Lyndon Tiu said:
>the ground directly below a big cave would have less gravity because gravity
>is related to mass and if you have a void, there will be less gravity. You
>weigh less (just a very tiny bit) standing on the surface above a big cave.
>You can measure this slight change in gravity to determine what could
>(speculative) be underneath the surface.
Long ago, back in the '70s, I wrote an article on this very effect and
its usefulness to cavers. I've copied it below for your enlightenment. ;-)
Mark Minton
THE BOVINE FLUX THEORY OF CAVE LOCATION
It has been well known among certain members of our Grotto that the
course of subterranean passages could be ascertained by observing the grazing
patterns of the local fauna, generally cattle, although any large species of
animal would do. Not only is this phenomenon useful in locating caves, but in
seeing how they relate to surface features and in predicting connections and
new entrances. The animals needn't actually be present, in fact, to take
advantage of this unique geologic aid. The areas most heavily grazed may be
obvious amid the less cropped flora, and the tell-tale dung piles are another
evidence (the cow pie are squared corollary, but I digress...).
The bovine flux theory, then, states that cattle tend to align
themselves above cave passages rather than above "solid" ground. Sound
scientific facts are responsible for this behavior. To wit, the force of
gravity is directly proportional to the mass of the object exerting said
gravity, here the earth. Now since a cavern is essentially a void in the earth
(air has a density of only 1.3 kg/m3 compared to 2.75 x 103 kg/m3 for
limestone) it follows that the gravity above a cave passage will be slightly
less than above "solid" ground, and hence less effort is required by the cow to
support its now decreased weight if it stands above the cave. (Even water, with
a density of 1x103 kg/m3, will give this effect, but to a much smaller extent
than with air filled passage.)
To calculate the magnitude of this effect, we call upon a little
elementary physics:
F = GM1M2 / R2
where F is the force due to gravity (weight), G is the universal gravitational
constant, M1 and M2 are the masses of the object and the body attracting it
(here cow and earth) and R is the distance between them. We see immediately
that the deeper the cavern lies below the surface, the smaller the effect. (In
going from 20 m to 80 m below the surface, the effect diminishes 94%.) Thus
only passages fairly near the surface will show significant bovine
correlation. It is also obvious that a large animal is required since as M1
decreases, the effect decreases. Thus man is, unfortunately (or fortunately -
could you see a cow caving!), not heavy enough (physically, not
philosophically) to perceive these small differences in gravity. For an
average, ball-park feeling of this effect, let us use 460 kg as the mass of a
cow. For our cave let us use 2 m by 1 m cross sectional area, and a segment 2 m
long (average length of a cow). This is a volume of 4 m3, or 11,000 kg of
limestone. Obviously larger passages will show higher correlation and smaller
passages lower. The effect of the above void beneath the cow at a depth of 50 m
below the surface results in a reduction of 1.35 x 10-7 newtons of force,
equivalent to a reduction of 1.38 x 10-8 kg in mass.
Exactly how a cow can distinguish this minute difference (only 3
billionths of a percent of its mass) is at present unknown, but is being
pursued by members of the scientific community. The many variables presented
(depth below the surface, passage dimensions, air- or water-filled) as well as
the effects of the various rock strata with their differing densities, all lead
to a relatively low correlation and reliability for this method. However, when
used in connection with other data, the bovine flux theory of cave location can
be a valuable scientific aid. And in more ideal locations, these variations can
be turned to advantage to predict yet undiscovered rooms, siphons, lakes, pits,
upper and lower levels, sandstone and mud fills, etc.
Perhaps if there is sufficient interest, a future paper will deal
with another valuable but little known item, the surface penetration theory of
cave extension, the use of which can lead to previously unknown connections and
correlations between seemingly unrelated systems.
