Common geophysical tests that are part of field exploration programs are used to increase the accuracy of drilling and laboratory study results. Typically, in large projects, the results of geophysical tests are used to complement information between boreholes and the overall interpretation of these results, and geotechnical sections of the site under study are prepared with the aid of information obtained from geotechnical explorations (borehole drilling, test pits, etc.) and geophysical surveys. In a particular project, due to special site conditions or project objectives, only geophysical studies may be used to determine the state of subsurface geological structures. Also, in many small and medium projects, the geotechnical study report of the project is prepared solely based on drilling information.

In general, most researchers recommend that when estimating the desired parameters must have high accuracy, it is better that the final interpretation of geophysical results be accompanied by geotechnical drilling.

The following goals can be achieved at a general level and as an estimate that are more descriptive in nature using geophysical surveys:

It should be noted that each of the geophysical methods has certain capabilities in determining the aforementioned goals. Before selecting any method, the advantages and disadvantages of that method and the capability and efficiency of the method in question in estimating the aforementioned goals should be considered.

Geophysical surveys include study of the physical properties of the earth and its surrounding environment using waves such as electrical, electromagnetic, magnetic, and acoustic waves. Today’s studies include:

In the seismic refraction method or boundary refraction, the velocity of the pressure wave is measured using a line of geophones placed at the site. Using the results, the depth of bedrock and the thickness of subsurface layers and possibly the groundwater level are determined. When sound waves pass through the interface between two layers that have different velocities, they refract. The wave angle that leaves the interface changes relative to the angle of incidence. This change in angle depends on the relative velocities. When the wave moves from a lower velocity layer to a higher velocity layer, the wave refracts at the upper surface of the lower layer. As the wave propagates, the refracted wave propagates along the upper surface. In this case, this wave enters the surface of the geophones. The velocity of sound waves in the lower layer is greater than that of the upper layer, so at some points, the refracted wave front overtakes and passes through faster than the direct wave. As a result, the refracted wave will be the first wave to reach the geophones, and this will continue until a faster, deeper wave exists. The time difference between the wave received by the geophones depends on the velocity of the lower layer. If the layer is flat, refracted waves are formed from a straight line whose amplitude is directly velocity dependent. The point at which refracted waves overtake direct waves is called the crossover distance and is used to estimate the distance from deep sections to the refraction surface. Refraction experiments are based on travel times generated by the recorded source motion at various offsets. The information obtained from refraction experiments consists of travel time versus offset curves. This information takes the form of depth to subsurface interfaces and velocities at which the wave travels within each of the subsurface layers. These velocities are controlled by the physical constants called elastic parameters.

Refraction observations require fewer wave generating sources and hence fewer receivers than reflection methods and are less expensive by comparison. Quantitative processing takes place on refraction observations. Since a small fraction of the earth's motion is recorded, modeling and interpreting what is recorded are simpler than other geophysical methods.

The seismic refraction method can only be useful if the motion generated and propagated increases with increasing depth into the earth. Seismic refraction observations are often interpreted in terms of layers. These layers have dips and topographies. Seismic refraction observations are only used for the travel times of the earth's motion at different offsets from the source. The model that is given for the subsurface results from modifying the observed travel times.

Instrument positioning, operation method and method for calculating bedrock depth by seismic refraction method (NHI, 2001)