Drilling for gravity

日期:2019-02-26 09:01:01 作者:康郇 阅读:

By Chris Watkins 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’s 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 (“Seeing with gravity,” New Scientist, 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’s magnetic field, which are detected by an exquisitely sensitive magnetic detector known as a SQUID—a superconducting quantum interference device. To eliminate any electromagnetic interference, the string is contained within a superconducting tube. The detector in Veryaskin’s gradiometer is incredibly sensitive, picking up displacements of 10-13 metres—far 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’s 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’s gravitational field over large areas, and indicate oil or mineral deposits far more quickly and cheaply than seismic methods. Veryaskin’s company, Gravitech Instruments, expects to have a prototype of the gradiometer within six months. “Once you’ve got a tool this sensitive,” says Simon Frasier of Gravitech, “there are any number of things you can do with it.” The sensor could, for example, be used to pinpoint a submarine’s location by taking a precise gravity reading and comparing this to a map of the Earth’s field, or to detect buried land mines,