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Electromagnetism and Groundwater

Electromagnetism and electromagnetic methods in groundwater exploration

By Darrel Dunn, Ph.D., Hydrogeologist

Introduction Electromagnetism and Groundwater Webpage

This webpage provides a brief description of electromagnetism followed by a description of the geomagnetic field that originates in the Earths core, the influence of the solar wind on the geomagnetic field, groundwater electromagnetism, and anthropomorphic background electromagnetism. Then applications of electromagnetism to groundwater exploration including seismoelectric/seismomagnetic geophysical surveying are described. The descriptions are relatively nontechnical and qualitative with some technical matter in parentheses. Rigorous treatment would involve advanced mathematics and quantum mechanics. Quantum mechanics is beyond the author's knowledge of physics.

Electromagnetism

Electromagnetism refers to phenomena associated with electric charges and currents and the electric and magnetic forces and fields associated with electric charge. These electric and magnetic forces are vectors in force fields. (They have magnitude and direction at each point in three-dimensional space.) Sometimes these force fields are represented by lines that are tangent to the force vectors. These are called field lines. A field line diagram shows a representative set of field lines. The electric charge generating a force field may be static or moving. Static charges generate electric fields derived from Coulomb force, which results from attraction between oppositely charged particles and repulsion between particles with the same charge. Ions in flowing groundwater are moving charges. Moving charges produce magnetic fields. These fields are not affected by the media that contain them, and the fields change virtually instantly when the movement or strength of the charges changes. The strength of the fields declines rapidly with distance from the source. A changing magnetic field creates an electric field, and a changing electric field creates a magnetic field. So the two fields are aspects of the same thing.

Geomagnetic Field

The Earth's geomagnetic field is a magnetic force field that is a combination of several magnetic fields. It extends from the Earth's core into the atmosphere and beyond. A worldwide system of stations continuously measure the field at points on the Earth's surface. The field is also studied by magnetic surveys using magnetometers mounted on ships, aircraft and other carriers. The Earth's outer core is thought to be composed of liquid iron. Convection in the liquid iron associated with the transferring internal heat to the Earths surface generates a magnetic field (Main Field) that contributes more than 90 percent of the magnetic force in the geomagnetic field. The Main Field changes very slowly. At the Earth's surface it has an intensity of 25,000 to 60,000 nT. (Geomagnetic flux density is usually measured in units of tesla. A nanotesla (nT) is a billionth of a tesla.)

Other magnetic fields contributing to the geomagnetic field include:

Fields generated by magnetized rocks in the Earth's crust.
Fields generated by charged particles moving in the magnetosphere and in the ionosphere,
Fields generated by ocean currents
Fields generated by lightning.

The magnetosphere is the part of the geomagnetic field in space around the Earth above the ionosphere. It is impacted by the charged particles in the solar wind to produce a comet shaped field surrounding the earth with the tail pointed away from the sun. The ionosphere is the part of the upper atmosphere where atoms and molecules are ionized by solar radiation. Its base is about 30 miles above the Earth's surface and its top is dynamic but extends to as much as 190 miles above the Earth's surface.

The geomagnetic field generated in the ionosphere varies much more rapidly than the Main Field. This rapid variation is caused by interaction with the charged particles of the solar wind. The magnetosphere is impacted by variation in the charged particles of the solar wind and this too can cause rapid variation in the geomagnetic field.. Figure 1 is an example of the variation in the geomagnetic field at one second increments. In this example the magnetic force varied as much as 0.06 nanoteslas in a second.

Figure 1. Total magnetic force at Boulder Magnetic Observatory near Boulder, Colorado. Graph of nanoteslas versus seconds.

Groundwater Electromagnetism

When groundwater flows through porous media it carries charged ions. These moving charged particles produce a magnetic field. This magnetic field is extremely weak. The strength of this electrokinetic effect depends on the flow rate and salinity of the groundwater. Henry and others (2014) describe an experiment where a magnetometer was placed in an aquifer at a depth of 1191 feet below another magnetometer in a very low noise location at the land surface. They found that: "Simultaneous measurements with two 3-axis SQUID magnetometers have allowed us to establish that the magnitude of a magnetic signal correlated with the water flow (expected due to the electrokinetic effect) is less than 0.13 nT (vertical component) and 0.26 nT (horizontal components)." The intensity of magnetic fields tend to decrease proportional to an inverse power of the distance. The magnetic force from this groundwater flow would be unlikely to be detected at the ground surface.

Electromagnetic Methods in Groundwater Exploration

Moving charged particles in groundwater produce extremely weak magnetic fields that extend to the ground surface, and very sensitive magnetometers are available to measure total magnetic flux density at any accessible point in the field. Consequently, under favorable conditions it might be possible to increase the probability of constructing successful groundwater test wells by measuring magnetic flux density at the ground surface. Two approaches might be considered:

Measure the natural magnetic flux, or
Measure induced magnetic flux resulting from artificial seismic pulses.

In the first approach the magnetic flux would be measured at points in a grid to attempt to identify an area where groundwater flow is converging on a relatively porous and permeable part of the aquifer system.

In the second approach, artificial seismic pulses are induced at the ground surface using dropped weights or explosives. These artificial seismic compressional waves cause increased velocity of the groundwater the instant the aquifer is compressed (due to instantaneous reduction of porosity). This approach has been applied using proprietary (seismoelectric) equipment.

Both approaches to using electromagnetic methods in groundwater exploration are adversely affected by three factors:

The extremely weak magnetic signals produced by moving groundwater,
The rapid decrease of magnetic field strength with distance from the source, and
The the noise in the magnetic signals measured at the ground surface.

The noise is produced by fluctuations in the geomagnetic field (see Figure 1) and by anthropic magnetic fields produced by anything that generates an electrical current. Various methods may be used to attempt to separate the magnetic signal produced by groundwater from the noise. The methods include:

Measuring the noise at a distant point,
Stacking (repeated measurements), and
Treating the magnetic measurements immediately preceding a seismic pulse as noise.

Due to the factors that adversely affect magnetic methods in groundwater exploration, uncertainty is present in the geophysical analyses of the magnetic data. This uncertainty would seem to increase with depth of the aquifer(s) and with the strength of the noise. In the seismic method the uncertainty would seem to increase as the strength of the seismic source decreases.

A geophysical method for groundwater exploration that involves natural electrical and magnetic fields is described in the webpage titled Audio-Magnetotelluric Surveying.

References for Electromagnetism and Groundwater

Henry, S. E. and others (2014): Monitoring Geomagnetic Signals of Groundwater Movement Using Multiple Underground SQUID Magnetometers; Inter-Disciplinary Underground Science & Technology Laboratory, EDP Sciences, HAL-01317678. 🔗

Jackson, J. D. (1962): Classical Electrodynamics; Wiley.

Love, J. J. (2008): Magnetic Monitoring of Earth and Space; Physics Today. 🔗

Posted March 2, 2024.

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