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Based on the property on which the method depends the geophysical techniques can be broadly classified as: 1. Gravity Prospecting 2. Magnetic Prospecting 3. Electrical and Electro-Magnetic Prospecting 4. Seismic Prospecting 5. Well Logging Techniques.
1. Gravity Prospecting Method:
The gravity method of geophysical prospecting is based on the fact that the gravitational attraction of sub-surface bodies directly up on their masses and inversely upon second power of the depth of their occurrence (i.e., their distance from the surface of the earth). This method utilizes the changes in the gravity field of the earth as measured in the surface of the earth.
If in a region sub-surface bodies whose density is different from surrounding rocks exist, then the gravity field on the surface deviates from the normal value (i.e., the field measured if the inhomogeneous would not have been present). From these deviations it is possible to locate the inhomogeneous body its parameters.
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Instruments used in gravity prospecting are:
1. Gravimeters,
2. Torsion balances, and
3. Gradientometer.
The pendulums measure the absolute value of the earth’s gravity field at any point, whereas the gravimeters measure its relative variation in the field from one place to another. Gradientometer are used for the measurement of the gradient of gravity and the curvature of the gravity equipotent surface.
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Gravity methods have limited use in the exploration of ground water, because the change in gravity field caused by the presence of ground water is very small.
Inspite of this, under favourable conditions the gravity methods are used for:
1. The determination of subsurface structures in the form of large ground water basins;
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2. Tracing tectonically disturbed zones;
3. Effects of ground water movement but not in all cases;
4. Detailed investigation of areas of artificial ground water recharge;
5. Mapping valley fills;
6. Mapping permeable zones in loose sediments; and
7. Mapping thin sediments lying over impermeable sub-stratum possible if the rock is crystalline.
2. Magnetic Prospecting Method:
The magnetic methods are based on the fact that the magnetic bodies present in the sub-surface contribute to the total magnetic field, which is measured on the surface of the earth. The contribution due to the magnetic body is directly proportional to the magnetic moment of the body and inversely proportional to the second power of the depth of its occurrence from the surface of the earth.
The controlling factor in magnetic is the magnetic susceptibility; in addition the natural magnetic remanence of the bodies also contributes to the magnetic field. The total magnetic field of the earth or one of its components (either vertical or horizontals components) is measured on the surface.
Bodies possessing magnetic moments different from those of the surrounding rocks give rise to the deviations in the measured field. From these magnetic anomalies it is possible to locate the anomalies object. Instruments used in magnetic prospecting are: Magnetometers are the instruments used in magnetic prospecting.
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The Schmidt and Torsion magnetometer measure the variation of the vertical or horizontal component of the earth’s magnetic field.
The flux-gate, proton procession and Rubidium-Vapour magnetometers measure the magnitude of the (total) magnetic field of the earth and latter find application for not only for surface exploration but also in air, on oceans and in boreholes.
The following are the ground water problems solved by magnetic methods:
1. Determination of sub-surface structure of large ground water basins,
2. Tracing technically disturbed zones, and
3. Effects of ground water movement.
3. Electrical and Electromagnetic Prospecting Method:
It is the only geophysical method, which is most commonly used for ground water prospecting and also solved other problems related to ground water prospecting. The electrical methods permit a variety of applications in mapping geophysical structures like contacts, faults, shear zones, basement relief, etc. The methods can also aid in the investigation of civil and military engineering.
Under the broad field of electrical methods we make use of the following methods:
1. Self-potential (SP) method.
2. Telluric method.
3. Magneto telluric method.
4. Equi-potential line method.
5. Potential-drop ratio (PDR) method.
6. Induced polarization (IP).
7. Electromagnetic method.
Simplest of all the geophysical methods is the spontaneous polarization (SP) method, which makes use of natural potentials existing inside the earth. Besides its use for exploration of minerals this method is also very helpful for the location of ground water discharge zones below sea, river lakes and also for finding the direction of ground water movement.
Suitabilities:
The resistivity methods are comparatively easy to apply and are inexpensive; they are suitable for a variety of purposes:
1. In their exploration of highly conductive supplied ores,
2. High resistivity minerals like quartz, etc.,
3. Mapping geological structures, and
4. In solving engineering and water supply problems.
Instruments:
Instruments generally used for SP and resistivity methods are simplest and unsophisticated.
Electromagnetic methods are undoubtedly better suited for location of conductive ore bodies and ground water investigations particularly in areas where the over-burden has high resistivity.
In the last few years, the IP method of prospecting has accounted for the largest activity in ground electrical methods.
The following are the ground water problems solved by electrical methods:
1. Determination of sub-surface structures of large ground water basins (dipole electrical sounding, vertical electrical sounding, frequency electromagnetic sounding and radio frequency sounding methods).
2. Determination of horizontal and vertical distribution of aquifers, the regional boundaries and correlation between aquifers, tracing tectonically disturbed zones, effects of ground water movement (dipole electrical sounding, vertical electrical sounding, frequency electromagnetic sounding and radio frequency sounding methods).
3. Tracing of salt, fresh water boundary in the aquifer in land or near seashore (electrical profiling, vertical electrical sounding, radio interference sounding methods).
4. Estimation of ground water salinity (vertical electrical sounding, frequency electromagnetic sounding, ratio interference sounding and induced polarization methods).
5. Detailed investigation of areas of artificial ground water recharges and also discharge zones below sea, rivers and lakes (electrical profiling, vertical electrical sounding, and ratio wave profiling, and induced polarization method).
Electrical resistivity method of ground water exploration and add a note on its merits and demerits:
In the electrical resistivity method, the electrical resistance determined by an electric current (I) to metal stakes (outer electrodes) driven into the ground and measuring the apparent potential difference (V) between two inner electrodes (non polarizing d.c. type) buried or driven into the land.
Gives an indication of the type and depth of the sub-surface material. Changing the space of electrodes changes the depth of penetration of the current and the apparent electrical resistivity ρa obtained at different depths by measuring the resistance R (=V/I), is plotted on a double log paper against the depth.
The depth at which current enters a formation of higher or lower resistivity is signalled by a change in the resistivity’s recorded at the ground surface. By proper interpretation of the resistivity data from the field curve so obtained and matching them with standard curves available (Mooney and Wetzel), master curves, it is possible to identify the water bearing formation and accordingly limit of the depth of well drilling.
There are mainly two common systems of electrode arrangement:
i. Wenner electrode arrangement, and
ii. Shlumberger electrode arrangement.
In the Wenner system, the electrodes are spaced at equal distances a Fig. 7.6 and the apparent resistivity ρa for a measured resistance R (= VII).
AB = Current Electrodes
MN = Potential Electrodes
a = Spacing of Electrodes
C = Centre of Electrode Spread
And the field curve is plotted on a semi log paper ρa versus a, being in Ohm-meters in logarithmic scale and a in meters in arithmetic scale.
In the Schlumberger system Fig. 7.7. The distance between the two inner potential electrodes (a) is kept constant for some time and the distance between the current electrodes (L) is varied. The apparent resistivity ρa for a measured resistance R (= VII).
And the field curve is plotted on a double log paper ρa versus L/2,
ρa being Ohm-meters and L/2 in metres
For example: L- 60, a – 10 metres configuration constant (k).
There are several advantages in using Schlumberger configuration over Wenner:
For example:
Different AB/2 separation it is not necessary to disturb potential electrode positions, which reduces the cost and the time of operation. Because in Schlumberger MN/2 is kept less than or equal to one-fifth AB/2 distance. The potential measured would be in the uniform part of the potential distribution, thus minimizing the possible errors of the media can be calculated by use of the following expression.
Vertical Electrical Resistivity Sounding:
Vertical Electrical Resistivity Sounding is used to study the vertical variation of the formation factors. Here potential probes are kept fixed at the centre of the line while the current electrodes are moved symmetrically outwards in steps. Since only two electrodes are moved, the field routine with the Schlumberger procedure is much more convenient than that with the Wenner array. Furthermore as potential electrodes are kept fixed, the effect of local shallow in homogeneities in their vicinity (due to soil, weathering, etc.) is constant for all observations.
A typical scheme of current electrode separation (2L) in the Schlumberger drilling procedure is to start with a probe separation of, say 2 mt, and, keeping this constant, take observations with 2L = 10, 20, 30 mt. If at any stage the voltage between the probes is deemed to be too small for accurate measurement, the probe separation is increased to, say 5 mt. and the observations are made with the next current electrode separation in the series.
If the basement is infinitely resistive than for larger AB/2 separations, the resistivity values increase asymptotically. This also indicates that any increase in AB/2. Separation thereafter would not change the current distribution and the depth of investigation is more than the depth to the basement.
The greatest success in the application of resistivity methods is to hydrogeology has been for demarcating salt-fresh-water boundaries. The method also had some success in locating alluvium-bed rock contacts in river valleys and in locating layers of gravel or sand below clay and salt.
Interpretation of VES Data:
After obtaining the field data, a proper interpretation is necessary with respect to sub-surface geology, structural geology, etc., interpretation generally consists of two parts i.e.:
i. Qualitative and
ii. Quantitative.
In quantitative interpretation, which aims at detection of the anomalous zones of a geologic section with its parameters accusatively estimated using the geophysical data and the interpretation techniques, the VES data may be interpreted analytically, or by using the technique of curve matching. Analytical method consists of computing resistivity values for different electrode separations and different earth conditions.
Curve Matching Techniques:
The field curves are matched with the readily available master curves to obtain the parameters. If perfect matching is found between the field curve and the master curve, the parameters of the latter are considered to be that of the former i.e., the field curve, procedure for the interpretation of three layered curves by using theoretical master curves available and the auxiliary point chart.
Interpretation of Three Layer Curves:
Interpretation of three-layer Vertical Electrical Sounding (VES) field curves can be achieved with the help of available two and three-layer master curves and the auxiliary point charts, this method is known as auxiliary point charts of interpretation.
(a) Direct Method (Using Two Layer and Three Layer Curves):
Plotting the field data on double logarithm graph sheet with a modulus of 62.5 mm, and matching the left-hand part of the curve by superposing the field curve on the set of two layer master curves. Keeping the axes parallel to the coordinates. The coordinates of the origin of the master curve, as read on the field curve, gives, h1 and P1 From the curve with which the match is obtained, and by interpolation if necessary, M2 = P1/P2 can be read; since P1 is known P2 can be calculated.
(b) Choose the right set of three layer master curves from the knowledge of P1, M2 – (P2/P1) already noted in step (a) and the most probable value of P3 (guessed at).
(c) Superposing the three layer field curve with the point (h1/P1) on the origin of three layer master curve set chosen in step (2a) and read the value of V2 (h2/h 1) from the curve with which the field curve matches to obtain V2, interpolation on the log scale may be necessary.
(d) If there is an exact match with a three layered curve, the value of P3 may also be read from the asymptotic value, thus, so far we have obtained, P1, h1, M2 (hence P2) V2 and P3 and the problem is then solved since h2 = V2 x h1.
Auxiliary Point Chart Method:
(a) Finding the value of P1, h1 and P2 from the two layer master curves, as explained in step (1).
(b) Matching the last part of the curve with a two layer master curve, putting a cross mark on the field curve, corresponding to the origin of the set of master curves. This gives P3 and he.
(c) Putting the points (h1 P1) on the origin of the auxiliary point charts and read the value of V2 corresponding to the cross mark. If the bottom layer is highly resistive basement, we can adopt the simplified procedure for interpretation as did in the previous section (2 layer match).
Transverse Resistance (T) and Longitudinal Conductance (S):
Consider a prism of unit cross-section with thickness h and resistivity (Fig. 7.8) then the resistance (T) normal to the face of the prism, and the Conductance (S) parallel to the face of the prism.
T = h/ρ….(1)
and S = h/ρ….(2)
Where from, we get h = √ST and ρ = T/S …….. (3)
Thus, each value of S and T determine a section with definite values of h and P by equation (3).
If the prism consists a parallel homogeneous and isotropic layer of resistivity’s of ρ1, ρ2, … , ρn and thickness h1, h2 …, hn (Fig. 7.8) and when the current is flowing normal to the base, the total transverse resistance of the prism.
And when the currents is flowing parallel to the base, the Longitudinal conductance.
Relationship of transvers resistance with transmissibility:
Equation 4 of the section above:
Where h and ρ are respectively thickness and resistivity of the formation. It is Known that with the increases in clay content, the resistively of a formation decreases and so also the permeability. Hence, the aquifer transmissibility TH = kh.
(Where K and h are permeability and thickness of the aquifer respectively) also decreases. Therefore, as a first approximation one can consider that T determinable from the interpretation of sounding, to be the filtration property (k) of the corresponding formation.
Based on the resistivities of the aquifer and also the ground water resistivities from the soundings done near boreholes one can establish the value of an aquifer, (ρ rock/ρ water) in a region. Such information can be helpful in predicting the quality of water in other areas of the same region. Since the ground water resistivity is closely related to the total saline content in it.
The readings given in Table 7.1 were obtained from NGRI Resistivity meter. Plot ρa versus AB/2 on log log paper (Double log paper).
Table:
Resistivity depth probe—Schlumberger Configuration (Example).
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National Institute of Astrophysics, Siddalaghatta road, Near Hoskote, Hoskote Taluk, Bangalore Rural District, Karnataka, India.
Resistivity surveys make use of the fact that the water increases the conductivity of rocks, and thereby decreasing their resistivity. Hence it can be established geologically, that the same rock formation exists for a certain depth, say 100 m, and by electrical testing it is found that the resistivity is decreasing below 20 m. Depth, then it can be easily concluded that water is present below 20 m depth.
In the figure the resistivity is decreasing below 20 m depth and up to 100 m depth. Whereas geology of the area confirming the existence of the same type of rock up to this 100 m depth; then it easily gives the inference that water, exists between 20 m and 100 m depth.
There are basically two types of instruments to conduct the electrical resistivity survey:
(a) N.G.R.I, resistivity meter:
A d. c. type meter manufactured by the National Geophysical Research Institute, Hyderabad (South India). In this instrument V and I are separately measured to obtain the resistance R (= VII). Generally battery packs with different voltages of 15, 30, 45 and 90 volts are employed.
(b) Terrameter:
An a.c. type of instrument manufactured by Atlas Cop co ABEM A.B, Sweden. The output is 6 watts at 100, 200 or 400 volts using low frequency (1-4 Hz) square waves. The terrameter directly gives the resistance R in Ohms. It is a good instrument for conducting rapid electrical surveys for locating sites for drilling bore wells.
Uses of electrical resistivity method:
Some of the Geophysical investigations that can be done by the electrical resistivity method for ground water studies is:
1. Correlating lithology and drawing geophysical sections.
2. Bedrock profile for sub-surface studies.
3. Fresh water-salt water interface by constant separation profiling.
4. Contact of geological formations.
5. Water quality in shallow aquifers and ground water pollution as in oil field brine pollution, pollution by irrigation waters and pollution by sea water intrusion, which cause change in conductivity.
Resistivity of consolidated and unconsolidated rocks ranges widely, being high for dense impervious rocks, medium for porous rocks containing water and low for clays and saline water as indicated below:
4. Seismic Prospecting Methods:
Seismic methods make use of the differences in elastic properties of rocks. These change with changes in lithology. Measurement of seismic wave velocities of rocks formations therefore provides a means to distinguishing different sub-surface lithological units.
In seismic methods waves are generated artificially in the ground. The seismic waves thus produced travel through the sub-surface layer of the earth. Suffer reflection or critical refraction and arrive at the sub-surface of the earth where they are detected by geophones.
From the time taken by the waves to travel through the sub-surface formations and from the seismic wave velocities of the media; it is possible to determine the depths to various elastic boundaries. Depending upon the types of waves recorded the investigation are classified as reflection and refraction methods.
The following ground water problems could be solved by this method:
1. Determination of sub-surface structure of large ground water basins,
2. Determination of horizontal and vertical distribution of aquifers,
3. The regional boundaries and correlation,
4. Tracing technically disturbed zones, and
5. Effects of ground water movement but not in all cases,
6. Mapping promising ground water in hard rock areas like:
(a) Saturated weathered rock,
(b) Fractures and fissures, and
(c) Mapping thick sediments lying over impermeable substratum.
5. Well Logging Techniques:
Geophysical methods of prospecting can be divided into three types.
They are:
i. Surface,
ii. Sub-surface, and
iii. Air-borne investigations.
Boreholes are the only possible approach to the hidden treasure before the actual exploitation sets in after the surface geophysical investigation to have the accurate parameters of the concealed body it is essential to incorporate borehole geophysical investigations.
Here physical properties of various formations are measured along the borehole in contrast to the physical field measurement on the surface. For the ground water investigations resistively logging, self-potential logging, radioactivity logging and acoustic logging are most commonly used.
In well logging physical properties are measured by lowering the compact instruments into the borehole. The instruments are mostly electronic and are accommodated in casing called ‘SONDE’. Using well log data lithological parameters like porosity, permeability of the formations, water saturation, salinity of the formation, etc. could be interpreted.