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

GEOPHYSICAL INVESTIGATIONS 53
resistivity values for the regions, and from them it is seen that
certain lbaitations ill the use of the resistivity method are indicated
in the respective areas. For example, ill the Coastal Plain it
should be possible to locate a Triassic dike beneath the surface
due to the marked chauge in resistivity between the diabase and
the Triassic sediments, but the method would not be feasible if
the diabase were inthe basement crystalline rock since the diabase
would have about the same resistivity as the surrounding
rock.
In this connection it may be noted that the areas of high resistivity
are characterized by marked communication disturbances
during periods of sun spot activity. At such times high voltages
are found to be induced in the telegraph and telephone lines to
such an exteut that the lines are put out of service and frequently
equipment is damaged. The explanatiou of this phenomenon is
that the marked changes in the earth's magnetic field which
accompany these periods of extra-terrestrial activity induce large
currents in the ground which serves as a surface conductor. In
regions of high surface conductivity such as the Coastal Plai_ no
damage results since little resistance is offered to the flow of cul'-
rent. Ill the Highlauds, however, where the resistivity is high,
the currents are forced through high-resistance material of poor
conductivity with the result that high differences in potential are
obt_dned (as nmch as 14 volts per mile) aud telegraph and transmission
lilies have induced ill them the accumulated voltage
between the points where they are grounded. As these points may
be twenty or more miles apart this potential may amount to several
hundred volts in a section of line, and it is these voltages
that cause the damage. As a result of the correlations made with
surface geology, both the telephone aL_d power companies know "
in which areas they are likely to have the most trouble and keep
extra repair crews in those areas.
Another factor effecting transmission line service is frequency
of lightning discharge. This, however, appears to be a topographic
control on the whole_ rather than a geologic one. This
latter type of disturbance affects the magnetic field in the discharge
area due to the secondary magnetic fields accompanying
the lightning discharge. This complicates the geological interpretation
of magnetic investigations ill such areas Since the polarization
will not be that induced by the earth's field but rather
that due to the lightning.
No other geophysical work is known to have been done in the
Highlands area and it is doubtful if other methods would give
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