A sapphire monolithic differential accelerometer as core sensor for gravity gradiometric geophysical instrumentation

Abstract

Gradiometric gravimetry is a survey technique widely used in geological structure investigation. This work demonstrates the feasibility of a new class of low frequency accelerometers for geodynamics studies and space applications. We present the design features of a new low noise single-axis differential accelerometer; the sensor is suitable to be used in a Gravity Gradiometer (GG) system for land geophysical survey and gravity gradient measurements. A resolution of 1 Eötvös (1 Eö = 10-9s-2) at one sample per second is achievable in a compact, lightweight (less than 2 kg) portable instrument, operating at room temperature. The basic components of the sensor are two identical rigidly connected accelerometers separated by a 15-cm baseline vector and the useful signal is extracted as the subtraction of the two outputs, by means of an interferometric microwave readout system. The structure will be engraved in a monocrystal of sapphire by means of Computer-Numerically-Controlled (CNC ultrasonic machining: the material was chosen because of its unique mix of outstanding mechanical and dielectric properties.