Better picture of what lies beneath the Earth's surface

Geologists could soon get a better picture of what lies beneath the Earth’s surface using a tool which measures minute changes in the planet’s gravity field from the air. Cambridge-based ARKeX, a geophysical service company, is developing an exploration gravity gradiometer (EGG) which, it says, can be mounted on aircraft or ships and will be more sensitive than existing gradiometers. It will also provide a cheaper alternative to seismic surveying.
Dr. John Lumley, ARKeX chief technology officer, told the Engineer that “EGG is a superconducting tool which works at 4º above absolute zero (-269º C). It consists of two superconducting proof masses [predetermined reference masses], which are levitated by the magnetic force generated by currents in superconducting coils. These form part of closed superconducting loops. The magnetic levitation height is determined by the strength of the external gravity field and essentially what you’re looking at is the small changes in levitation height as you travel across the Earth’s surface, which gives you a measure of the gravity field.”

The tool contains two superconducting proof masses rather than one because while a single mass would detect the motion of an aircraft and the change in gravity field, two would each experience the same acceleration due to the plane but see slightly different gravitational fields from the Earth. It is this difference in gravity field changes that is measured. The EGG therefore measures how fast the gravity is changing as a function of position across the Earth, rather than the gravity coming from the Earth, and the results are displayed as a set of voltages as a function of time.
ARKeX exploited two basic principles of physics in EGG’s development — the Meissner effect, which prevents any magnetic field getting inside the superconductor, and magnetic flux quantisation, a phenomenon whereby a current goes around the superconducting loop without ever decaying. “The main advantage of the superconducting system is those currents stay forever so the magnetic field generated by these currents stay absolutely stable at all times. The only time they will change is when the gravity is changing, causing the matters to bounce up and down on them. What this means is that this system is incredibly stable. The low noise and high stability is the main advantage of working at low temperatures,” said Lumley.

The device itself is in a cryostat, which Lumley said was essentially a vacuum flask containing liquid helium, which is the cooling agent used to get the temperature down to 4º Kelvin. The EGG sits in a vacuum and is thermally coupled to this helium, which is held in the cryostat, which in turn is held in a framework that forms part of what is known as a stabilized platform. “As the EGG travels across the Earth’s surface, it is important for its operation that it is maintained to be pointing vertical at all stages, and the stabilised platform does that. Otherwise it generates a noise signal which is enormous,” he said. The whole assembly, which weighs about 400 kg, can be used in a low-flying aircraft or on a boat, and measures the gravity gradient field of wherever it is, be it over land or on the sea. “When it is on the sea, effectively it would be measuring the gravity field from the water, from the sand, from the rocks below the sand. If there is any oil or gas below that it will measure the field from all of those objects,” said Lumley. He did, however, admit that in a marine application, the EGG would be better used as a complementary dataset to surveying techniques such as seismic.

Since the device can also be flown, Lumley said one of its benefit is its ease of accessibility over a wide range of terrain compared with existing methods. “Seismic, particularly overland, is incredibly expensive and is bedevilled with all sorts of ground trespassing. An aircraft does not have those trespass issues. Also, if you are trying to do this over very swampy land, it is usually impossible to get any survey technique in at all,” he said.
Last month the company raised more than £15 million during its third round of funding. The investment was led by Oslo’s Ferd Venture and included existing investors Energy Ventures, Scottish Equity Partners, and members of ARKeX senior management.