An optomechanical MEMS geophone with a 2.5 ng/Hz noise floor for oil/gas exploration.

High-precision geophones play crucial roles in terrestrial applications such as oil and gas exploration as well as seismic monitoring. The development of optomechanical precision measurements provides a new design method for geophones, offering higher sensitivity and smaller dimensions compared to traditional geophones. In this work, we introduce an optomechanical microelectromechanical system (MEMS) geophone based on a plano-concave Fabry‒Perot (F-P) microcavity, which has a high sensitivity of 146 V/g. The F‒P microcavity consists of a movable mirror on the sensing element and a fixed hemispherical micromirror fabricated from silicon-on-insulator (SOI) and monocrystalline silicon wafers, respectively. The experimental results show that the geophone has a low noise floor of 2.5 ng/Hz (with a displacement noise floor of 6.2 fm/Hz) within the frequency range of 100~200 Hz, a broad bandwidth of 500 Hz (-3 dB), and a measurement range of ±4 mg. To mitigate common-mode noise originating from the laser source and environmental factors such as temperature and air fluctuations, a balanced detection method is employed. This method substantially reduces the noise floor, nearly reaching the thermal noise limit (2.5 ng/Hz). Furthermore, a compactly packaged optomechanical MEMS geophone with a diameter of 40 mm is demonstrated. The high performance and robust features hold great potential for applications in oil and gas exploration.