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Groundwater investigation techniques 193

complementary, surface geophysics is generally em- investigation: at the start for conceptualizing the
ployed at an early stage in a hydrogeological invest- main controls on groundwater flow in the model
igation, before boreholes are drilled, while logging area and in indicating the type and length of field data
techniques are employed later to obtain detailed that will be required to construct a model; and at
information on aquifer and fluid properties (see Fig. the end for predicting future aquifer response under
6.28) (Barker 1986). There is a large literature relating different groundwater conditions (Rushton 1986,
to the application of geophysical methods in hydro- 2003). With a well-constructed model, the ability to
geology and the objective here is to direct the reader predict groundwater flow patterns, for example the
to textbooks and articles that expound the various effects of different groundwater abstraction patterns
downhole and surface techniques. Downhole geo- on sensitive aquatic systems (Box 5.6), or the shape of
physical techniques are discussed by Beesley (1986), wellhead capture zones for protecting groundwater
BSI (1988), Chapellier (1992) and Sharma (1997) and quality (see Fig. 7.8), or future aquifer response to
surface geophysical techniques by Griffiths and King changing recharge amounts under climate change
(1981), Barker (1986) and Kearey and Brooks (1991). (see Section 8.5), makes groundwater modelling an
Various hydrogeological applications are demon- indispensable tool for managing local and regional
strated in the following papers: groundwater resources.
• The application of geophysical borehole measure- In the process of constructing a groundwater
ments in crystalline rocks using acoustic televiewer model, the primary aim is to represent adequately
and caliper measurements, electrical resistance, ther- the different features of groundwater flow through
mal techniques and vertical seismic profiling (Wilhelm the aquifer within the model area or domain. In
et al. 1994a). this respect, the important features to consider in
• A gravity survey and resulting Bouguer anomaly governing the response of an aquifer to a change
map of the subsurface position of a buried channel in in hydrogeological conditions include: aquifer inflows
the Chalk aquifer of East Anglia (Barker & Harker (recharge, leakage and cross-formational flows); aqui-
1984). fer outflows (abstractions, spring flows and river
• An investigation of saline intrusion using borehole baseflows); aquifer properties (hydraulic conductivity
logging, seismic reflection profiling, vertical electrical and storage coefficient); and aquifer boundaries (con-
resistivity soundings (VES) and electromagnetic induc- stant or fixed head, constant flow or variable head,
tion surveying in a coastal sand and gravel aquifer and no-flow boundaries).
(Holman & Hiscock 1998; Holman et al. 1999). Given the complexity of regional groundwater
• An evaluation of lithological, stratigraphical and flow problems, the equations of groundwater flow
structural controls on the distribution of aquifers (eqs 2.40, 2.46) cannot be solved by analytical
using VES, transient electromagnetic (TEM), tenso- methods (Section 2.12) and, instead, approximate
rial audio-magnetotelluric (AMT) and nuclear mag- numerical techniques are used. These techniques
netic resonance (NMR) depth sounding and inversion require that the space and time co-ordinates are
measurements (Meju et al. 1999, 2002). divided into some form of discrete mesh and time
• The detection of leaks from environmental barriers interval. Common approaches to defining the space
using electrical current imaging (Binley et al. 1997). co-ordinates in the model domain are the finite-
• The application of cross-borehole transmission radar difference and finite-element approximations. The
and electrical resistance tomography to characterize finite-difference approach is based on a rectilinear
groundwater flow and solute transport in the unsatur- mesh whereas the finite-element approach is more
ated (vadose) zone (Binley et al. 2002a, 2002b). flexible in allowing a spatial discretization that can
fit the geometry of the flow problem. For each cell in
the mesh and, for transient simulations, at each time
5.9 Groundwater modelling step, the unknown heads are represented by a set of
simultaneous equations that can be solved iteratively
Numerical modelling of groundwater flow can be by specifying initial head conditions. Model runs
undertaken at the start or end of a hydrogeological are performed by a computer program that employs