The following points highlight the two main geophysical methods of soil exploration. The methods are: 1. Seismic-Refraction Method 2. Electrical Resistivity Method.
1. Seismic-Refraction Method:
The seismic-refraction method is based on the principle that elastic shock waves travel at different velocities in different materials. Shock waves are generated at a point on the ground surface, using a sledge hammer. These waves travel deep into the ground and get refracted at the interface of two different materials and to the ground surface. The time of arrival of these waves at different locations on the ground surface are recorded by geophones, which pick up the refracted waves. The geophones convert the ground vibrations into electrical impulses and transmit them to a recording apparatus.
When the distance between the vibration source and the geophone is short, the arrival time will be that of a direct wave. When the distance exceeds a certain value (depending on the thickness of the stratum), the refracted wave will be the first to be detected by the geophone [see Fig. 14.21(a)]. This is because the refracted wave, although longer than the direct wave, passes through a stratum of higher density (and hence higher seismic velocity).
A plot is made between distance on the x-axis and time on the y-axis as shown in [Fig. 14.21(b)]. Points B and C in Fig. 14.21(b) represent the distance at which the refracted wave from the second and third strata arrive at the geophone, marked by a change in the slope of the graph. The slope of line AB gives the reciprocal of seismic velocity in the top layer (1/v1, that of BC gives (1/v2), and that of CD gives (1/v3), etc. The thickness (H1) of the top stratum of the soil is given by –
where D1 is the distance corresponding to point B, where the seismic velocity changes from v1 to v2.
If there is a small or large variation in the thickness of the top stratum, H1 represents the average thickness. Shepard and Haines (1944) gave Eqs. (14.12) – (14.15) for determining depths H1 and H2 in a three-layer stratum –
where l1 is the length AB1 = Intecept of line AB on the y-axis and l2 = AC1 – AB1 = B1C1 [see Fig. 14.21(b)].
The seismic-refraction method is based on the following assumptions:
i. The density, and hence the seismic velocity, of successive layers of soil increases with depth.
ii. Each stratum of soil is homogeneous and isotropic.
iii. The boundaries between strata are distinct horizontal or inclined planes.
The electrical resistivity method consists of measuring the resistivity of the soil strata and correlating the resistivity to the properties of the soil. The principal application of the electrical resistivity method is in investigating foundations of dams and other large structures, particularly in exploring granular river channel deposits or bedrock surfaces. The method is also used for locating fresh or salt water boundaries.
The electrical resistivity method is of the following two types:
i. Electrical profiling method.
ii. Electrical sounding method.
In this method, four electrodes, usually in the form of metal spikes, are driven into the ground at the same spacing. The two outer electrodes are known as current electrodes, and the two inner electrodes are known as potential electrodes, as shown in Fig. 14.22.
A direct current (DC) of 50-100 milliamperes (mA) is applied between the outer electrodes, and the voltage drop or the potential difference between the inner electrodes is measured using a potentiometer. The mean resistivity of the soil up to a depth of D cm below ground surface is obtained from Eq. (14.16) as follows –
where D is the distance between electrodes in centimeters (cm), V the voltage drop between inner electrodes in volts (V), and I the current flowing between outer electrodes in amperes (A).
The electrodes are moved as a group, at the same spacing between them, as shown in Fig. 14.23(a), and different profile lines are run across the area. The test is repeated after changing the spacing to determine the mean resistivity up to a depth equal to the new spacing. The method is also known as resistivity mapping method. The type of soil or rock stratum encountered will be estimated using the measured resistivity from Table 14.9.
This method is similar to the electrical profiling method, except that the electrode system is expanded about a point P by increasing the spacing between the electrodes in successive operations. For example, the electrode spacing is increased with every successive test, as shown in Fig. 14.23(b).
As the depth of current penetration is equal to the electrode spacing, the change in the mean resistivity is correlated to the changes in the strata at that location. The midpoint (P) moves forward by a distance of 4D in successive tests of the electrical profiling method. However, the midpoint (P) remains at the same position in the electrical sounding method.
The electrical resistivity method is found to be less reliable than the seismic-refraction method, since the resistivity of a particular soil or rock can vary over a wide range of values depending on the density, voids or fractures, and degree of saturation of the soil.
Limitations of the electrical resistivity method are the following:
i. The method is capable of detecting only the strata having different electrical resistivities.
ii. The results are considerably influenced by surface irregularities, wetness of the soil, and electrolyte concentration of groundwater.
iii. As the resistivity of different strata at the interface changes gradually and not abruptly, the interpretation becomes difficult.
iv. Services of an expert in the field are needed.
v. The equipment is very costly.