MEMS‐based 3‐C borehole gravimeter development

Abstract

AbstractA 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.