Geoscience Reference
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zone. In the next section, we will show how seismo-
electric data from the vadose zone can be used to
quantitatively determine its water content.
in the left panel of Figure 7.5 is the arithmetic mean of
nine repeated borehole logs acquired over a 10-month
period from December 2002 to October 2003.
In this case study, we report on seismoelectric sound-
ings conducted in July 2003 within 25 m of each other
at four locations centered on the axis spanned by two
boreholes, purpose-drilled 15 m apart to 30 m depth
(Figure 7.5). At each of the four seismoelectric sounding
locations, ten sets of data were recorded. The commer-
cially available Groundflow EKS GF2500 TM was used
for the seismoelectric data acquisition. The seismic source
consisted of sledgehammer blows on an aluminum plate.
This source demonstrated good shot-to-shot repeatabil-
ity. Two 0.5 m long copper rods, spaced 2 m apart, at
0.5 and 2.5 m distance from and in-line with the shot
point, served as the two receiving electrodes. There are
therefore eight electrodes in total (4 dipoles, 2 on each
side of the shot point). This system was also used by
Kulessa et al. (2006). Since seismoelectric conversions
are focused within the first Fresnel zone of the down-
going seismic wave (see Chapter 4), the survey setup
used in this investigation is favorable to the detection
of the seismoelectric conversions.
The seismoelectric experiment was complemented by
two types of seismic surveys. Split-spread surface seismic
data were recorded simultaneously with the seismoelec-
tric data, using 24 (40 Hz) vertical geophones connected
to a Geometrics Geode seismograph. Geophone spacing
was 1 m, and the seismic array was centered on the
respective seismoelectric shot point, aligned parallel to
the axis spanned by the seismoelectric array at a distance
7.2 Case study: Sherwood sandstone
7.2.1 Experimental results
This case study describes work that was performed in a
sand quarry near the village of Great Heck, Yorkshire
(United Kingdom), by B. Kulessa and L. J. West.
A summary of previous works at this site can be found
in West and Truss (2006). This quarry was chosen
because the glacial till cover was absent, so that the top
of the 14.5 m thick vadose zone of the Sherwood Sand-
stone was directly accessible. Furthermore, spatially
highly resolved (0.25 m intervals), volumetric water
content-depth profiles down to 11.25 m were available
from a borehole using time-domain reflectometry
(TDR). Detailed lithological profiles derived from sand-
stone core and gamma-ray borehole measurements (West
& Truss, 2006, their Figure 5) were also available. The
vadose zone at this site consists mostly of horizontally
stratified deposits of fine- to coarse-grained sandstones
typically 0.3
,
ranges from ~0.15 to 0.33 (Figure 7.5). The existence
of these water content variations is facilitated by substan-
tial differences in the grain sizes of the sandstone units
and, as such, results in variations of the capillary entry
pressure (West & Truss, 2006) as a function of sandstone
unit composition. The volumetric water content shown
-
1.5 m thick. Volumetric water content,
θ
Water content (%)
SE 4
SE 3
SE 2
SE 1
1 0
20
30
BH -N
BH -S
0
2
4
6
8
10
12
14
VSP 2
VSP 1
5 m
Water table
Figure 7.5 Mean volumetric water content-depth profile in the vadose zone (left panel; from West & Truss, 2006) and present field
acquisition geometries (right panel). The four seismoelectric sounding locations (SE 1 -
SE 4 ) were complemented by VSP surveys using
borehole geophone spacing of 0.5 m down to the water table at 14.5 m. Hammer-and-plate shot points at VSP 1 (for geophone
deployment in the northern borehole (BH-N)) and VSP 2 (for geophone deployment in the southern borehole (BH-S)) were used
as the seismic source. Only selected geophone positions are indicated for figure clarity.
 
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