Joint Use of Seismic and Electromagnetic Methods in Geophysical Surveys

Multi-channel Analysis of Surface Waves (MASW) is a seismic wave propagation method which involves the measurement of Rayleigh waves propagating along the surface of a medium. The method is non-intrusive, fast and practical and it has been successfully utilized for the in-situ evaluation of shear modulus and layer thicknesses of soils and, more recently, pavement systems. The method is also widely utilized as a tool for monitoring stiffness during construction, for maintenance inspections and even for the detection of voids and sinkholes. Time Domain Reflectometry (TDR) is an electromagnetic method based on the measurement of the propagation velocity of a step voltage pulse along a probe inserted in the soil. Electrical properties of the soil, i.e. dielectric permittivity and bulk electrical conductivity, are determined and can be related to some geotechnical properties, e.g. the volumetric water content and potentially the soil density. Seismic wave propagation methods such as MASW are sometimes used in conjunction with electromagnetic methods, in an attempt to reduce the uncertainty associated with each individual method, and to provide an enhanced characterization of the investigated soil. It is still unknown however, whether they are mostly complementary methods or whether they share the assessment of common mechanical/geotechnical properties. In this work the potential and the limitations of the joint use of the MASW and TDR techniques were investigated through an in-situ near-surface programme measurement at two different soil sites, up to a depth of 1 metre. A Dynamic Cone Penetrometer test was performed and the Particle Size Distribution curve determined to extend the soil characterization, and where possible soil samples were taken at various depths in order to measure the dry density and the volumetric water content. The two techniques measured similar trends, augmenting the results obtained by each method and showing the potential for an enhanced and more complete assessment of the soil properties. In addition, bulk electrical conductivity was shown to be related to the shear modulus for the soils studied.