ADVERTISEMENTS:

Geophysics, in the past few years, has reached a place of vital importance to the scientific development and protection of the world’s precious ground water supply. Geophysical investigations of the buried strata can be made either from the land surface or in a drilled hole in the formation.

The surface methods include: 1. Electrical Resistivity Method 2. Seismic Refraction and Reflection 3. Other Survey Methods.

**1. Electrical Resistivity Method****: **

In the electrical resistivity method, the electrical resistance determined by applying 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-polarising d.c. type, i.e., porous pots filled with CuSO_{4} solution, and metal stakes in a.c. type) buried or driven into the ground; Fig. 8.1, gives an indication of the type and depth of the subsurface material.

ADVERTISEMENTS:

Changing the spacing of electrodes changes the depth of penetration of the current and the apparent electrical resistivity ρ_{a}, obtained at different depths by measuring the resistance R(= V/I), is plotted on a semi-log or log-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 resistivities recorded at the ground surface. By proper interpretation of the resistivity data from the field curves so obtained and matching them with standard curves available (Mooney and Wetzel, master curves), it is possible to identify the water bearing formations and accordingly limit the depth of well drilling.

There are mainly two common systems of electrode arrangement. In the Wenner system, the electrodes are spaced at equal distances, a, Fig. 8.i (.a), and the apparent resistivity ρ_{a} for a measured resistance R(= V/I) is given by-

ρ_{a} = 2πaR …(8.1)

And the field curve is plotted on a semi-log paper ‘ρ_{0} versus a’, ρ_{a} being in ohm-metres in logarithmic scale and α in metres in arithmetic scale. In Schlumberger system, Fig. 8.1(6) the distance between the two inner potential electrodes ‘b’ is kept constant for some time and the distance between the current electrodes (L) is varied. The apparent resistivity ρ_{a} for a measured resistance R (= V/I) is given by-

And the field curves is plotted on a log-log paper ρ_{a} versus L/2, p_{a} being in ohm-metres and L/2 in metres.

**There are basically two types of instruments to conduct the electrical resistivity survey: **

(i) NGRI 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(= V/I). Generally battery packs with different voltages of 15, 30, 45 and 90 volts are employed.

ADVERTISEMENTS:

(ii) Terrameter, an a.c. type of instrument manufactured by Atlas Copco ABEM AB, 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 resistivity surveys for locating sites for drilling borewells.

**Depth Sounding and Traversing:**

Two methods of investigation are generally employed in the electrical resistivity method of traversing.

**(i) Resistivity Depth Probing or Sounding to Detect Vertical Changes: **

Here the centre of the electrode spread remains fixed and the spacing between the electrodes is progressively increased until the maximum required depth is reached, Fig. 8.2.

**(ii) Traversing or Profiling Method:**

Here the electrode separation is kept constant for two or three values (say a = 10 m, 15 m, or 20 m) and the centre of the electrode spread is moved from one station to another station (grid points) to have the same constant electrode separations.

Traversing or profiling is used to detect subsurface changes in horizontal direction or the lateral spread. Profiling can be carried out along a series of parallel lines and a resistivity contour map of the area showing iso-resistivity lines can be prepared. This will indicate areas of high resistivity and will be useful in identifying aquifer formations, Fig. 17.6.

**Use of Electrical Resistivity Method: **

**Some of the geophysical investigations that can be done by the electrical resistivity method for ground water studies are: **

(i) Correlating lithology and drawing geophysical sections.

(ii) Bed rock profile for subsurface studies.

(iii) Fresh water-salt water interface by constant separation profiling.

(iv) Contact of geological formations.

(v) 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 electrical conductivity.

Resistivity data for the rock formations in Karnataka are given in Table 8.1.

**Example 1:**

The readings given in Table 8.2 were obtained from an NGRI resistivity meter. Plot p_{a} versus L/2 on log-log paper (Schlumberger field curve) and interpret the results.

**Solution:**

‘ρ_{a} versus AB/2′ is plotted on a log-log paper, Fig. 8.2. A 15 cm well drilled at the site to a depth of 30 m gave a yield of 530 1pm at a depth of 21 m.

**Example 2: **

The following readings (Table 8.3) were obtained during a terrameter survey. Plot p_{a} versus a on a semi-log paper and interpret the results.

ρ_{a} versus a is plotted on a semi-log paper and the Wenner field curve is shown in Fig. 8.3.

**Master Curves for Layered Media: **

Resistivity data may be interpreted from master curves for a small number of earth layers assuming them as horizontal of uniform thickness and resistivity. They are prepared for particular electrodes configuration, like Wenner, Schlumberger, various thickness and resistivity ratios being assumed for the individual layers.

**Resistivity curves for 3-layers are generally divided into four type as: **

ρ_{1} > ρ_{2} < ρ_{3} – high-low-high – H-type

ρ_{1} < ρ_{2} < ρ_{3} – low-low-high – A-type

ρ_{1} < ρ_{2} > ρ_{3} – low-high-low – K-type

ρ_{1} > p_{2} > p_{3} – high-low-low – Q-type

Such ideal conditions like horizontal beds of uniform thickness and that the lowest bed extends in depth to infinity may not exist in the earth, but they are of help for comparison and interpretation of field curves obtained.

**Two-Layer Case: **

For a layer of thickness ‘h’ overlaying an infinitely thick homogeneous substratum of resistivity ρ_{a}, a family of curves is given by Tagg, Fig. 8.4,

ρ_{1}/ρ_{a }vs. h/a for ρ_{2} > ρ_{1}, k = positive

ρ_{a}/ρ_{1 }vs. h/a for ρ_{2} > ρ_{1}, k = negative

Where ρ_{1} = ρ_{a} as a → 0 i.e., at small electrode spacings

ρ_{2} = resistivities for various electrode spacings by Wenner configurations resistivity coefficient k = ρ_{2} – ρ_{1}/ρ_{2} + ρ_{1} …(8.3)

For particular value of a and ρ_{1}/ρ_{a}, the values of h/a are read from the ‘master curves’ for different values of k. Multiplying the h/a values by the corresponding a, h values are obtained. These are plotted as ‘k vs. h’ (see Fig. 8.6 in Example 8.3).

If the curves for different electrode spacings ‘a’ intersect near a point, it can be assumed as a simple two-layer case, and the coordinates h and k of this point can be read. From this k, the resistivity of the substratum can be obtained from-

ρ_{2} = ρ_{1} (1 + k/1 – k) …(8.3(a))

And h = thickness of the surface layer.

**The limitations of this method are: **

(i) The value of ρ_{1} obtained at small electrode spacings (a → 0) may not truly represent the resistivity of the top layer unless it is homogeneous and isotropic.

(ii) It involves numerous steps and is time consuming.

It is now customary to plot the master curves and the field curve on a log-log paper and for the Wenner or Schlumberger configuration. The curve of best fit is one which is parallel to the relevant master curve.

Master curves for three-and four-layer configurations have been published by Moony and Wetzel (1956), by Orellana and Mooney (1966), and also by European Association of Exploration Geophysicists (1963) which allow greater flexibility in the choice of resistivity patterns.

New methods of plotting the field resistivity data by ‘inverse slope’ and ‘direct slope’ techniques have been developed for determination of absolute resistivity and thickness of layers.

**Example 3:**

**Results of resistivity depth sounding at Adirampatnam (Thanjavur Dist. T.N.) are given below: **

**Interpret the data:**

(a) By cumulative resistivity plotting.

(b) By using Tagg’s master curves.

**Solution: **

The field data processed as follows to obtain Σρ_{a} for plotting the cumulative resistivity curve (ρ_{a} vs. a), Fig. 8.5, and ρ_{1}/ρ_{a} to obtain the field curves ‘k vs. h’ for different values of k using Tagg’s master curves, Fig. 8.6.

**Data for Drawing ‘k Vs. h’ Curves, for (b): **

**a = 10 m, ρ _{1}/ρ_{a} = 0.55; for this value, from Tagg’s master curves, Fig. 8.4 for different values of k, h/a values are read and multiplied by ‘a’ (= 10) to obtain h values:**

**Interpretation: **

(a) The cumulative resistivity curve is drawn, Fig. 8.5.

Tangents are drawn to the curve and the values of ‘a’ at which the slope of the curve changes, give the depths to the top of each layer.

Thus,

I layer, a = 3 m, thickness = 3 m (top soil)

II layer, a = 10 m, thickness = 10 – 3 = 7 m

III Layer, a = 23 m, thickness = 23 – 10 = 13 m

IV layer, a = 23 m, thickness = infinite.

It is a four-layer case.

(b) For a = 10, 20, 30 m, ‘k vs h’ curves intersect at nearly P, Fig. 8.6. The coordinates of Pareh = 4.2 m and k = 0.49; from Eq. (8.3a),

ρ_{2} = ρ_{1} (1 + k/1 – k) – 1 × (1 + 0.49/ 1 – 0.49) ≈ 3 m

Thickness of surface layer h = 4.2 m

Resistivity of the substratum ρ_{2} = 3 Ωm

It is assumed for two layers the top layer being of thickness 4.2 m, and the substratum extending to infinity below this layer.

**2. Seismic Refraction Method: **

Seismic reflection methods provide information on the deep seated strata (750 m) while the seismic refraction methods cover only a few hundred metres below the ground surface. Hence, the seismic refraction method is used in ground water investigation. The elastic waves caused by the detonation of explosives near the ground surface or a sledge hammer striking a metal plate on the ground, travel downwards into the various rock layers and are refracted back to the surface from the junctions between different rock layers.

The waves are picked up at various points on the ground surface by a geophone, Fig. 8.7, and recorded. This record shows when the energy commenced and when it was picked up at the surface. By knowing the arrival times of different waves at different distances from the energy source, the velocity of propagation of the wave through each rock layer can be calculated. The velocities are characteristic of particular rocks in particular conditions, i.e., dry, jointed, saturated with water, weathered, etc.

The refracted waves arrive at the surface only on the condition that the velocity of the propagation in the underlying layer is higher than that in the overlying area. Each layer through which the refracted wave travels horizontally must have a thickness that is great enough to permit transmission of the wave. The deeper a horizon is buried, the thicker it must be to properly refract the shock wave.

A time-travel curve (time versus distance from source to geophone) is drawn, Fig. 8.7 and by knowing the distance X_{1} to the first point on the curve where a change in slope is indicated, the depth to the rock layer can be computed from the equation-

Where V_{1} and V_{2} are the velocities of propagation through the earth and the rock layer respectively. Using the intercept time t_{1}, the depth Z_{1} is given by the equation-

The critical angel i_{c} is given by-

sin i_{c} = V_{1}/V_{2}

For angles of incidence greater than critical, there are no refractions into the deeper layers but the waves are totally reflected. Travel time is usually measured in milliseconds. The disturbances, commonly amplified, are recorded photographically on a moving film.

By moving the source of the shock wave along a line, a profile of the underlying bed rock surface (or other layer, such as the water table) can be obtained.

**The refraction method is faster and often finds application in:**

(i) Locating the ground water table.

(ii) Determining depth to bed rock or impermeable layer and configuration (volume of material).

(iii) Locating a buried stream channel (cut into bed rock).

(iv) Locating faults that could act as ground water barriers.

Generally water table delineation is confined to loose alluvium. In the Deccan, the alternation of soft and hard layers, however, limits the method to obtaining depths and velocities down to the first high velocity layer.

The velocity of propagation varies from as low as 120 m/sec in dry top soil to more than 6000 m/sec in very dense rocks such as granite, limestone and basalt. The velocities in saturated strata are somewhat greater than in unsaturated strata. The average velocities of seismic waves in different formations are given in Table 8.4.

**Example 4:**

In a refraction shooting, nine geophones were placed along a straight line at distances of 40, 60, 80, 100,140, 180, 220, 260 and 320 metres from the shot point. The seismic record gave the following data (Table 8.5). Draw the time-distance graph and determine the velocity of the shock wave and thickness of each layer.

**Solution:**

The time-distance graph is drawn as shown in Fig. 8.8. The velocity of shock wave (direct wave) in the top soil layer, i.e., reciprocal of slope-

V_{1} = 80/0.15 sec = 533 m/sec

The velocity of the shock wave (refracted wave) in the second layer (weathered rock)-

V_{2} = (180 – 80)/(0.20 – 0.15) sec = 2000 m/sec

The velocity of the shock wave (refracted wave) in the bottom hard rock layer-

V_{3} = (320 – 180)/(0.225 – 0.200) sec = 5600 m/sec

The thickness of the first layer using the critical distance formula-

The thickness of the first layer using the intercept time formula-

The thickness of the second layer using the intercept time formula-

An approximate equation for Z_{2} presented by Geophysical Specialities Company (1960), in modified form, is-

Where X_{2} is the horizontal distance of the second intersection point on the time-distance graph.

As compared with Z_{2} = 54.8 m obtained from the intercept time formula.

**3. Other Surveying Methods: **

**(a) Soil Temperature: **

The high specific heat of ground water can cause a shallow aquifer to act as a heat sink or heat source that influences the near-surface temperatures to a measurable extent. Measurements of soil temperatures are made at about 45 cm below the land surface using an electronic thermometer—a thermistor at the end of a long probe.

**(b) Magnetometer: **

Basically, a magnetometer measures the intensity and direction of magnetic forces. ‘Minimag’, supplied by UNICEF, has a suspension wire magnetometer that measures vertical and horizontal field components with an accuracy of ± 20 gammas (γ) if tripod is used. They are useful in granite areas where vertical or nearly vertical dykes are frequent. In basaltic areas they are of little or no use. As dykes normally have another mineral composition than the surrounding rock, an anomaly in the magnetic field can be observed, Fig. 8.9. The dykes sometimes also serve as underground barriers for the ground water.

**(c) Gravity:**

Gravity is directly related to the density and volume of earth materials beneath the point being measured. Gravity studies, because they are relatively insensitive to small changes in geology, have been used in ground water studies to map large, buried valleys. They have also been used to locate sink holes and caverns in limestone areas.

The most common type of gravity instrument is the gravimeter, which measures the direct effects of the pull of gravity on a mass suspended by a delicate spring. Changes in the length of the spring are related directly to the vertical intensity of the gravity field. Optical or electrical methods are used to amplify movements of the spring so that very slight changes can be measured. Gravity is measured in gals (1 gal = 1 cm/sec^{2}) or milligals (1 milligal = 0.001 gal).

**(d) Remote Sensing: **

This method includes infra-red photography, aeromagnetic surveys at low altitudes and other methods which use photographic techniques, conducted above the earth surface, either by aircrafts or satellites. Infra-red photography can detect temperature difference in water. Aerial magnetic surveys can delineate subsurface rock structures which might control the flow of ground water.

For ground water the most widely observable anomalies (i.e., changes in the properties of rocks due to the presence of the resource—water) are electrical conductance of the rock and velocity of sound through particular rock types. As such the resistivity and seismic methods are widely employed and are also relatively cheap. Electrical resistivity surveys are generally used for preliminary exploration of rather large areas. The interpretations based on such surveys should be confirmed by test drilling.