The potential of hydroacoustic methods widely used in coastal engineering geophysical survey is limited in the presence of gas-saturated bottom sediments. Under unfavourable conditions it is advisable to use electric prospecting methods. This article analyses the efficiency of modern electric resistivity tomographic (ERT) technologies of the surface and seabed observation systems for studying the geological sections in coastal water areas. Basic geoelectric and electric resistivity tomographic models are synthesized to assess the influence of water salinity and lithological composition of sediments on the results of marine electric prospecting. Petrophysical modelling data showed that, along with the influence of pore water mineralization on the ratio of specific electric resistivity values of dispersed soils, the mineral composition of clay minerals has a significant effect. This effect is manifested as a shift in the position of the inversion point of specific electric resistivity values of sandy-clayey soils with increasing cation exchange capacity typical of different mineralogical types of clays. Results of numerical modelling of electric resistivity tomography sections using surface and seabed observation systems demonstrated that the seabed measurement system provides reliable information on geoelectric structure of sandy-clayey sediment sections, while the sections obtained from the ERT survey on water surface exhibit distortions in the geoelectric section structure and false anomalies. The advantage of seabed ERT for studying the geological structure of coastal marine areas was ascertained. Experimental work in the Luga Bay water area in the Gulf of Finland confirmed the efficiency of the seabed ERT for increasing the reliability of cross-well geological interpolation when constructing composite geological and geophysical sections. The use of seabed ERT ensured a continuous tracking of geoelectric boundaries corresponding to different lithological species in seabed sandy-clayey sediments.
The paper focuses on the relation between geoelectrical characteristics of the soil: resistivity, polarizability and normalized polarizability (ρ, η, Mn) – and its superficial conductivity (SC or σ") using methods of vertical electrical sounding with induced polarization (VES-IP), electric profiling with induced polarization (EP-IP) and frequency characteristic (FC-IP). The authors demonstrate that superficial conductivity can be determined not only from spectral IP data, but also from soil resistivity obtained through petrophysical measurements. In this case normalized polarizability (Mn) is equal to superficial conductivity (SC). Superficial conductivity, in its turn, is proportionate to clay content of the soil. Increasing clayiness reduces hydraulic conductivity. It has been demonstrated that interpretation of EP-IP results benefits from combined study of the plots of three abovementioned parameters (ρ, η, Mn). In the aeration zone, incomplete humidity has a significant effect on geoelectrical parameters of the soil. Petrophysical modelling helps to investigate the impact of humidity.
Alexandrovsky ancient settlement is situated in the Kaluga region near the river Voria and Alexandrovka village. Self potential (SP) measurements with new non-polarizing electrodes of the second generation instead of traditional copper-copper sulfate electrodes were performed in 2013 and increased the quality of measurements. SP data were analyzed together with electrical resistivity tomography (ERT). The authors found filtration SP anomalies and proved that the main SP anomaly has diffusion-adsorption origin, determined that the Alexandrovsky site of the ancient settlement has decreased SP values.
Difference in apparent resistivity values determined in galvanic and inductive electrical and electromagnetic methods depends on macroanisotropy of layered cross-section and decreased penetration depth of galvanic methods without changing penetration depth of inductive methods. Joint influence of these two factors in case of high contrast of layers' resistivities results in difference in several tens of apparent resistivity values.
New field technology for studying anisotropy of hydraulic properties in fractured rocks on azimuthal self potential (SP) measurements, developed by D.N. Wishart and L. Slater, is considered and used to study water filtration in loose grain sediments of river floodplain. Azimuthal SP diagrams allow finding direction of groundwater filtration and can add to traditional azimuthal resistivity survey.
