Abstract
Gravity gradiometry is undergoing a renaissance. Gravity
gradients measured by the Eotvos torsion balance were an
important tool for geophysical prospecting from the early
nineteen hundreds to the nineteen thirties. The development
of the gravimeter led to the abandonment of the torsion
balance which, although it provided very accurate
measurements, was tedious to use and could not be used at
sea..
During the last few years, a number of gradiometers have
been developed, At least one manufacturer has an instrument
which is very accurate, makes measurements relatively
quickly and has been successfully deployed on land, at sea,
and in aircraft. The instrument consists basically of pairs
of accelerometers, the difference of reading between them
eliminates inertial accelerations but records gravity
gradients. It thus has the advantage that it is easily used
on moving platforms. In addition it can be shown that the
gradiometer is considerably superior to the gravimeter in
recording signals with wavelengths under about 12
kilometers.
The current research project has two foci. One focus is
to investigate changes in gravity gradients with time in
heavy oil fields. In these fields when steam is injected to
make the oil less viscous and drive it towards producing
wells, an important problem is to determine the location of
the steam oil interface. Tine lapse measurements are
expected to be able to trace the interface.
The second focus relates to developing methodology for
inverting and interpreting gravity gradients. The gravity
vector has three components and since each component can
vary in three orthogonal directions, there is a total of
nine gradient components, of which five are independent.
Figure 1 shows synthetic computation of gradients over an
irregularly shaped hill. With the exception of the vertical
gradient Uzz, none of the gradients show an obvious spatial
relationship with a hill. A non linear inversion scheme has
been devised which can invert the various gradient
components to recover the topography of the hill (Figure 2).
Investigations and enhancements related to the inversion
scheme are the focus of ongoing research.
It is likely that airborne gradiometry will become the
gravity measuring device of choice in the future for a wide
range of applications, and future research in the use of
this technology could be very rewarding.
|
