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

PART D
GEOPHYSICAL METHODS OF EXPLORATION
In thissectiona very briefoutli_en,illbe given o_ the geophysicalmethods
commonly employed in investigatingeological
problems so that the reader may have some idea as to what is
involved in making such measurements, what quantities are measured,
and how these measurements are interpreted_in terms of
geology. E_/amples of results obtained by some of}he geophysical
methods are included, showing both in p]an and l_rofile the type
of change that may be expected in crossing certaizt geologic
bodies. In pla n the results are generally contoured in much the
same way that topography is indicated on a map by lines of equal
elevation; that is, lines are drawn through all points of equal
value. For example, all points where the reading is zero are connected
and the same is done for the points where the value is ten
and similarly where the readings are twenty, etc., and thus the
areas of high and low values are indicated.
Another method of indicating these areas of change is by means
of gradient vectors: that is, by arrows whose lengths are proportional
to the increase in value observed, and which point in the
direction of the areas of high value.
_{AGNETIC
METHODS
Magnetic mess,lrements are largely dependent upon the earth
itself, which is essentially a large magnet with a north and south
/3o]e similar to an ordinary bar magnet. As a resu/t we find that
freely suspended small magnets align themselves parallel to the
lines of magnetic force surrounding the earth, trod this fact is
made use of in the compass. It is also a matter of observation
that non-magnetic bodies may become polarized as when magnetic
cranes are used in handling scrap iron whereby each piece of
scrap becomes a magnet itself, and that the field strength of the
earth is greatest at the poles just as in a b_r magl_et whose poiz_ts
of maximum attraction are at the ends, the poles, rather than on
the sides.
These conditions not only make many geologic features discoverable
through the phenomenon of induced magnetism but also
furnish the methods of finding them. Two of the terrestriaI magnetic
quantities which can be used in geologic investigations are
"declination", also known as "variation", and "iz_c}ination".
The first refers to the difference in degrees of azimuth between
the direction of magnetic north as indicated by a compass and
true north; and the second to the phenomenon of magnetic dip
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NEW JERSEY GEOLOGICAL SURVEY