MEMS‐based 3‐C borehole gravimeter development

Abstract

A micro-electro-mechanical-based 3-C borehole gravimeter has been developed in China for mineral and hydrocarbon exploration. The 3-C borehole gravimeter is composed of a three-axis gravity sensor chip based on a 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. Silicon wafer is etched by the micro-nanofabrication technique to form a high collimation groove. The size of the gravity-detecting mass block in the sensitive element plays a decisive role in the thermal noise level of the instrument. Deep silicon processing technique can produce a thicker silicon mass block (500 μm), which can obtain a larger mass block in the same area compared with the traditional silicon surface processing technique (10-100 μm). The out diameter of the final tool is 50 mm with 10,000 mGal measurement range, 155˚C temperature and 100 MPa pressure rating. Apart from the 3-C micro-electro-mechanical gravity sensor, a 3-C fluxgate magnetic sensor is also integrated into the 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 the downhole 3-C gravity and 3-C magnetic data during the data processing stage. The prototype tool was tested in a borehole up to 850 m depth, and the 3-C gravity sensor inside the tool can measure the vibration of the environment during the stationary measurement downhole. The first test data set shows repeatability near 800 m depth, which verifies the adaptability of the micro-electro-mechanical-based gravimeter to the down instrument environment.