Mems Based 3-C Borehole Gravity Meter Development

A MEMS (Micro-Electro-Mechanical) based 3-C borehole gravity meter is being developed in China for mineral and hydrocarbon exploration. The 3-C borehole gravity meter is composed of a three-axis gravity sensor chip based on deep silicon etching technique, high precision capacitive displacement sensing and weak signal detection circuitry. The gravity sensing chip is a silicon-based integrated spring-mass block system. The silicon wafer is etched by micro-nanofabrication technique to form a high collimation groove. The size of the gravity detecting mass block in the sensitive unit plays a decisive role in the thermal noise level of the instrument. Deep silicon processing technique can produce thicker silicon mass block (500 µm), which can obtain larger mass block in the same area compared with traditional silicon surface processing technique (10–100 µm). The out diameter of the final tool will be 50 mm with 5 μGal resolution, 20 μGal repeatability, 10,000 mGal measurement range, 155℃ temperature and 100 MPa pressure rating. Apart from 3-C MEMS gravity sensor, a 3-C fluxgate magnetic sensor is also added to downhole tool. This allows us to measure both 3-C gravity field and 3-C magnetic field downhole simultaneously, and conduct joint inversion of both downhole gravity and magnetic data.