in Space and Time: A Case Study: AAPG Division of Environmental Geosciences Journal, v. 2, no. 2, p. 74-84.

Abstract: Gasoline and other hydrocarbons leaking from underground storage tanks are common groundwater pollutants. The extent of the contaminant plumes is often assessed using only monitoring wells. However, surface geophysical surveys can also help map areas of contaminated soil and groundwater. Geophysical methods are appealing because they are inexpensive, quick, and nondestructive to the environment, and they can be performed at most locations. Electrical resistivity and very low frequency electromagnetic induction data were collected at a 9-km2 site of shallow hydrocarbon contamination in central Utah County, Utah. Electrical resistivity data were also collected over a two-year period. Previously installed monitoring wells facilitated analysis of water chemistry to enhance interpretation of the geophysical data. The electrical resistivity and very low frequency electromagnetic data correlate well and were used to map the contaminant plume. Contour maps were constructed from both the apparent resistivity and interval resistivity data, the latter determined from the apparent resistivities by iterative modeling. These maps outline the hydrocarbon plume. The plume was delineated as an area of low apparent and low interval resistivities. Apparent and interval resistivity data were plotted as a function of time against hydrocarbon concentration data collected during the same period of time. The results indicate that interval resistivity data is a reliable method for monitoring changes in hydrocarbon concentration over time

Eze, C.L., Mamah, L.I., and Isreal-Cookey, C., 2004, Very low frequency electromagnetic (VLF-EM) response from a lead sulphide lode in the Abakaliki lead/zinc field, Nigeria: International Journal of Applied Earth Observation and Geoinformation, v. 5, no. 2, p. 159-163, doi:10.1016/j.jag.2004.01.004.

Abstract: Six very low frequency electromagnetic (VLF-EM) lines were surveyed across the expected strike of a lead sulphide lode in a mine in the Abakaliki lead/zinc field of Nigeria. The aim of the survey was to investigate the lateral extension of the lode and establish the potential of this tool for mapping sulphide deposits in an area underlain principally by shales. VLF-EM field values were filtered using Fraser filter and the results plotted directly over the survey lines. The plots were examined for significant anomalies that could be related to the lode. About 40 m length of the lode was successfully mapped by this simple, inexpensive tool. The anomalies strike NW–SE following the strike of one of the predominant set of faults. A depth of 10 m and a dip of 90° were calculated for the conductor.

Gnaneshwar, P., Shivaji, A., Srinivas, Y., Jettaiah, P., and Sundararajan, N., 2011, Very-low frequency electromagnetic (VLF-EM) measurements in the Schirmacheroasen area, East Antarctica: Polar Science, v. 5, no. 1, p. 11-19, doi:10.1016/j.polar.2010.09.001.

Abstract: To assess the feasibility of the very-low-frequency electromagnetic (VLF-EM) method in the Schirmacheroasen area of East Antarctica, and to investigate its response, VLF-EM measurements were performed along four traverses. The preliminary results reveal the locations of geological boundaries and shear zones/faults, which may indicate that VLF anomalies are due to shear zones or alteration zones located along contacts between different rock types. The strength of the VLF anomaly decreases over the polar ice cap. The inphase component of the VLF anomaly, when processed and interpreted with an analytic signal approach, yields a depth range of 15–30 m, whereas Fraser and Hjelt filter analyses yield a depth range of 25–60 m. The VLF-EM responses along all four traverses, along with their interpretations, are presented here as a case study.

Jeng, Y., Lin, M., and Chen, C., 2004, A very low frequency-electromagnetic study of the geo-environmental hazardous areas in Taiwan: Environmental Geology, v. 46, p. 784-795, doi:10.1007/s00254-004-1071-7.

Abstract: This study utilized the very low frequency-electromagnetic (VLF-EM) technique, a passive electromagnetic prospecting method working in the very low frequency range (15–30 KHz) to investigate the geo-environmental problems of shallow, low conductivity sedimentary layers in Taiwan. Field examples successfully demonstrate the advantages of using this method in locating non-mineralized shallow fault zones. The zero-crossings of in-phase and quadrature measurements in 2-D contour maps clearly locate the position of subsurface anomalous source bodies. Further analysis of the measured VLF single profile peaks reveals that this method is useful in determining subsurface structures and conductivity. The advantages of nondestructive, noninvasive, and low consumption of power make this method extremely friendly to the environment. The authors anticipate this method will have more profound impacts on the interactions between prospecting technology and the earth.

Monteiro Santos, F.A., Mateus, A., Figueiras, J., and Gonçalves, M.A., 2006, Mapping groundwater contamination around a landfill facility using the VLF-EM method — A case study: Journal of Applied Geophysics, v. 60, no. 2, p. 115-125, doi:10.1016/j.jappgeo.2006.01.002.

Abstract: An electromagnetic survey to detect leachate flow and to map its spatial distribution using the VLF-EM method was carried out around a landfill that is operating since 1998. The survey comprised twelve, roughly E–W, VLF-EM profiles in the western part of the landfill where conductive anomalies had been detected in a previous resistivity survey (Wenner and dipole–dipole arrays). The VLF-EM data were interpreted qualitatively, using the Fraser and the Karous–Hjelt filters. The quantitative interpretation of the data was done with a 2-D code that performs the inversion of the tipper data, using the results of the previous resistivity survey to constrain the inversion. Fraser filtered data and relative current density pseudosections indicate the presence of shallow and deep conductive zones that cross the landfill facility along known fracturing directions. The 2-D resistivity models calculated from tipper data indicate that these zones may have resistivity lower than 400 Ω m that is in a good agreement with the results obtained from the previous resistivity data. Chemical analysis of both surface and groundwater collected inside and outside the landfill confirms the presence of a halo of water contamination around the landfill location. This enables to infer that the anomalies detected by VLF-EM data are due to contaminated groundwater flowing in connected fractures.

Oryński, S., Okoń, M., and Klityński, W., 2016, Very Low Frequency Electromagnetic Induction Surveys in Hydrogeological Investigations; Case Study from Poland: Acta Geophysica, v. 64, p. 2322-2336, doi:10.1515/acgeo-2016-0092.

Abstract: In 2011, a geophysical survey was carried out in the surroundings of the Jagiellonian University in Cracow, using a Very Low Frequency method. The measurements were designed to determine the reason of frequent flooding of the lowest level of the building. The main objective of the study was to find out from where and in which way the rainwater seeps into the building and how this problem can be solved in the least invasive manner. The aim of geophysical methods was also to provide necessary information that will enable the construction of a hydro-geological model of the local environment. The interpretation revealed the presence of a sandy gutter surrounded by impermeable clay. There is a big resistivity contrast between those layers. Their location and approximate dimensions were determined.

Ramesh Babu, V., Ram, S., and Sundararajan, N., 2007, Modeling and inversion of magnetic and VLF-EM data with an application to basement fractures: A case study from Raigarh, India: Geophysics, v. 72, no. 5, p. 1SO-Z83, doi:10.1190/1.2759921.

Abstract: We present modeling of magnetic and very low frequency electromagnetic (VLF-EM) data to map the spatial distribution of basement fractures where uranium is reported in Sambalpur granitoids in the Raigarh district, Chhattisgarh, India. Radioactivity in the basement fractures is attributed to brannerite, U-Ti-Fe complex, and uranium adsorbed on ferruginous matter. The amplitude of the 3D analytical signal of the observed magnetic data indicates the trend of fracture zones. Further, the application of Euler 3D deconvolution to magnetic data provides the spatial locations and depth of the source. Fraser-filtered VLF-EM data and current density pseudosections indicate the presence of shallow and deep conductive zones along the fractures. Modeling of VLF-EM data yields the subsurface resistivity distribution of the order of less than 100 ohm-m of the fractures. The interpreted results of both magnetic and VLF-EM data agree well with the geologic section obtained from drilling.

References

Bayrak, M., 2002, Exploration of chrome ore in Southwestern Turkey by VLF-EM: Journal of the Balkan Geophysical Society, v. 5, no. 2, p. 35-46.

Mussett, A.E. and Khan, M.A., 2000, Electromagnetic Methods, in Looking into The Earth: An Introduction to Geological Geophysics: New York, Cambridge University Press, p 210-231.

Paterson, N.R. and Ronka, V., 1971, Five years of surveying with the Very Low Frequency Electro magnetics method: Geoexploration, v. 9, no. 1, p. 7-26, doi:10.1016/0016-7142(71)90085-8.

Sharma, S., Biswas, A., and Baranwal, V., 2014, Very Low-Frequency Electromagnetic Method: A Shallow Subsurface Investigation Technique for Geophysical Applications, in Sengupta, D., ed., Recent Trends in Modelling of Environmental Contaminants: New Dehli, Springer India, p. 119-141.