There is no doubt that water is a unique and invaluable natural resource on our planet Earth. And the fact that we need it in our daily activities. Also by nature plants and other living things depend on it for their existence. So water is LIFE!
There are many sources of surface water ranging from oceans , Lakes, seas, to polar ice . However due to periodic global climatic changes and other factors which cause depletion of safe and clean surface water. Many people mostly living in rural areas of the developing world, for example some African and Asian countries, fail to access the portable water of adequate quality and quantity.
However groundwater is the next most significant source behind oceans and polar ice. It is approximately 50 to 70 times more plentiful than surface water (Fetter 1994). And the fact that most groundwater is not frequently experiencing surface and bacteriological pollution, so it is an alternative way for gaining clean and safe water.
It is a key for those who call themselves hydrogeologists to understand the characteristics, occurrence, interaction and movement of groundwater in the subsurface.
Unfortunately the geophysical investigation has found a proven way for searching groundwater resources through groundwater exploration projects.
What is groundwater exploration?
Groundwater exploration
Groundwater exploration is the systematic process of searching for underground productive aquifers (groundwater resources) which can yield an adequate and productive water well supply. This is a technical definition, however in layman terms, we can say that exploration is the process of searching or finding something. So groundwater exploration is the process of finding groundwater.
Basically in most cases groundwater exploration must involve two (2) Principal techniques which are hydrogeology and geophysics. So by default hydrogeologists or anyone who wants to carry out groundwater investigation must have a geophysical background as a supportive tool for his / her groundwater search investigation.
However this post is tailored to give you a brief description regarding suitable geophysical methods that are mostly applied in groundwater exploration. And we have to be specific that this post is going to tell you about surface geophysical methods and not borehole geophysical methods.
Groundwater is the water that is found within the subsurface that occupies the pore spaces of rocks. However specifically when hydrogeologists talk about groundwater by default in most cases they are dealing with sedimentary rocks. This is because sedimentary rocks have adequate primary and secondary porosity and permeability to store and transmit groundwater respectively.
Why is groundwater exploration important?
Importance of groundwater exploration.
Groundwater exploration is important because,
1. It explores new groundwater resources. Through the use of hydrogeological and geophysical investigation new productive aquifers can be found and made available into respective regional to district hydrogeological records..
2. It increases water supply and availability to the community end - users. Since after exploring new productive aquifers , the successful project can be developed into productive water well supply. Then community water users get access to more quality and quantity water.
3. It assists in planning the future exploitation of groundwater resources. Since groundwater investigation can supply related groundwater data such as availability and distribution of productive aquifers in a given location, so as to support the future planning of consuming these groundwater resources.
4. Groundwater is plentiful and less susceptible to pollution. It is approximately 50 to 70 times more plentiful than surface water (Fetter 1994). Also groundwater is not frequently susceptible to surface and bacteriological pollution. So it provides another alternative way for the availability of clean water.
What are geophysical Techniques applied in groundwater exploration?
Geophysical methods for groundwater explorations
Geophysical methods for groundwater exploration are Electrical Resistivity, Seismic Refraction, Electromagnetic (EM), VLF and TEM), and Magnetic Method. Because there are already detailed posts regarding the basic theory of each geophysical method described in this discussion. We will go through basic theory, advantages and limitations of each geophysical method in groundwater exploration in a simplified view, while directing you to the other relevant posts at the end or within each method.
1. Electrical Resistivity
Electrical resistivity investigations for groundwater are based on the principle of applying electric current to the earth through two electrodes and measuring the potential difference between two or more other electrodes. The distance between the electrodes and the measured potential difference are the data used to make interpretations of subsurface conditions.
The basic procedure in resistivity surveying is to measure the potential drop on the ground surface associated with a known current flow into the earth and then calculate the apparent resistivity from the equation.
Resistivity theory states that the lines of current flow will be deflected toward a good conductor (lower resistance) which is similar to ground-water flow where the flow lines are more dense in the aquifer of higher permeability (which is equivalent to geometric Factor).
The potential difference (∆v) between the potential electrodes is proportional to the current density of the small near surface element of material.
∆V = ITr
where ∆V is voltage drop , Tr is true resistivity, I is current density- (the current passing through a unit cross-sectional area)
Advantages of Electrical Resistivity Method
One of the advantages of the electrical Resistivity Method is its popularity and having a great historical track in searching for groundwater resources. Also simplicity and cost effective this method compared to other geophysical methods utilized in groundwater exploration projects.
Limitations of Electrical Resistivity
1. Overlapping in the resistivity of various materials.
Since formulation of Electrical Resistivity related equations assumes the homogeneous nature of the Earth, However in real field resistivity data acquisition involves the heterogeneous nature of the Earth section. And the fact that conductivity of geological materials varies widely with many factors, for example moisture content and dissolved solids content may cause changes of the ground-water condition investigation. Another case is that resistivity of unsaturated material overlying the aquifer, thickness and depth to the saturated aquifer may prevent aquifer conditions investigation. So the practice of trying to invert ground conductivity directly from measured field resistivity data without having other reference ground trust may lead into serious interruption.
2. One dimension (1D) subsurface investigation
Here I will talk about one of the most utilized techniques in groundwater exploration known as Vertical Electrical Sounding (VES). This method gained popularity and till today is used as standard in some countries for groundwater exploration and gives good results! However VES gives only one dimension (1D) such that one point variation of the subsurface resistivity with depth. So if you have a large working area using this technique you will be required to carry many VESes so it will be time consuming and Labor intensive. The more convenient and effective way is to use 2D or tomographic 3D Resistivity techniques such as ERT.
3. Need for Ground Contact
As the rule of thumb in order to operate the Electrical resistivity method the current and potential electrodes must be plugged directly into the ground. Think if the terrain is covered by igneous layers or any other hard materials, then plugging the electrodes into such an environment would be so challenging! Another challenge related to ground contact is that we can't implement it from the Air, such as using Airborne simply to say that No Airborne Resistivity Surveying!!?
4. Extraneous electrical currents
Extraneous electrical currents simply means of external origin either natural or artificial which may cause errors in apparent resistivity readings. As we know that there are natural earth currents are such as telluric currents. Also in some cases when ore bodies found below the aquifer oxidation may induce currents that may interfere with the reading of the resistivity related to groundwater within such an aquifer. Also Artificial underground of electrical installations such as electric railroads may interfere with resistivity reading.
It is a good approach to use a good Resistivity meter that has a well designed system for attenuation of these currents. You can check the post regarding Common noise in resistivity survey
2. Seismic Refraction
Seismic refraction techniques are designed to obtain data on the near surface (about 30 meters) although it can capture data at depths in excess of 200 meters if more powerful seismic sources are utilized.
Seismic refraction provides data on the refraction of seismic waves at the interface between subsurface layers and on their travel time within the layers. Through seismic refraction data it is possible to estimate the thickness and depth of geologic layers (including the water table) and to assess their properties. Also, changes in the lateral facies of aquifer material can sometimes be mapped with this method (Sendlein and Yazicigil, 1981). See basic principle of seismic refraction.
Despite the recent advances in instrumentation and the development of new field techniques for shallow, high-resolution seismic reflection techniques that overcome most of the problems. However, it can still be assumed that seismic refraction should be the method of choice, when it comes to shallow groundwater exploration (hydrogeological investigations).
Advantage of using Seismic refraction in groundwater exploration
Although seismic refraction has generally lower resolution than seismic reflection, it generally has been the preferred seismic method in shallow hydrogeological investigations for a number of reasons (Zohdy et al., 1974):
The First Reason is that Refraction methods generally produce reasonable results in areas of thick alluvial or glacial fill and where large velocity contrasts exist, such as buried bedrock valleys.
Another reason is that Personnel and equipment requirements for seismic refraction are generally simpler and less expensive compared to seismic reflection surveys.
Also interpretation of Seismic refraction data is less ambiguous compared to the electrical Resistivity method. Interpretation techniques involve the use of time - distance plots matching
Note: For more details on time - distance plots, checkout seismic refraction (t - x) plots guide.
Limitations of Seismic Refraction
1. Velocity increase with depth
An important assumption for seismic refraction to be valid is that the velocities of the corresponding layers must increase with the depth of burial. Such that the layer buried at 20 feets depth should have (transmit) low seismic velocity than the layer buried at depth of 30 feets. This means that the seismic refraction method will detect only horizons which increase in velocity with depth.
However in practice this statement is not generally true. The seismic velocity is always governed by the elastic modulus of the material medium which in turn is related with density of the material medium through which they propagate.
It is generally true that the more dense the material medium, then the higher velocity than less dense material medium. So there is a rise in the Masking Problem. For Example a dense rock layer near the surface will mask weathered rock (low velocity) underneath. Another example is that Low velocity sands cannot be detected if they lie beneath denser high velocity soil materials.
2. Blind zoning
Another Masking problem regarding seismic refraction method is Blind zones, sometimes layers may obey assumption of velocity increases with depth (see figure 1 below, V1< V2 < V3 <V4) such that velocity through blind layers (also known as hidden Layers) are lower compared to those below them but unfortunately are not detected because they are relatively thin. See figure 1 below despite layer 3 having higher velocity than layer 2 such that V2 < V3, however it (layer 3) will not show in the time distance plot because it is thinner compared to other layers (1, 2, and 4) so it is masked. Sander (1978) examines the significance of blind zones in ground-water exploration.
Figure 1 : Masked Blind zones
3. Electromagnetic Method
Electromagnetic method gained popularity and have proven success record in groundwater exploration projects. Inost cases EM is used for reconnaissance survey to narrow down the survey area into potential areas susceptible to have groundwater for further follow up using other geophysical methods such as Electrical Resistivity or borehole geophysical logging and
1. Transient Electromagnetic Method (TEM)
TEM is used to measure conductivity of the subsurface based on the principle that the time varying magnetic Field generated by the transmitter coil .This field propagate into the subsurface, when a conductor (layers with potential water) is present, this Field induces a small current which in tum generates a secondary magnetic field. Secondary magnetic Field is then detected by a receiver coil placed at a short distance away from the transmitter coil. You can read the detail regarding basic principle of Electromagnetic (EM) Method
Note: However in most cases Frequency Domain EM (FEM) method is applied to investigate the shallow conductivity condition using ground conductivity meters such as Horizontal and Vertical coils. Ground conductivity meters vary the depth of penetration by changing the coil spacings. See figure 2 below.
Figure 2: FEM profiling using horizontal and vertical coils.
2. Very Low Frequency (VLF) method
Very Low Frequency (VLF) measures the ratio of electrical to magnetic fields generated by military radio communication transmitters (around 15 to 25 kHz). It is named as Very Low Frequency since radio waves are of a very low frequency, since they are often of higher than those used in EM induction methods.
Advantages of Electromagnetic (EM) Method
1. Rapid data acquisition over large areas and thus is ideally used for Regional and reconnaissance studies that in turn allow rapid assessment of bulk resistivity of the subsurface of the large area. However this is for the case of the Airborne Electromagnetic (AEM) Method.
2. It is a Non contact method. This means that TEM does not need direct contact of its instruments with the ground during data acquisition hence it can be used in groundwater mapping in remote areas. However, the VLF method needs ground contact during measurements like the DC Resistivity Method except that it uses a remote transmitter and utilizes only two potential electrodes.
3. Depth of Penetration. Transient EM methods have great depth of Penetration since the secondary Magnetic Field can be measured when the Primary Field is switched off. Also depth of Penetration is not dependent on transmitter receiver coil separation spacing as in case of Resistivity sounding but the frequency of applied Primary Field. This means deep depth can be achieved while using a constant coil spacing.
4. Qualitative interpretation, through EM and VLF profiling provides means for mapping of subsurface vertical fractures in fresh zones associated with groundwater movement, see figure 2 above.
5. Contaminant mapping , VLF, is an excellent method for investigating contaminated sites; as a result, it is the second most commonly used electromagnetic method for such applications after Electromagnetic Induction (EMI).
Note: An advantage of VLF measurements over EM and DC resistivity methods is that VLF uses a remote transmitter.
Limitations of Electromagnetic (EM) Method
One of the limitations regarding the VLF EM method is that it experienced a Static Effect Problems, since it utilizes only two potential electrodes that should be in contact with the ground surface. However this problem can be minimized through high spatial density.
Another limitation regarding the VLF EM method is that measurements must be adjusted to account for differences in surface elevation before readings in sloping terrain can be compared.
4. Magnetometry
Magnetometry method involves measuring the intensity of Earth's magnetic Fields.
Advantage of Magnetic Method in groundwater exploration
1. It is cheap and relatively simple method
Ground magnetic surveying is simple to carry out and involves less logistics compared to seismic reflection. Since it only needs to have a simple and precise Magnetometer device either proton precession or fluxgate magnetometer. However Aeromagnetic surveys can be challenging to conduct and expensive since they involve complex data acquisition planning and interpretation.
2. Reconnaissance survey. The main use of magnetic methods in groundwater exploration is to narrow down the site into pinpointed areas for further follow up by using other geophysical methods such as electrical Resistivity surveys.
3. It is more useful in basaltic volcanics environment
Magnetometry technique is successfully applied in basement areas that contain igneous rocks having larger proportions of magnetic minerals than in most sedimentary formations. So this made magnetic methods to be used in mapping igneous structures such as dykes that may provide the pathways for groundwater movements.
However there is another main use of magnetic methods in ground-water contamination studies that is to locate buried metal drums that may be a source of contamination. Where drums are buried in shallow trenches, trench boundaries also can be located with magnetometer surveys (Gilkeson et al., 1986).
Limitations of Magnetic Method.
Ambiguity Nature.
Like any other potential Field technique such as gravity Method, also magnetic methods have been proved to be faced with ambiguity especially when it comes to quantitative interpretation.
Ambiguity means magnetic data always gives possible solutions regarding the nature of the subsurface geology that produced it. This means that it is difficult to interpret magnetic data quantitatively without relying on other reference data. So also it is difficult to calculate the depth of buried objects with a magnetometry method without relied on more sophisticated analytical and numerical methods.
There are many basic and specialized geophysical techniques that are utilized in groundwater exploration including contamination studies, however these 4 methods, Electrical Resistivity, Seismic Refraction, Electromagnetic (VLF and TEM), Magnetometry methods which discussed in this guide already have shown a proven record and highly recommended regarding this issue.
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