Ultra-sensitive electrostatic planar acceleration gradiometer for airborne geophysical surveys

Abstract

We propose a new concept of gravity gradiometer, GREMLIT, for the determination of the spatial derivatives of gravitational acceleration during airborne surveys. The core of this instrument is the acceleration gradiometer composed of four ultra-sensitive electrostatic planar accelerometers, inheriting from technologies developed for the GRACE and GOCE satellite gravity missions. Data from these missions have greatly improved our knowledge of the Earth’s gravity field and its time variations. However, resolving wavelengths of a few 10 km or less, beyond the reach of the satellite resolution, is of utmost importance to study a number of crustal geophysical processes and geological structures. We first present the benefits for a new gravity gradiometer, then we describe the planar acceleration gradiometer, which put together with three orthogonal gyroscopes, constitutes the gravity gradiometer GREMLIT. The acceleration gradiometer enables measurement at one point of the horizontal spatial derivatives of the acceleration horizontal components. We explain the measurement principle and describe the computation of the gravity gradients along with the necessary ancillary measurements. From a detailed error budget analysis of the accelerometers, an expected spectral sensitivity below is found in the [10−3, 0.2] Hz measurement bandwidth. To maintain such performance in flight, we finally discuss the adaptation of the acceleration gradiometer to the turbulent airborne environment. To limit the saturation of the accelerometers, we propose to cancel the common-mode output of the acceleration gradiometer by integrating the instrument on a double-gimbal platform controlled by the common-mode. We demonstrate on a real case study that with such a solution, it is technically possible to prevent the saturation of the accelerometers at least 95% of the time and it is not damaging to the airborne survey.

New Matching Method for Accelerometers in Gravity Gradiometer

“…The first gravity gradiometer is the Loránd Eötvös’ torsion balance, which was designed by Hungarian physicist Baron von Eötvös. Although it is useful in oil and gas exploration, the use of this bulky terrestrial gradiometer has been practically phased out because of its low measurement efficiency [ 14 , 15 ]. Since the 1970s, many novel gravity gradiometers have been developed, such as: Gravity Field and Steady-State Ocean Circulation Explorer (GOCE), which is a satellite gravity gradient instrument launched by the European Space Agency.…”

“…It has three pairs of ultra-sensitive accelerometers mounted at the end-points of three orthogonal axes. The noise of these accelerometers is approximately 1–11 pm [ 14 , 16 ]. Full Tensor Gradiometer (FTG; based on rotating accelerometers), which was designed and manufactured by Lockheed Martin, is now widely used in hydrocarbon and mineral exploration [ 17 ].…”

“…The performance objective of the instrument is [ 21 , 22 ]. GREMLIT (French Aerospace Lab, Palaiseau, France), a compact planar gravity gradiometer based on four ultra-sensitive electrostatic planar accelerometers (inherited from technologies specifically developed for the GOCE and Gravity Recovery and Climate Experiment missions), was designed by the Onera-French Aerospace Lab and is used for airborne surveys [ 14 ]. AI (Earth Observation Programmes, European Space Agency, Noordwijk, the Netherlands) are gravity gradiometers that are based on cold-atom interferometry-based accelerometers and used to measure all diagonal elements of the gravity gradient tensor.…”

“…GREMLIT (French Aerospace Lab, Palaiseau, France), a compact planar gravity gradiometer based on four ultra-sensitive electrostatic planar accelerometers (inherited from technologies specifically developed for the GOCE and Gravity Recovery and Climate Experiment missions), was designed by the Onera-French Aerospace Lab and is used for airborne surveys [ 14 ].…”

A seesaw-lever force-balancing suspension design for space and terrestrial gravity-gradient sensing

“…16 Recently, an airborne gravity gradiometer based on the GOCE accelerometers was introduced. GREMLIT 17 has four differential accelerometers providing a noise floor of 1 E= ffiffiffiffiffiffi Hz p , demonstrating directly a performance degradation of approximately 250 in accommodating terrestrial gravity on a mobile platform.…”