Introduction
Groundwater exploration is the investigation of underground formations to understand the hydrologic cycle, know the groundwater quality, and identify the nature, number and type of aquifers. … Surface geophysical method is one of the groundwater investigation methods.
The use of geophysics for both groundwater resource mapping and for water quality evaluations has increased over the years in large part due to the rapid advances in microprocessors and associated numerical modeling solutions.
Electrical Resistivity Method application for ground water exploration
Wenner Array
The Wenner array consists of four collinear, equally spaced electrodes. The outer two electrodes are typically the current (source) electrodes and the inner two electrodes are the potential (receiver) electrodes. The array spacing expands about the array midpoint while maintaining an equivalent spacing between each electrode. The advantages of the Wenner array are that the apparent resistivity is easily calculated in the field and the instrument sensitivity is not as crucial as with other array geometries. Relatively small current magnitudes are needed to produce measurable potential differences. The disadvantages are that for each sounding, all of the electrodes have to be moved to a new position. In order to image deep into the earth, it is necessary to use longer current cables; handling the cables and electrodes between each measurement can be cumbersome, especially in difficult terrain. The Wenner array is also very sensitive to near surface in homogeneities which may skew deeper electrical responses. The Wenner array is a labor-intensive survey because of the cable lengths required and the movement of the electrodes during the survey. Substantial lengths of cable energized with current at high voltage present a safety hazard.
Apparent Resistivity is calculated by the formula
ρ_a=2πa (∆V/I)
Where,
〖 ρ〗_a=Apparant Resistivity
a = Electrode spacin
∆V=Potential difference
I= Current
Apparent Resistivity is calculated by the formula
〖 
Where
〖 ρ〗_a=Apparant Resistivity
AB = Current Electrode spacing
MN = Potential Electrode spacing
∆V=Potential difference
I = Current
Schlumberger Array
The Schlumberger array consists of four collinear electrodes. The outer two electrodes are current (source) electrodes and the inner two electrodes are the potential (receiver) electrodes. The potential electrodes are installed at the center of the electrode array with a small separation, typically less than one fifth of the spacing between the current electrodes. The current electrodes are increased to a greater separation during the survey while the potential electrodes remain in the same position until the observed voltage becomes too small to measure. Typically, expanding the current electrodes occurs roughly six times per decade. The advantages of the Schlumberger array are that fewer electrodes need to be moved for each sounding and the cable length for the potential electrodes is shorter. Schlumberger soundings generally have better resolution, greater probing depth, and less time-consuming field deployment than the Wenner array. The disadvantages are that long current electrode cables are required, the recording instrument needs to be very sensitive, and the array may be difficult or confusing to coordinate amongst the field crew. Substantial lengths of cable energized with current at high voltage present a safety hazard. The Schlumberger array is a labor-intensive survey because of the cable lengths required and the movement of the electrodes during the survey.
Objectives of the Survey
1- To investigate the aquifer characteristics, thickness, layer wise soil type, ground water quality (by conductivity value) and water table depth using Vertical Electrical Sounding (VES) resistivity survey technique.
2- To determine the geoelectric and hydrogeologic characteristic of the aquifer present in the study area.
3- Detecting and mapping water saturated layer in various type of shallow as well as deep aquifer.
Literature Review
Adequate groundwater exploration can be considered as one of the primary components of the groundwater management practices. The assessment of the aquifer potential both on the basis of quality and quantity is of utmost importance. Resistivity survey can be used as an initial step for assessing the aquifer potential to promote the sustainable groundwater usage.
Electrical Method
Electrical and electromagnetic techniques have been extensively used in groundwater geophysical investigations because of the correlation that often exist between electrical properties, geologic formations and their fluid content. Most electrical techniques induce an electrical current in the ground by directly coupling with the ground. The resulting electrical potential is then used to measure the variation in ground conductivity, or its inverse, resistivity. Different materials, and the fluids within them, will show different abilities to conduct an electric current. In general, sequences with high clay content show higher conductivity as do saturated sequences and especially sequences where saline (or sometimes other contamination) fluids are present. Common field practice for electrical surveying relies on directly placing an electrical current into the ground (direct current electrical resistivity surveying) and measuring the response (the electrical potential drop) to that current over a set distance. The typical results of electrical surveys are electrical profiles or geo-electric images and geo-electric depth soundings. The profile or transect method for mapping lateral resistivity changes is now largely replaced by electromagnetic techniques as the electrical technique is slow (when probes have to be placed directly into the ground) and thus is not cost effective relative to the electromagnetic techniques. Electrical methods are still widely used however for conducting soundings and electrical cross-sections. Electrical techniques can be divided into a number of types based on the configuration of the electrodes that are used to input the electrical currents into the ground and the nature of the electrical signature. Only the basis of direct-current electrical resistivity techniques will be discussed here without a review of the different electrode configurations.
Direct Current Resistivity Method
The direct-current (DC) electrical resistivity method for conducting a vertical electrical sounding (VES) has proved very popular with groundwater studies due to the simplicity of the technique and the ruggedness of the instrumentation. Before the vertical electrical sounding was used a failure rate of over 82% was recorded for boreholes. With the geophysics and a combination of geological and photo geological inspection this was dramatically reduced to less than 20% failure. Van Over meeren showed the use of electrical measurements in mapping boundary conditions in an aquifer system in Yemen. Groundwater is defined as the subsurface water that occurs beneath the water table and flows through voids in the soils and permeable geologic formations that are fully saturated.
Methodology
1. A leveled terrain in the VES station was located and the Schlumberger array was used for the present work.
2. The four electrodes were positioned symmetrically along a straight line i.e. the current electrodes (C1 and C2) on the outside and the potential electrodes (P1 and P2) which are also the inner electrodes place in between C1 and C2.
3. The CRM-500 AUTO-C was set and further adjustments were made such as: setting the number of circle to 1, reading of resistance values in ohms and setting current on auto-C so that the current will be injected as per ground requirement.
4. To change the depth range of penetration, the current electrodes were displaced outwards while the potential electrodes remained fixed.
5. When the ratio of the distance between the current electrodes to that of the potential electrodes became too large, the potential electrodes were displaced outwards otherwise the potential difference becomes too small to measure with sufficient accuracy.
6. The configuration used is given in the VES table.
7. The maximum current electrode spacing (AB/2) was 300m and the CRM 500 AUTO – C was used to measured and record the resistance of the subsurface.
8. The values of the resistance obtained in the field were multiplied with their respective Geometric factor (k) which gave the required apparent resistivity results.
9. The required data was plotted on a log-log graph sheet and the resultant curve was quantitatively interpreted. Also IPI2WIN software is used for interpreting the data acquired from field.
10. Specific yield for the given aquifer is defined as the ratio of volume of ground water that can be extracted to the total volume of water in an aquifer. Specific yield depends upon grain size, shape, distribution of pores and compaction of the formation. Based on the certain assumption about the pumping data including drain water volume, pressure, time duration etc. The specific yield of reservoir has been estimated. However, the precise specific yield can be determined only after having the real & actual pumping data of the wells.
11. The GPS was used in locating the longitude and latitude of each VES station.
12. These steps were repeated at subsequent VES.
13. The collected sounding data is interpreted by the team of experienced Geophysicist & geologist.
Material and Methodology
Materials
1. CRM-500 AUTO – C
2. Coiled wire
3. Tape
4. Hammer
5. GPS
6. Current and potential electrodes
Calculation
The data were calculated using the equation below. For calculating Geometric factor (k) for schlumberger array, the following formulae was used.
AB is the distance (in meter) between the two current electrodes. MN is the distance (in meter) between two potential electrodes. Themidpoint is a constant point for the current electrode and the potential electrodes. Π is a constant = 3.142. Apparent resistivity is calculated using ohms law
Apparent resistivity (ρ_a) = Resistance (R) × Geometric Factor (K)
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