Bulletin 54. Geophysical Methods of Exploration and their ...

GEOPHYSICAL INVESTIGATIONS 5l
the North Branch of Raritan River in the district southwest of
Mendham ; one was on the South Branch of Raritan River in the
vicinity of Califon; and the fifth was on Museonetcong River
southeast of the town of Washington.
The purpose of the surveys was to determine the topography of
the rock floor which at each of the aforementioned sites is concealed
by an overburden of mmonsolidated materials. Because of
the marked difference in porosity, density and other physical
characteristics between the overburden and the underlying ancie_t,
cmnpaet rock, it was believed that electrical methods of
prospecting could be used with success.
The prospecting method adopted was the electric potential
method in which two metallic stakes are driven into the ground,
a current is forced through the ground between them, and the
position of the equipotential curves is mapped.
Results of the work were apparently satisfactory in part and
were summarized in a report from which the following statement
relative to the _Ternoy Site near Califon, Hunterdon County, is
quoted :
"Vcraoy Site
• West of ststio_s 98, ]09 and 118 the overburden is conductive
(400-800 ohms) and the bed rock resistive. The electrical diagrams
are regular and easy to interpret. This area may be considered
as having been studied quite in detail.
Further east, determinations 132 and 133 are also satisfactory,
but measurements 108, 24, 25, 26, 27 and 28 are on much less
regular ground, so that the values given for the depth to bed rock
might be modified, somewhat, were additimml work to be carried
out in the valley on the prolongation of llne 2+50 East, and to
the south of this line. Such additional work would have been
desirable, but was not performed due to lack of time, only."
The foregoing statement is quoted in full because the Vernoy
Site was subsequently core-drilled and it was found that bed rock
was very much deeper thau had been anticipated from the electrical
prospecting. This• failure tan probably be attributed to
hurried work (see above), and the use of a technique not well
adapted to the problem in hand whose results at best could only
give a qualitative estimate on depth. If the work had been done
a few years later it is certain that the electrical resistivity method
would have been used rather than the equipotential one tried. If
done now, it is probable that a seismic investigation would be
made,
NEW JERSEY GEOLOGICAL SURVEY