A prototype superconducting gravity gradiometer

We report the successful laboratory test of a single-axis gradiometer designed to measure a diagonal component of the earth\´s gravitational gradient tensor. It Consists of a pair of accelerometers mounted with their sensitive axes vertical and in line. The difference in displacement of the accelerometers is proportional to the component of the tensor gradient and is sensed via the modulated inductance of a superconducting coil coupled by a superconducting transformer into an RF biased SHE SQUID with energy sensitivity 4 × 10^-29J/Hz. Rejection of in-line common mode accelerations is achieved by trimming the natural resonant frequency of each accelerometer: the restoring force acting on an accelerometer test mass is partly magnetic and can be trimmed by adjusting the persistent currents in a pair of force coils. A common mode rejection ratio exceeding 95 dB has been achieved in the presence of linear accelerations ms^-2, and a laboratory generated gradient of 30 Eö rms has been detected with a signal to noise ratio of about 100. The dependence of this signal on the distance between source and detector has the expected Newtonian form. Under quiet conditions the background noise level of the instrument is at present 3 Eö/ . ( 1 Eö = 10^-9s^-2.) This is close to the practical limit achievable for such a single axis configuration: a three axis instrument for geophysical application is under development.