Potential Application of MEMs-Based Inertial Gravimetry to Planetary Geodesy and Geophysics.

Abstract:

Inertial gravimetry, the use of an inertial measurement unit (IMU) to conduct gravity surveys, represents an effective, low-cost method for probing the subsurface structure and gravity field of planetary bodies. Additionally, IMUs are more durable than traditional gravimeters in a wide variety of environments making them suitable for planetary exploration. However, a significant challenge is poor stability and instrument drift over long periods of time. While inertial surveys have been conducted for geodetic purposes terrestrially for nearly three decades; only recently was the first extraterrestrial survey performed by Lewis et.al. [2019] using the micro-electro-mechanical systems (MEMS) accelerometers within the IMUs on the Curiosity Rover. The use of MEMS-based IMUs could represent an even further cost reduction but introduces additional challenges, namely the relatively low signal-to-noise ratios of MEMS devices and strong temperature dependencies. To address these challenges, an instrument that utilizes a tri-axial MEMS IMU has been developed by Texas A&M University and NASA. The instrument, named HELIX, will serve as a proof-of-concept for the application of MEMS inertial systems in planetary geodesy and geophysics.

Calibration and noise reduction procedures performed at Johnson Space Center show promising results when compared to an interpolated gravity value based on National Geodetic Survey observations, with an average difference of 6 milligals. The raw data consists of 1Hz samples taken over a 16.75-hour period to assess long-term stability and temperature effects on the accelerometers. The raw data is subjected to a linear minimum variance unbiased estimator to solve for internal tilt-related bias and scale factor errors and is then averaged by five minutes. Non-linear least squares is used to model the time-dependent instrument drift and temperature effects. Finally, wavelet denoising is used to remove the remaining noise since wavelets excel at modeling the transient effects that characterize white and vibrational noise. This method will be applied to a survey of the High Island salt diapir and, if successful, will represent the first detection of a subsurface structure by a MEMS IMU and prove the viability of MEMS-based inertial gravimetry.