Seismoelectrical Method
Encyclopedia
The seismoelectrical method (also called the electroseismic method) is based on the generation of electromagnetic field
s in soils and rocks by seismic wave
s. Although the method is not reported to detect groundwater flow
, it does measure the hydraulic conductivity
, which is related to permeability and, therefore, to the potential for groundwater flow.
As the seismic (P
or compression
) waves stress earth materials, four geophysical phenomenon occur:
The dominant application of the electroseismic method is to measure the electrokinetic effect or streaming potential (item 2, above). Electrokinetic effects are initiated by sound waves (typically P-waves) passing through a porous rock inducing relative motion of the rock matrix and fluid. Motion of the ionic fluid through the capillaries in the rock occurs with cations (or less commonly, anions) preferentially adhering to the capillary walls, so that applied pressure and resulting fluid flow relative to the rock matrix produces an electric dipole. In a non-homogeneous formation, the seismic wave generates an oscillating flow of fluid and a corresponding oscillating electrical and EM field. The resulting EM wave can be detected by electrode pairs placed on the ground surface.
Surface seismic sources and measurement electrode pairs are generally used to measure the electrokinetic effect. Borehole
systems also have been recently developed. A hammer blow or small explosive (black powder) are typically used for the seismic source. Two, short (few meters) electrode pairs are typically located collinear and symmetric to the seismic source. Stacking of repeat seismic "shots" is often used to improve signal to noise.
The computer-receiver contains a preamplifier, analog-to-digital converter and power supply system to measure the voltages induced in the grounded dipoles.
The hammer-and-plate seismic source is used along with two pairs of electrodes arranged in a straight line with electrodes offset (from the center of the array) by 0.25 and 2.25 m. The seismic source is positioned in the center of the array. Larger electrode spacing can be used; however, they are typically centered in pairs around the shot point and are generally less than about 10 m in length. Timing of the measurement is achieved by a hammer trigger (or other mechanical trigger). Measurements are made for a period of 400 ms after triggering. The last 200 ms of the record is used as a sample of background noise
and is subtracted from the first 200 ms of signal in order to remove the first, third, and fifth harmonics of the background noise at the receivers. This noise is typically from power lines.
repetitive hammer (or explosive) shots and removal of power line and other noise as described above. This processing is conducted on the computer-receiver while in the field. Depending upon field conditions, up to 20 soundings can be conducted in a day. It is expected that measurement time will increase proportionally with stacking times, and that long stacking times would be needed in the vicinity of power lines.
The signal files are processed to give a sounding plot of permeability and porosity against one-way seismic travel time
. Values for a simple seismic velocity model are required to allow time-to-depth conversion. The resulting logs against depth are displayed in the field.
Output from the electroseismic inversion is typically a plot showing hydraulic conductivity versus depth. This interpretation assumes a one-dimensional layered earth. In theory, the inversion also can derive fluid conductivity and fluid viscosity from the rise time of the signal. Where many soundings are measured in close vicinity, a pseudo 2-D cross-section can be generated.
The maximum depth of penetration of electroseismic measurements is stated to be about 500 m depending upon ambient noise and water content. Lateral resolution is not stated in the literature but is expected to be on the order of the depth of exploration. No information was found regarding the vertical resolution of the method.
properties such as hydraulic permeability or fluid chemistry. This theoretical potential for hydrogeological applications, however, is so far confirmed only by a very limited number of reliable field studies. In addition, these studies usually show only one example of a successful detection of seismoelectric signals. As a consequence, the seismoelectric method is still far from being routinely used.
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
s in soils and rocks by seismic wave
Seismic wave
Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...
s. Although the method is not reported to detect groundwater flow
Groundwater flow
Groundwater flow is defined as the "...part of streamflow that has infiltrated the ground, has entered the phreatic zone, and has been discharged into a stream channel, via springs or seepage water". In hydrogeology it is measured by the Groundwater flow equation.- See also :*Subsurface...
, it does measure the hydraulic conductivity
Hydraulic conductivity
Hydraulic conductivity, symbolically represented as K, is a property of vascular plants, soil or rock, that describes the ease with which water can move through pore spaces or fractures. It depends on the intrinsic permeability of the material and on the degree of saturation...
, which is related to permeability and, therefore, to the potential for groundwater flow.
Operation
When a seismic wave encounters an interface, it creates a charge separation at the interface forming an electric dipole. This dipole radiates an electromagnetic wave that can be detected by antennae on the ground surface.As the seismic (P
P-wave
P-waves are a type of elastic wave, also called seismic waves, that can travel through gases , solids and liquids, including the Earth. P-waves are produced by earthquakes and recorded by seismographs...
or compression
Longitudinal wave
Longitudinal waves, as known as "l-waves", are waves that have the same direction of vibration as their direction of travel, which means that the movement of the medium is in the same direction as or the opposite direction to the motion of the wave. Mechanical longitudinal waves have been also...
) waves stress earth materials, four geophysical phenomenon occur:
- The resistivity of the earth materials is modulated by the seismic wave;
- Electrokinetic effects analogous to streaming potentials are created by the seismic wave;
- Piezoelectric effects are created by the seismic wave; and
- High-frequency, audio- and high-frequency radio frequency impulsive responses are generated in sulfide minerals (sometimes referred to as RPE).
The dominant application of the electroseismic method is to measure the electrokinetic effect or streaming potential (item 2, above). Electrokinetic effects are initiated by sound waves (typically P-waves) passing through a porous rock inducing relative motion of the rock matrix and fluid. Motion of the ionic fluid through the capillaries in the rock occurs with cations (or less commonly, anions) preferentially adhering to the capillary walls, so that applied pressure and resulting fluid flow relative to the rock matrix produces an electric dipole. In a non-homogeneous formation, the seismic wave generates an oscillating flow of fluid and a corresponding oscillating electrical and EM field. The resulting EM wave can be detected by electrode pairs placed on the ground surface.
Surface seismic sources and measurement electrode pairs are generally used to measure the electrokinetic effect. Borehole
Borehole
A borehole is the generalized term for any narrow shaft bored in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid or gases , as part of a geotechnical investigation, environmental site...
systems also have been recently developed. A hammer blow or small explosive (black powder) are typically used for the seismic source. Two, short (few meters) electrode pairs are typically located collinear and symmetric to the seismic source. Stacking of repeat seismic "shots" is often used to improve signal to noise.
Commercial systems
The only company found that produces a commercially available electroseismic prospecting system is GroundFlow Ltd. in the UK. This system has been mainly used for field demonstration purposes and research and development studies. Very few detailed case histories are available to web manual the performance of the system. The electroseismic system made by GroundFlow is called "GroundFlow 3000" and the main equipment components are:- A combined computer-receiver
- Antenna cables and electrodes
- Trigger cable.
The computer-receiver contains a preamplifier, analog-to-digital converter and power supply system to measure the voltages induced in the grounded dipoles.
The hammer-and-plate seismic source is used along with two pairs of electrodes arranged in a straight line with electrodes offset (from the center of the array) by 0.25 and 2.25 m. The seismic source is positioned in the center of the array. Larger electrode spacing can be used; however, they are typically centered in pairs around the shot point and are generally less than about 10 m in length. Timing of the measurement is achieved by a hammer trigger (or other mechanical trigger). Measurements are made for a period of 400 ms after triggering. The last 200 ms of the record is used as a sample of background noise
Background noise
In acoustics and specifically in acoustical engineering, background noise or ambient noise is any sound other than the sound being monitored. Background noise is a form of noise pollution or interference. Background noise is an important concept in setting noise regulations...
and is subtracted from the first 200 ms of signal in order to remove the first, third, and fifth harmonics of the background noise at the receivers. This noise is typically from power lines.
Data acquisition, processing, and interpretation
Measurements are recorded using the instrument and system setup described above. Data processing consists of stackingStacking
Stacking may refer to:* Stacking * Stacking , a 2011 game from Double Fine involving matryoshka dolls* A gang signal made with the hands* Sport stacking, played using plastic cups* A film directed and produced by Martin Rosen...
repetitive hammer (or explosive) shots and removal of power line and other noise as described above. This processing is conducted on the computer-receiver while in the field. Depending upon field conditions, up to 20 soundings can be conducted in a day. It is expected that measurement time will increase proportionally with stacking times, and that long stacking times would be needed in the vicinity of power lines.
The signal files are processed to give a sounding plot of permeability and porosity against one-way seismic travel time
Travel time
Travel time may refer to* Travel, movement of people between locations* Travel journal, record made by a voyager* Propagation speed, term in physics to measure things such as the speed of light or radio waves...
. Values for a simple seismic velocity model are required to allow time-to-depth conversion. The resulting logs against depth are displayed in the field.
Output from the electroseismic inversion is typically a plot showing hydraulic conductivity versus depth. This interpretation assumes a one-dimensional layered earth. In theory, the inversion also can derive fluid conductivity and fluid viscosity from the rise time of the signal. Where many soundings are measured in close vicinity, a pseudo 2-D cross-section can be generated.
The maximum depth of penetration of electroseismic measurements is stated to be about 500 m depending upon ambient noise and water content. Lateral resolution is not stated in the literature but is expected to be on the order of the depth of exploration. No information was found regarding the vertical resolution of the method.
Limitations
The electroseismic method is apparently susceptible to electrical noise from nearby higher voltage power lines. Typical electroseismic signals are at the microvolt level. The electroseismic signal is proportional to the pressure of the seismic wave. Thus, it would seem possible to increase the signal by using stronger seismic sources. This is not mentioned in the literature. Electroseismic soundings have a maximum exploration depth of about 500 m in the reported literature.Examples of successful field studies
The propagation of seismic waves in porous rocks is associated with a small transient deformation of rock matrix and pore space which can cause electromagnetic fields of observable amplitude if the pores are saturated. Seismoelectric field measurements are expected to help localize permeable layers in porous rocks and provide information about anelasticViscoelasticity
Viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just...
properties such as hydraulic permeability or fluid chemistry. This theoretical potential for hydrogeological applications, however, is so far confirmed only by a very limited number of reliable field studies. In addition, these studies usually show only one example of a successful detection of seismoelectric signals. As a consequence, the seismoelectric method is still far from being routinely used.