Near Surface Geophysics - Volume 21, Issue 4, 2023

This paper presents a successful application of the mise‐à‐la‐masse (MALM) or applied potential method in association with induced polarization (IP). MALM is known, since 1920, to be able to delimit electrically conductive massive mineralization. The method is used at the point where a drill hole intercepts a massive, mineralized body, to estimate its extent and shape. It is thus used as a complementary mineral exploration tool. IP is normally used in conjunction with resistivity. However, the acquisition of IP data together with the MALM method is rarely found in the literature. This survey shows that it is possible to successfully detect and delimit areas of disseminated sulphides using MALM with IP. In this paper, the MALM method is presented and discussed in detail. A short description of the IP method is also provided. Data are from the Camaquã Project (Caçapava do Sul, RS, Brazil) and the Santa Maria (lead, zinc and associated silver) deposit, 5 km south of Caçapava do Sul. In a well that intercepted a massive mineralization (borehole 8011) with electrical continuity, ∆V measurements delineated the extent, shape and orientation (NS) of a conductive sulphide body. Mineralization intercepted by other two wells (8011 and 8103) did not present enough electrical continuity to exhibit potential contours that could be associated to massive sulphides; it was, however, successfully detected by IP.

Many coastal areas are affected by groundwater salinization due to the unsustainable use of groundwater resources. For a cost‐effective quantitative assessment of groundwater resources, electrical resistivity (ER) tomography is often used as a standalone geophysical technique. In this paper, we present an application of the integration of direct‐current ER and full‐decay‐induced polarization (IP) method at the Pontina Plain (Central Italy). The case study is a coastal area in Central Italy prone to salinization due to both geological and anthropogenic factors. To achieve these goals, we inverted full‐decay time‐domain electrical data for Cole–Cole parameters. The resulting multi‐parameter model provides a first approximation prediction of the permeability, employing well‐established empirical relationships with the electrical parameters. We demonstrated that our approach: (i) can locate highly conductive zones directly related to saline intrusion inland using the resistivity as a fast proxy; (ii) can remove the ambiguity in the detection of clay/silt layers in the near‐surface; and (iii) permit a prediction of the permeability, employing full‐decay inversion of time‐domain electrical data. However, the extremely conductive environment prevents the use of IP data for the reconstruction of deep layers or detection of the salt wedge front. Therefore, this approach can be used for hydro‐geophysical screening and monitoring of salinization‐prone sites, where strong limitations to direct inspection exist due to external constraints (e.g., protected lands).

Rice is a staple crop in the Vietnam Mekong Delta (VMD) in which more than half of Vietnam's rice is produced. However, rice production in the VMD is threatened by increasing saltwater intrusion due to land subsidence and climate change induced sea level rise. Saltwater intrusion into lowland areas through the canal system or capillary rise of saline water from near surface saline water tables may result in salt accumulation in the topsoil. Therefore, it is important to disentangle the two effects and their relative importance to implement appropriate strategies for water and salinity management for adapting rice production systems of the VMD to climate change. Here, we report on the possibility of using geoelectrical methods to evaluate the potential threat of subsoil salinity to rice production. To evaluate the level of subsoil salinity, we measured soil electrical resistivity using an ARES II to a depth of 40 m in a case study comprising five locations in the VMD. Electrical resistivity measurements were calibrated to soil types, which were identified through evaluating 1 m core sections obtained by drilling down to 40 m depth. The relationship between drilling data and soil resistivity was determined by applying clustering and principal component analysis. Resistivity values smaller than 3 Ω m were clearly identified as indicative for a saline water table. The results show a direct link between the depth of the saline water table and the proximity to the sea, but not to the rice production system (single, double, or triple cropping). This study proved for the first time the applicability of the electrical resistivity tomography method for identifying groundwater tables and evaluating subsoil salinity on an agricultural field scale in the VMD.

Radiomagnetotellurics (RMT) is an electromagnetic method that uses signals from radio transmitters broadcasting in the 10 kHz to 1 MHz frequency range. Due to its limited frequency range, RMT is commonly used as a shallow‐depth investigation tool. However, in remote areas, there is a lack of radio transmitters and only signals from very low frequency (VLF) antennas (10–30 kHz frequency range) can be measured. This can give rise to low signal‐to‐noise ratio. To overcome this disadvantage of RMT, a controlled‐source RMT (CSRMT) can be applied to measure signals of the low‐frequency (LF) and mid‐frequency ranges (30–1000 kHz). Moreover, the wider frequency range of the CSRMT method (down to 1 kHz) leads to a deeper sounding depth. We present the first RMT and CSRMT validation studies using two perpendicularly located horizontal electric dipoles to realize a 3D inversion of CSRMT data. The survey area in Alexandrova village in Kaluga region, Russia, a previously investigated area, was selected for a validation study. We acquired the data along 8 profiles with 175 stations. Transmitter lines for the CSRMT case were about 900 m long, and the minimum and maximum distances of the stations from transmitters were 450–1000 m, respectively. We applied 2D and 3D inversions over the far‐field data and compared with the previous results. The available geophysical information as well as the borehole data indicate a high agreement between the obtained models and the geological structure. We can confirm that the CSRMT method is a reliable approach for near‐surface explorations and that, the existing advanced and tested inversion tools for magnetotellurics, can be used to invert the RMT and far‐field zone CSRMT data leading to comparable results.