PhD Scholarship
Non-Invasive, Non-Destructive Mapping of Water Content Changes and
Contaminant Movement in an Alluvial Gravel Vadose Zone
David C. Nobes
Department of Geological Sciences
University of Canterbury
and
Murray E. Close
Environmental Science and Research (ESR) Ltd, Christchurch
SUMMARY
As part of the Integrated Research for Aquifer Protection (IRAP) project, a PhD
scholarship is being offered jointly supported by ESR Ltd and the Department of Geological
Sciences (DoGS) at the University of Canterbury (UoC). An array of research methods and
approaches is being used to characterize the movement of water and water-borne chemicals
through the unsaturated zone, focussing on a site near Lincoln, Canterbury. This is a difficult
zone to characterize; most attention in the past has either focussed on the soil layer or the zone
below the water table because they are less complex.
The aims of the PhD project are to characterise the stratigraphy and variability of the
subsurface, including identifying preferential flow paths, and to examine whether the local
characterisation can be applied more widely across the region. The research project would
primarily use ground penetrating radar (GPR), multi-electrode resistivity (MER) and possibly
electromagnetic (EM) geophysical imaging, combined with and calibrated by borehole and
sediment sampling information, to examine spatial and temporal variations in water content
across the site. A significant associated aspect of the project is the examination of the
constitutive relations used to relate subsurface physical properties, such as the porosity and
water content, to the geophysical responses.
Amongst the inter-related questions to be answered are:

How well can we characterise the physical properties of the subsurface?

How good are our spatial and temporal resolutions, including the depth of penetration in
alluvial gravels?

Can we identify the subtle lithological variations across the site noninvasively?

What combination of range and scale of geophysical methods, such as ground penetrating
radar and multi-electrode resistivity, are necessary and/or sufficient to provide the
information needed?

Can additional information be obtained from the integration of two or more simultaneous,
or nearly simultaneous geophysical surveys at the same site?

What is the best or most efficient method of obtaining water content data from other
similar sites in Canterbury?
ESR/U of C Joint PhD Proposal

Can we identify preferential flow paths, such as macropores, and how detailed must our
surveys be to do so?

Can we adapt statistical methods to extract additional information, particularly
hydrogeological parameters, from the geophysical data?

What are the lithological influences and controls on the extraction of hydrogeological
information?
The intense study of the one site, using complementary invasive and non-invasive methods,
provides a rare opportunity to explore the questions associated with the movement of water
and water-borne chemicals through the unsaturated zone, from the surface to the water table.
RESEARCH GOALS
Based on previous work, we can outline a number of clearly defined research goals:

To accurately determine the stratigraphy and stratigraphic variation, a three-dimensional
(3D) GPR and MER surveys should be completed. As much as possible, the work should
be done within a short time frame so that changes in water content are negligible. The 3D
survey should be repeated at least once to compare the results.

To aid in this mapping, a range of GPR frequencies should be used: 50 MHz signals will
yield the deeper and larger scale layering and structure; 100 MHz signals will yield more
detail while still showing some of the layering and structure apparent in the 50 MHz
results; and 200 MHz signals should be tested to see what detailed information can be
obtained that is not available in the 100 and 50 MHz results. In particular, 200 MHz results
would be better for comparisons with trench and logging data.

A range of electrode separations should also be used, again to look at greater depth ranges,
and to look at the changes in the electrical properties as a function of scale.

To supplement the GPR and MER surveys, EM maps of electrical conductivity should be
acquired.

To obtain more detailed stratigraphic information, common mid-point (CMP) surveys
should be done at a number of locations using a range of frequencies, as was done by Close
et al. (2004). Specifically, 50, 100 and 200 MHz antennas should be used. A more dense
spatial coverage should be used to obtain more accurate velocity information. To obtain
stratigraphic information at depth, in addition to more dense spatial coverage, which will
increase the signal stacking at later times, but more signal stacking at each location (trace)
should be done to improve the signal-to-noise at depth.

Models of GPR velocity and MER resistivity as a function of water content should be
tested by using the water content as determined from shallow neutron logging and from
direct soil measurements. We do not know the velocities when the soil is dry or saturated.
Even after an extended period of drought, there can be residual water content, and after a
prolonged period of rain, the soil may not be 100 % saturated, although it is likely to be.
Using absolute velocity values can only yield qualitative results without additional
information or constraints, and the calibrated water content is one such constraint.
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ESR/U of C Joint PhD Proposal

Finally, the geophysical results should be combined and, if possible, interpreted and
modelled jointly, particularly GPR and resistivity. For example, the GPR stratigraphy can
be used as a constraint in modelling the resistivity response, and the electrical properties
can be used to constrain the radar signal attenuation. The electrical properties depend on
the water content, water quality and clay content in a systematic way (modified Archie’s
law, as in McNeill, 1990) that is different from the radar velocity. In principle, by
combining the two sets of geophysical results, we can determine the porosity and saturation
independently. In practice, the dependence is not simple, but depends on whether the water
saturation is increasing (rain) or decreasing (Knight and Endres, 1990). A 3D electrical
tomography survey would complement the GPR surveys, but both surveys would need
neutron probe data in addition to trench and log data for calibration.

Finally, the results should be extrapolated to other sites to test how well such surveys are
representative of the broader region.
RESEARCH TIMETABLE
The proposed timeline for the research project is as follows:
Year 1:
 Compile and collate previous work.
 Repeat GPR, resistivity and EM surveys in each season, but at a level of detail that is
greater than the pilot surveys.
 Acquire direct measurements of the in situ soil properties as much as possible at the
same time as the geophysical surveys are done.
 Process the geophysical results.
 Begin comparisons of calibrated results with models of physical properties as a function
of, for example, water content.
Year 2:
 Repeat GPR, resistivity and EM surveys in each season for a second year, to better
delineate interannual and seasonal variability in the geophysical response.
 Repeat GPR, resistivity and EM surveys at new sites to test the extrapolation of the
results from one site to another within a region.
 Repeat the soil property measurements.
 Write up previous year’s results for publication and presentation at national and
international conferences.
 Continue comparisons of direct soil property measurements with models of the
geophysical properties.
Year 3:
 Complete processing of results and writing of thesis and papers for journal publication
and conference presentation.
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ESR/U of C Joint PhD Proposal
REFERENCES
Close, M. E., Nobes, D. C. and Pang, L., 2004. Presence of preferential flow paths in shallow
groundwater systems as indicated by tracer experiments and geophysical surveys. In
Hyndman, David & Bridge, John (eds), Aquifer Characterization, SEPM Special
Publication, 80: 79-91.
Knight, R. and Endres, A, 1990. A new concept in modelling the dielectric response of
sandstones: Defining a wetted rock and bulk water system. Geophysics, 55: 586-594.
McNeill, J. D., 1990. Electromagnetic methods. In S. N. Ward (ed), Geotechnical and
Environmental Geophysics, Vol. I: Review and Tutorial, Society of Exploration
Geophysicists, Tulsa, Oklahoma, 191 - 218.
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