1D VES, 2D imaging survey and 3D ERT survey


The greatest limitation of the resistivity sounding method is that it does not take into account horizontal changes in the subsurface resistivity. A more accurate model of the subsurface is a two-dimensional (2-D) model where the resistivity changes in the vertical direction, as well as in the horizontal direction along the survey line. In this case, it is assumed that resistivity does not change in the direction that is perpendicular to the survey line. In many situations, particularly for surveys over elongated geological bodies, this is a reasonable assumption. In theory, a 3-D resistivity survey and interpretation model should be even more accurate.

However, at the present time, 2-D surveys are the most practical economic compromise between obtaining very accurate results and keeping the survey costs down.Typical 1-D resistivity sounding surveys usually involve about 10 to 20 readings, while 2-D imaging surveys involve about 100 to 1000 measurements. In comparison, a 3-D survey might involve several thousand measurements. The cost of a typical 2-D survey could be several times the cost of a 1-D sounding survey, and is probably comparable with a seismic refraction survey.

In many geological situations, 2-D electrical imaging surveys can give useful results that are complementary to the information obtained by other geophysical methods. For example, seismic methods can map undulating interfaces well, but will have difficulty (without using advanced data processing techniques) in mapping discrete bodies such as boulders, cavities and pollution plumes. Ground radar surveys can provide more detailed pictures but have very limited depth penetration in areas with conductive unconsolidated sediments, such as clayey soils. Two- dimensional electrical surveys can be used in conjunction with seismic or GPR surveys as they provide complementary information about the subsurface.