PROSPECTING for oil, gas and mineral deposits should become a less
hit-and-miss affair thanks to an accurate and robust instrument that can spot
the valuable resources by their impact on the Earth鈥檚 gravitational field.
Most gravity sensors, or gravimeters, use combinations of masses, balances
and accelerometers to pick up the tiny changes in gravity caused by the
variations in the densities of the rock below. But these instruments cannot
distinguish between acceleration and gravity, making it difficult to use them on
the move. A more promising approach is to measure how the strength of the
gravitational field changes with distance
(鈥淪eeing with gravity,鈥� New 杏吧原创, 14 September 1996, p 24),
using a sensitive instrument known as a gradiometer.
Alexey Veryaskin, a physicist working in New Zealand, has developed a
gradiometer that is built round a superconducting string of atoms kept supercool
by liquid helium. Changes in the gravitational gradient along the string, which
is suspended vertically, cause tiny deformations. The deformations show up as
tiny changes in the string鈥檚 magnetic field, which are detected by an
exquisitely sensitive magnetic detector known as a SQUID鈥攁 superconducting
quantum interference device. To eliminate any electromagnetic interference, the
string is contained within a superconducting tube.
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The detector in Veryaskin鈥檚 gradiometer is incredibly sensitive, picking up
displacements of 10-13 metres鈥攆ar smaller than the width of an atom. It
is also easier to isolate the string from the effects of motion, which can
confuse other types of gradiometer. Veryaskin says the sensor can separate
changes in gravity from perturbations caused by acceleration because the string
is measuring gradients along its length.
The first major use of the sensor will be for geological surveys. A Veryaskin
gradiometer lowered down a borehole would determine the density of the
surrounding rock, indicating what mineral deposits are present. As Veryaskin鈥檚
device is only 20 centimetres long and largely immune to inertial forces, it can
be used on the move without having to be constantly recalibrated. An airborne
survey could map the Earth鈥檚 gravitational field over large areas, and indicate
oil or mineral deposits far more quickly and cheaply than seismic methods.
Veryaskin鈥檚 company, Gravitech Instruments, expects to have a prototype of
the gradiometer within six months. 鈥淥nce you鈥檝e got a tool this sensitive,鈥� says
Simon Frasier of Gravitech, 鈥渢here are any number of things you can do with
颈迟.鈥�
The sensor could, for example, be used to pinpoint a submarine鈥檚 location by
taking a precise gravity reading and comparing this to a map of the Earth鈥檚
field, or to detect buried land mines, or search for archaeological remains.