GEOPHYSIAL METHODS - SITE EXPLORATION.

Geo-physical methods are used when the depth of exploration is very large, and also when the speed of investigation is of primary importance. Geo-physical investigations involve the detecttion of significant differences in the physical properties of geological formations. These mehods were developed in connection with prospecting of useful minerals and oils. The major method of geo-physical investigations are: gravitational methods, magnetic methods, seismic refraction method, and electrical resistivity method. Out of these, seismic refraction method and electrical resistivity methods are the most commonly used for Civil Engineering purposes.



Seismic refraction method

In this method, shock waves are created into the soil at their ground level or a certain depth below it by exploding small charge in the soil or by striking a plate on the soil with a hammer. The radiating shock waves are picked up by the vibrarion detector (also called geophone or seismometer) where the time of travel of the shock waves gets recorded. A number of geophones are arranged along a fine (Fig. 2.16). Sorne of ihe waves, known as dírect or pruna?y waves nave! directly from the shock point along the ground surface and are picked first by the geophone. The other waves which travel through the soil get refracted at the interface of two soil strata. The refracted rays are also picked up by the geophone. If the underlying layer is denser, the refracted waves travel much faster. As the distance between the shock point and the geophone increases, the refracted waves are able to reach the geophone earlier than the direct waves. By knowing the time of travel primary and refracted waves at various geophones, the depth ofv arious strata can be evaluated, by preparing distance-time graphs and using analytical methods.

Seismic refraction method is fast and reliable in establishing profiles of different strata provided the deeper layer have increasingly greater density and thus higher velocities and also increasingly greater thickness.

Different kinds of materlais such as gravel, clay hardpan, or rock have characteristic seismic velocities and hence they may be identified by the distance-time graphs. The exact type of material cannot, however, be recognised and the exploration should be supplemented by boring or soundings and sampling.

FIG. 2.16  SEISMIC REFRACTION METHOD.

Electrical Resistivity Method

The electrical resistivity method is based on the measurement and recordíng of changes in the mean resistivity of various solis.

Each soil has its  own resistivity depending upon its water content, compaction and composition; for example, it is low for saturated silt and high for loose dry gravel or solid rock.

The test is conducted by driving four metal spikes to serve as electrodes into the ground along a straight line at equal distance. A direct voltage is imposed between the two outer electrodes, and the potential drop is measured between the inner electrodes. The mean resistivity Q (ohm-cm) is computed from the expression

The depth of exploration is roughly proportional to the electrode 

FIG. 2.17 RESISTIVITY METHOD.

spacing. For studying vertical changes in the strata, the electrode system is expanded, about a fixed central point, by increasing the spacing gradually from an initial small value to a distance roughly equal to the depth of exploration required. The method is known as resistivity sounding.

To correctly interpret the resistivity data for knowing the nature and distribution of soil formation, it is necessary to make preliminary trial or calibration tests on known formations.