Agriculture Reference
In-Depth Information
3.2 Characterisation of soil heterogeneity
Conventional techniques, such as soil sampling and core drillings provide more or less
disturbed samples on which one can measure porosity, unsaturated hydraulic conductivity
and Pf curve (if undisturbed) or grain size distribution and water content on disturbed
samples. The Pf curve describes the relationship between soil suction and soil water content,
this curve determines the unsaturated hydraulic conductivity (ref). Well established pedo-
transfer functions such as Hazen's equation (ref.) can be used to calculate hydrogeological
parameters such as the hydraulic conductivity. The samples can also be used for bio-geo-
chemical characterisation interactions between contaminants and soil. A number of spatially
collected samples can then be used to establish geostatistical properties of the different
hydrogeological parameters (discussed in the next section). More recently advanced direct
push technology, which provides opportunities for in-situ measurements e.g. of hydraulic
conductivity by the use of specialised probes at the end of the direct push probe. The
disadvantage of these methods alone is that they are destructive, time consuming,
expensive; and do not give a continuous image of the subsurface. Deep geophysical
exploration has been around since the beginning of the last century, and is a common
method for geological characterization in oil exploration, mapping of lithostratigraphy,
fracture patterns in bedrock and is described in several text books (e.g. Kearey & Brooks,
2002). In the last couple of decades geophysical techniques such as those described in Table
3 have become more common for hydrogeological applications, for further reading see e.g.
Regli, et al, 2002; Hubbard & Rubin, 2000, Kowalsky et al., 2001; Rea and Knight, 1998;
Rubin & Hubbard, 2005; Veerecken et al., 2006.
As an example we discuss briefly the principle of the electrical resistivity method. The
electrical properties of soils are a function of the soil type, water content, soil temperature
and ion content of the soil water. Measurements of soil bulk electrical resistivity are most
commonly conducted by placing a set of electrodes in the ground along a line on the surface
or in vertical boreholes. By inserting a known current and measuring the resultant voltage
consecutively over the set of electrodes, one can after an inversion of the collected data
obtain an image of the distribution of electrical resistivities in the soil volume next to the
electrodes (see eg. Reynolds, 1997). A single measurement may reveal geological features of
the subsurface, while the comparison of images taken at different times (time-lapse
measurements) can help quantify spatial and temporal variability caused by changes in
water (Daily et al., 1992) and ionic contents.
The advantage of geophysical techniques over the more conventional and invasive
techniques is that they are non-destructive and provide continuous images of the
subsurface. The challenge of geophysical methods however is the ambiguity of their
interpretation. The non-invasive geophysical methods map zones or layers of different
physical characteristics (Table 3). The interpretation of such data requires that the data is run
through an inversion code which basically “suggests” a likely distribution of the specific
geophysical responses in a 1, 2 or 3D space. The results are optimised with respect to
measurements conducted on the surface or in boreholes. Forward and inverse modelling of
the system that is being studied is required for optimising the configuration of
measurements, this technique can also be used in a stochastic framework in order to include
uncertainty and coupling to soft and hard data for hydrogeological characterisation (Rubin
and Hubbard, 2005). Another recent development to reduce the non-uniqueness of the
interpretation is to combine different geophysical data sets collected at the same location
and time through joint inversion (e.g. Gallado & Meju, 2004; Linde et al., 2006). However
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