Detecting small-scale targets by the two-sided gradient transformation

Abstract

The two-sided three-electrode arrangement uses equally spaced electrodes permitting the use of automated devices for fast measurements. A new transformation method named as the 'two-sided gradient' (TSG) is derived by a modification of the gradient (G) transformation of Karous and Pernu(1985). The gradient transformations may remove the contribution of the 1-D earth from the apparent resistivity data, improving thus the effect of the lateral resistivity variation. This helps to the fast detection of small-scale targets. Tests on synthetic data show that the TSG transformation is superior to the G transformation in delineating the shallow lateral discontinuities. However, both transformations do not provide estimates for the depth and the exact size of the target. A field application was carried out in the archaeological site known as 'Alacahoyuk', a religious site of Hittites hosting the ruins of temples. The two-dimensional inversion of the two-sided three-electrode apparent resistivity data lead to the location of a part of the city-wall. The archaeological excavation next unearthed the wall. Thus, the validity of our interpretation was checked. The 2-D inversion of the two-sided three-electrode data estimated correctly the location, the depth and the dimensions of the city-wall. The 2-D inversion results predicted the termination of the wall inferred by the results in some profiles. On the contrary, the TSG transformation indicated the continuation of the city-wall and the subsequent excavations proved this fact. The field survey proved that the TSG transformation is a fast and efficient method in delineating small-scale targets. Then, it can be used as a complementary method for the 2-D inversion methods.