Vertical Electrical
Soundings for Water Well Location in a
Complex Bedrock Environment, Diablo Range, California.
by Jeremy C. Wire
Introduction
General
Vertical electrical soundings, when combined with other geophysical methods, geologic
mapping, and available well data can greatly assist the location and completion of
successful water wells in bedrock areas of complex hydrogeology typical of the Diablo
Range in California. The vertical electrical sounding (VES) method is usually considered
more suitable for the subsurface investigation of geologic environments consisting of
horizontal or nearly horizontal layers, such as occur in unconsolidated sedimentary
sequences. Abrupt lateral changes in lithology and electrical properties brought about by
steeply dipping beds, fracture and fault zones, or highly variable thicknesses of
weathered bedrock materials can make interpretation of VES admittedly very difficult in
this type of geologic setting.
However, with appropriate field techniques in completing the soundings combined with
the application of appropriate geoelectrical and geological models, VES results can focus
test drilling at locations and to target depths which will result in successful water
wells, even at sites where "dry" or marginal wells were previously drilled. The
case history presented herein illustrates one typical successful application.
Site Investigation
Hydrogeologic Conditions
The 73-acre site is located in the foothills of the Diablo Range northeast of San Jose,
off Little Felter Road (Figure 1), at an elevation of about 2000 feet. Typical of the
area, the site is underlain by a variety of bedrock types assigned to the Franciscan
Assemblage, including sandstone (greywacke), chert, and a sheared sandstone and shale
unit, which we refer to as "melange". Where fractured, the greywacke and chert
can generally store and transmit ground water to wells in quantities sufficient for
domestic or stock water purposes; the "melange" unit because of its sheared
rock, with a high clay content and association with serpentinite usually contains little
or no ground water.
A segment of the Calaveras Fault Zone lies very close to the site; several associated
fault traces were suspected, based on regional geologic mapping, to extend across it.
Faulting usually creates subsurface "compartments" which affect storage and flow
of ground water through the bedrock units. The geologic structure in the area is complex,
with different rock units juxtaposed along individual fault traces. Examination of a
limited number of outcrops present in the parcel revealed complex joint and fracture
patterns in the greywacke and chert, with steeply dipping bedding.
Preliminary Geophysical Surveys
Prior to performing the VES, in order to locate any potential fault traces forming
compartments in the bedrock, several geophysical traverses were made with a ground
magnetometer and gamma-ray spectrometer at locations shown on Figure 1. The results of
these surveys will not be discussed here; sufficient to say that at least one fault trace
was interpreted, separating the site into two main compartments, each of which were
further explored by VES.
The geophysical signatures of the bedrock materials traversed by the surveys also
suggested that probable "melange" was present on the west side of the site, with
greywacke on the east side. Six VLF/EM soundings were also completed at selected
locations. Although such soundings do not penetrate as deeply as the VES, they serve to
characterize the bedrock electrically which helps to identify locations that are favorable
for deeper exploration by VES.
Vertical Electrical Soundings
The locations of the soundings are shown on Figure 1 and the sounding curves and
interpreted geoelectric models are contained in Figures 2 through 4. The soundings were
made using a Syscal R2 unit and the Schlumberger Array.
Current electrode spacings (AB/2) varied from 200 to 400 feet. It should be noted that the
geoelectrical models on the figures do not necessarily indicated the total depth of
penetration because the last layer may continue below the deepest point shown.
VES-1 was completed for correlation purposes near an existing old well which produced
some water from a reported shallow depth less than 200 feet, and which is located west of
a possible fault trace identified previously. Materials of 60 ohm-meters, probably
greywacke, are noted to a depth of about 160 feet, where rock of nearly 200 ohm-meters was
found Based on our experience in the area, this relatively high value suggests the
presence of generally in fractured rock, such as unaltered serpentinite, in a melange
setting. The high resistivity also suggests lack of saturation. Probably all the
production of the existing well comes from the materials of lower electrical resistivity
above 160 feet.
Sounding VES-2 was located to the southeast of VES-1, across the suspected fault trace.
This sounding shows materials of 50 to 80 ohm-meters, probably fractured greywacke, to the
total depth explored. The relatively steep ascending curve, however, suggests
"tight" formation. Although the materials are probably water-saturated, based on
their electrical resistivities, the sounding location was not considered further
(particularly in view of the results of VES-3) for a well location. We considered that
possible barrier effects from the adjacent fault trace might diminish the yield in time.
Sounding VES-3 was completed in the northeastern portion of the site, in an outcrop
area of fractured greywacke. Materials of very high resistivity, probably unsaturated
greywacke, extend to depths of about 110 feet, where materials of 80 to 90 ohm-meters
extend tot he total depth explored. These resistivities are in the proper range for
"production"; the flatter ascending curve when compared with that of VES-3
suggests that the rock mass may be more fractured, with a potentially higher degree of
saturation.
Results
Sounding VES-3 was recommended for further evaluation by test drilling. Subsequently, a
test hole was drilled to a depth of about 350 feet by air-rotary methods. The indicated
water production was favorable, so that 5-inch slotted casing was installed, gravel
packed, and sealed against surface contamination. Upon air-lift, the well produced an
estimated 30 gallons per minute (gpm). Long-term yield will probably be about one-half
this amount, or 15 gpm, which is high for the area and is more than sufficient for the
intended use.
Conclusions
This is one example of a successful water well location project which benefitted from
the application of hydrogeology and geophysical surveys, with particular emphasis on the
role of VES to characterize favorable subsurface environments for groundwater production.
In the complex bedrock geologic environment of the Diablo Range, the need for costly
random drilling, resulting in dry holes or marginal production from wells, can largely be
eliminated by the judicious application of these kinds of geological and geophysical
studies. |
