6. Conclusions

ERT is a useful near-surface imaging technique, which mainly include data acquisition, numerical modelling and tomographic inversion. For surface data acquisition, dipole-dipole, Schlumberger and gradient arrays are applicable for high-resolution image; particularly, the full-range gradient array may complement to gradient array for better data coverage and completeness of data information. For cross-hole data acquisition, pole-pole (A-M), bipole-pole (AM-N) or pole-bipole (N-AM) and bipole-bipole (AM-BN) can be employed, and the geometry factor and numerical modelling may be applied for designing efficient and effective arrays and exam the imaging capability of ERT for specified targets.

GQG approach is a new version of finite-element modelling. It uses Gaussian abscissae to discrete the model domain and Gaussian weights to compute the volume integral. Therefore, it is much easier to match arbitrary free-surface topography and subsurface interface and computation of the subdomain integrals. It does not require a small size of element and complex element mesh generator. The accuracy of modelling depends on the number of Gaussian abscissae in subdomains. The more abscissae are employed in subdomains, the more accurate modelling result is generated but costs more computer time.

Tomographic inversion is generally implemented by a standard conjugate gradient algorithm, which requires to compute the gradient and the Hessian matrix of an objective function. Two types of objective functions can be applied. One is the l1 norm and the other is the l2-norm. The former is less sensitive to an outlier in data but hardly computes the gradient at zero misfit. The latter has no problem to compute the gradient and the Hessian matrix, and it is much easier to implement a standard conjugate gradient algorithm. With high-quality data, the two inversions converge. The field experiments show that surface and cross-hole ERT can be

applied to map the base of alluvial overburden and the base of pisolite, as well as the clay contamination within pisolite.
