Geoscience Reference
In-Depth Information
of ~1 m. Test surveys confirmed that cross-talk between
the electrodes and the geophones and the cables was not
of concern. This innovative approach allowed us to
simultaneously record seismoelectric and seismic data
sets in response to the same hammer blow. Vertical seis-
mic profiles (VSP) of the vadose zone were also acquired
using a three-component Geostuff BHG-2 borehole geo-
phone with a hammer-and-plate source. The geophone
sonde was lowered at 0.5 m intervals down the northern
borehole (labeled BH-N in Figure 7.5), and the plate was
hit at the surface at a distance of 14.5 m immediately
adjacent to the southern borehole (shot point labeled
VSP
1
in Figure 7.5), and vice versa for geophone deploy-
ment in the southern borehole (BH-S) and shots adjacent
to the northern borehole (VSP
2
).
7.2.2 Results
The seismoelectric traces, recorded with the two dipoles
of electrodes located on either side of the shot point, typ-
ically had maximum absolute amplitudes of several tens
of millivolts (Figure 7.6a). Strong, longer-period compo-
nents commonly dominated the first 15
-
20 ms of the
measured signals, onto which many shorter-period,
inphase (e.g., between ~2 and 3, 5 and 6, and 10 and
11 ms in Figure 7.6a), and out-of-phase (e.g., between
11 and 20 ms in Figure 7.6a) components were superim-
posed. After ~35 ms a strong 50 Hz, powerline-related
noise dominated the signals until the end of the recording
period (Figure 7.6a). The 50 Hz noise component appears
to be inphase at the two dipoles on either side of the
shot point. The associated split-spread seismograms were
40
30
20
Dipole A
10
0
-10
-20
-30
Dipole B
-40
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
Time (ms)
(a)
Geophone Position (m)
-10
-5
5
10
0
20
Figure 7.6
Recorded seismoelectric and
seismic data.
a)
Raw seismoelectric
sounding data recorded at SE
3
(see
Figure 7.4) for electrodes A and B,
respectively, spanned by two electrodes at
distances of 0.5 m and 2.5 away from, and
on opposite sides of, the shot point.
b)
Corresponding split-spread seismogram
using 24 40 Hz vertical geophones spaced
1 m. The locations of the seismoelectric
electrodes are indicated by arrows, and
the shot point is indicated by a star.
40
60
80
100
(b)
