Modeling Coastal Salinity in Quasi 2D and 3D Using a DUALEM‐421 and Inversion Software

Abstract

Rising sea levels, owing to climate change, are a threat to fresh water coastal aquifers. This is because saline intrusions are caused by increases and intensification of medium-large scale influences including sea level rise, wave climate, tidal cycles, and shifts in beach morphology. Methods are therefore required to understand the dynamics of these interactions. While traditional borehole and galvanic contact resistivity (GCR) techniques have been successful they are time-consuming. Alternatively, frequency-domain electromagnetic (FEM) induction is potentially useful as physical contact with the ground is not required. A DUALEM-421 and EM4Soil inversion software package are used to develop a quasi two- (2D) and quasi three-dimensional (3D) electromagnetic conductivity images (EMCI) across Long Reef Beach located north of Sydney Harbour, New South Wales, Australia. The quasi 2D models discern: the dry sand (<10 mS/m) associated with the incipient dune; sand with fresh water (10 to 20 mS/m); mixing of fresh and saline water (20 to 500 mS/m), and; saline sand of varying moisture (more than 500 mS/m). The quasi 3D EMCIs generated for low and high tides suggest that daily tidal cycles do not have a significant effect on local groundwater salinity. Instead, the saline intrusion is most likely influenced by medium-large scale drivers including local wave climate and morphology along this wave-dominated beach. Further research is required to elucidate the influence of spring-neap tidal cycles, contrasting beach morphological states and sea level rise.

1D sequential inversion of portable multi‐configuration electromagnetic induction data

“…; Davies et al . ) or the 2D distribution in a vertical plane from a recorded profile (Kang et al . ).…”

Mapping soil salinity in 3‐dimensions using an EM38 and EM4Soil inversion modelling at the reconnaissance scale in central Morocco

“…More recently, using a quasi‐3d inversion model, Davies et al . () showed how saline wedges in a swash‐zone (i.e. upper part of the beach between backbeach and surf zone) can be discerned with Zare et al .…”

Contributions of hydrogeophysics to the hydrogeological conceptual model of the Albufeira-Ribeira de Quarteira coastal aquifer in Algarve, Portugal

“…Such methods retrieve the subsurface electrical resistivity that is of special interest for hydrogeological purposes since it allows, e.g., to discriminate between freshwater and saltwater, between soft-rock sandy aquifers and clayey material, between hard rock porous/fractured aquifers and lowpermeability claystones and marlstones, and between water-bearing fractured rock and its solid host rock (Ernstson et al 2009). The applicability of the methods used in this study to the characterization of coastal aquifers is well documented and described in detail in articles, as for instance: Goldman et al (1991) and Yechieli et al (2001) for TDEM, Davies et al (2014) for FDEM and Nguyen et al (2009) and Wilson et al (2006) for ERT. However, the interpretation of the electrical resistivity survey is subject to the equivalence problem.…”