Application of geophysical borehole logging techniques to examine coastal aquifer palaeohydrogeology

Abstract

Geophysical logging is shown to be a useful technique to support investigations of coastal aquifer hydrogeology. Formation logging can identify the geological layering and fluid logging can characterize the salinity distribution. The measurements also reveal wellbore flow to be common in coastal boreholes, which can mask the salinity stratification in the aquifer matrix. Geophysical logging can be used to guide water sampling and to provide information on the palaeohydrogeology. In combination with water sampling and age determinations, it has shown modern groundwaters overlying Holocene age groundwaters, in turn overlying groundwaters of Pleistocene age, within 150 m of the surface in some of the aquifers studied. Sea-level change in response to Pleistocene glaciations and deglaciations is recognized as a major control on the salinity of groundwaters and on the development of permeable flow routes in coastal aquifers. The permeable routes that developed by groundwater circulation to older and deeper base levels are now partly or wholly occupied by groundwaters of modern flow systems, and can be the focus for saline intrusion. The effects of Pleistocene sea-level change on aquifer development appear to be worldwide and are being increasingly recognized. Examples are described to illustrate the development of flow horizons in relation to rock layering, structure and base levels.

The practicalities of using CFCs and SF6 for groundwater dating and tracing

“…Low concentrations of the CFCs, SF 6 or other anthropogenic tracers may be used to indicate cross-flow and short-circuiting in multi-screen or poorly-constructed wells, especially if used in conjunction with geophysical logging techniques (Buckley et al, 2001). …”

Transboundary geophysical mapping of geological elements and salinity distribution critical for the assessment of future sea water intrusion in response to sea level rise

“…The different probes typically measure changes in the physical characteristics of the surrounding formation, which reflect changes in lithology and/or porewater salinity. The use of a combination of different geophysical logging probes, such as gamma (NG), induction (IN) and fluid temperature and conductivity (FTC), has proved very efficient for investigation of important characteristics of coastal aquifer systems (Buckley et al, 2001). These tools have therefore been used in the present study.…”

“…However, borehole geophysics provides important data for the corroboration and interpretation of seismic (Rasmussen, 2009;Scharling et al, 2009) and AEM surveys (Mullen and Kellett, 2007), e.g. for separating saline layers from clay layers and for measurement of detailed changes in salinity and lithology profiles at extraction wells (Buckley et al, 2001).…”

“…This inductive response of the ground makes TEM methods well-suited for detecting conductive layers where eddy currents are stronger, such as groundwater-saturated formations (Auken and Sørensen, 2003;Viezzoli et al, 2010), and even more so when formations are saturated with saltwater (Fitterman and DeszczPan, 2004;Auken et al, 2010b;Kafri and Goldman, 2005;Kirkegaard et al, 2011;Kok et al, 2010;Teatini et al, 2011). Clay content also increases the conductivity, which makes interpretations less than straightforward and calls for the use of supplementary information from other sources such as well logging (Buckley et al, 2001). The diffusion of the electromagnetic field implies that the resolution capacity decreases with depth, which makes it difficult to obtain detailed structures using only TEM data.…”

“…However, geophysical well logging is necessary where there is a need for detailed assessment of the salt/freshwater distribution and estimates of e.g. chloride concentrations at the intake of water supply wells (Buckley et al, 2001;Mullen and Kellett, 2007), and where there is a need for determining the exact location of lithological boundaries.…”