Integrated MASW and ERT Imaging for Geological Definition of an Unconfined Alluvial Aquifer Sustaining a Coastal Groundwater-Dependent Ecosystem in Southwest Portugal

Abstract

This paper integrates multichannel analysis of surface waves (MASW) and time-lapse electrical resistivity tomography (ERT) to define aquifer geometry and identify transient groundwater features of the Cascalheira Stream Basin Holocene alluvial aquifer (aquifer H), which contributes to the Santo André Lagoon, part of a coastal groundwater-dependent ecosystem (GDE), located in southwest Portugal. MASW measures shear-wave velocity (VS), allowing one to obtain steady geological models of the subsurface, and ERT measures subsurface electrical resistivity (ER), being subjected to ambient changes. MASW enables disambiguation of geological structures in low ER environments, such as coastal areas. This research covered one natural year and involved one MASW campaign, four ERT campaigns, and additional geological field surveys and groundwater monitoring to assist interpretation of results. In the area, the conjugate NW–SE and NE–SW strike-slip fault systems determine compartmentalization of geological structures and subsequent accommodation space for Holocene sedimentation. MASW and ERT surveys show how the NW–SE system deepens these structures toward the coast, whereas the NE–SW system generates small horsts and grabens, being one of these occupied by aquifer H. From upstream to downstream, aquifer H thickness and width increase from 10 m to 12 m and from 140 m to 240 m, respectively. Performance of VS and ER models was satisfactory, with a normalized error of the VR and ER models in the 0.01–0.09 range, meaning that a quantitative quota of uncertainty can be segregated from the overall uncertainty of groundwater models without substantially affecting its simulations accuracy. This methodology seeks to improve the design of shallow groundwater research in GDE preservation policies.

Usefulness of Compiled Geophysical Prospecting Surveys in Groundwater Research in the Metropolitan District of Quito in Northern Ecuador

“…This paper reinterpreted the 2D VS models for the shallow geological definition following the interpretative criteria reported by Paz et al [63] and Alcalá et al [64]. These authors propose that subsurface VS propagation is a site-specific steady property determined by effective compaction and therefore is dependent on the age and depth of each geological material piled vertically [65][66][67][68].…”

“…These authors propose that subsurface VS propagation is a site-specific steady property determined by effective compaction and therefore is dependent on the age and depth of each geological material piled vertically [65][66][67][68]. The different relationships between VS and age and depth in different lithologies described in the scientific literature [63][64][65][66][67][68][69][70][71] were used to reinterpret the VS models.…”

Regional calibration and electromagnetic conductivity imaging for assessing the dynamics of soil salinity

“…The vertical-equispaced VS values from all MASW sections were georeferenced and interpolated to create 2D VS layers at different elevations (called MASW maps) (Figure 4). [62]. These authors propose that VS propagation in sediments is a site-specific steady property determined by effective compaction and as such it is dependent on the age and depth of each geological material piled on vertical [63][64][65][66].…”

“…Some MASW maps at different elevation regarding the sea level in Adra town, showing observed and inferred normal and strike-slip fault systems, and location of MASW sections M1 to M13. This paper uses the MASW sections and maps for geological definition following the interpretative criteria reported by Paz et al (2020)[62]. These authors propose that VS propagation in sediments is a site-specific steady property determined by effective compaction and as such it is dependent on the age and depth of each geological material piled on vertical[63][64][65][66].…”

Delineation of Potential Groundwater Zones Using Shear Wave Velocity in Eastern Deccan Volcanic Province, India

“…Electrical resistivity tomographic (ERT) and electromagnetic (EM) are among the suitable geophysical techniques commonly used to understand the shallow to deeper aquifer systems and mapping of fractures and weathering zones for groundwater applications (Dahlin and Owen 1998;Suzuki and Higashi 2001;Hamzah et al 2006;Rai et al 2011Rai et al , 2012Rai et al , 2013Tamma rao et al 2014;Martorana et al 2014;Rai et al 2015). However, the ERT/DC resistivity techniques cannot easliy distinguish the conductivities of pore water and geological material although geological settings are favorable to high conductivity pore water because of contaminants (Degnan and Brayton 2008;Martorana et al 2014;Paz et al 2020). In view of the drawback in the methodology of ERT/EM, by default, we chose to use MASW to study the basaltic terrain.…”