Elsevier

Engineering Geology

Volume 108, Issues 3–4, 8 October 2009, Pages 161-168
Engineering Geology

Review
Contribution of electrical resistivity tomography to the study of detrital aquifers affected by seawater intrusion–extrusion effects: The river Vélez delta (Vélez-Málaga, southern Spain)

https://doi.org/10.1016/j.enggeo.2009.07.004Get rights and content

Abstract

The coastal aquifer of the Plio-Quaternary delta sediment deposits of the Vélez river (province of Málaga, Spain) presents a highly irregular basement morphology and widely varying fill thickness (10–80 m between neighbouring sectors). The basin, which is tectonically controlled, is filled with lutite facies alternating with channel-filling rudites. This detrital aquifer is affected by seasonal seawater intrusion–extrusion processes due to increasing withdrawal of groundwater for human consumption and irrigation during dry periods.

A study was performed to improve the hydrogeological knowledge of this coastal aquifer system. The study examined the morphology of the impervious substratum, the facies distribution and the position of the seawater wedge. For this purpose, an Electrical Resistivity Tomography (ERT) geophysical technique was used and the tomographic data were calibrated using geological observations and borehole studies. An analysis was carried out to compare the direct information obtained from the 35 boreholes with the indirect data corresponding to the four electrical tomography profiles. In the study, over 9660 resistivity data points were processed.

The ERT profiles perfectly corroborated the information derived from the boreholes. The profiles made it possible to detect thickness changes, lithological changes and the presence of faults. Moreover, from a hydrogeologic standpoint, this research technique is capable of detecting the position of the phreatic level and, in coastal aquifers such as the one examined in this study, the possible horizontal or vertical penetration of seawater intrusion. Therefore, the electrical geophysical prospecting based on ERT can be highly useful in areas lacking sufficient geological information and/or mechanical borehole data.

Introduction

In studying the thickness and geometry of depositional systems, a common procedure is to make use of information from geological research, drilling and exploitation boreholes. However, these methods are expensive and time consuming, preventing their use on a large scale. Moreover, these types of data are spatially limited. In contrast, geophysical measurements can provide a less expensive way to improve the knowledge of a set of boreholes (Maillet et al., 2005). For this reason, in many cases, geophysical prospecting techniques can provide complementary data that enable geological correlation, even in sectors where there are no data from boreholes (Gourry et al., 2003, Colella et al., 2004, Maillet et al., 2005, Sumanovac, 2006, Massey and Taylor, 2007, Naudet et al., 2008). Borehole can provide direct information about the subsoil, but is spatially localised. However, indirect geophysical methods generate continuous data throughout a given profile. Both aspects are of particular interest in this study because block tectonics give rise to significant lithological variations between nearby areas. An accurate determination of the fill geometry is important from a hydrogeological standpoint when assessing available water reserves. It also helps in the understanding of the spatial relations between fresh, brackish and salt water, which commonly coexist in coastal aquifers.

In the last decade, a renewed interest in the geoelectrical method has been observed due to the development of multi-electrode arrays, fast acquisition systems, and new inversion algorithms. As a consequence, this method has been re-applied to a wide spectrum of geological studies. However, relatively few electrical resistivity investigations have been performed in fluvial environments totally or partially saturated with salt or brackish water. Electrical Resistivity Tomography (ERT) has been applied in a sand-infilled paleochannel located in the Rhône Delta (Maillet et al., 2005). Massey and Taylor (2007) used ERT in a study of the coastline of South Devon, South-west England, which is believed to be subsiding rapidly. However, ERT has rarely been used for complex tectonic reconstruction and control of saline intrusions.

This study set out to determine the reliability of indirect data obtained by ERT. To do so, these data were compared with those obtained from a well-documented depositional system, for which information was available from both boreholes and indirect methods. The system in question is that of the delta sediments of the Vélez river, near the town of Torre del Mar (SE Spain) (Fig. 1). A comparison was made with data from the boreholes in the area, on some occasions complemented with piezometric, physical and chemical data on the groundwater, and information from electrical tomography profiles. The aim of this study was to test the efficiency of the ERT technique in determining the geometry of complex depositional systems affected by seawater intrusion while providing knowledge about the hydrogeological setting of the detrital aquifer of the Vélez river basin.

Section snippets

Geological and hydrogeological context

The Vélez river basin, with a surface area of 610 km2, is located in the south-eastern part of the province of Málaga (Spain) on the Mediterranean coast (Fig. 1). The southern sector consists of alluvial deposits with an area of 20 km2 which form a delta at the river mouth. The alluvial deposits are assumed to be related to the fluvial activity of the Vélez river and its tributaries. In lithological terms, these deposits are made up of gravel, sand and silt of varying thicknesses, which may

Materials and methods

Electrical resistivity surveys have progressed from conventional vertical soundings to techniques such as ERT, which provides two and even three-dimensional high-resolution electrical images of the surface (Colella et al., 2004).

This electrical geophysical prospecting method consists of determining the distribution of a physical parameter that is characteristic of the subsoil (the resistivity) on the basis of a very large number of measurements of apparent resistivity made from the ground

Results and discussion

Table 1 shows the depth of the Palaeozoic basement and the total depth of the 35 mechanical research boreholes. The boreholes were drilled in rotation while obtaining continuous samples from the core barrels under direct circulation (location shown in Fig. 1). Fig. 2 is based on the data obtained from these boreholes, showing 19 stratigraphic columns correlated in three profiles. The columns were nearly parallel to the tomography profile, including the two in the NW–SE direction and the one in

Conclusions

The analysis of the information obtained from the mechanical boreholes drilled in this region revealed the existence of important variations in the thickness of the sedimentary coverage, which ranged from 10 to 80 m in nearby areas. These stratigraphic colums correlated, leading to the conclusion that the geometry of the river basin is highly irregular and determined by tectonic processes. The filling is comprised, fundamentally, of lutite facies and sands. In addition, there are rudite facies

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