Measuring acceleration using interferometry with reduced environmental effects

A downhole apparatus for measuring acceleration, the downhole apparatus comprising:

• a carrier configured to be conveyed through a borehole penetrating the earth;
• a control volume disposed on the carrier;
• a reference cavity disposed in the control volume and comprising an optical medium and a first fixed reflecting surface and a second fixed reflecting surface disposed a distance dREF from the first fixed reflecting surface;
• a sense cavity disposed in the control volume and comprising the optical medium and a fixed reflecting surface and a non-fixed reflecting surface disposed a distance dSENSE from the fixed reflecting surface, the non-fixed reflecting surface being configured to be displaced when subject to an acceleration force;
• a laser light source configured to illuminate the reference cavity and the sense cavity;
• a photodetector configured to detect light emitted by the reference cavity and the sense cavity;
• a gas cell configured to provide a reference wavelength of light to the laser light source;
• a master clock configured to output a master time signal;
• a synchronous controller configured to (a) vary a wavelength of light emitted by the light source and/or an index of refraction of the optical medium and (b) synchronize with the master time signal;
• a plurality of reference sensors configured to sense properties of the control volume;
• a calibrator configured to apply a stimulus of known magnitude to the sense cavity that results in a change in dSENSE for providing a calibration response;
• an interferometer sensor configured to measure using the light detected by the photodetector, for each variation of the wavelength of light emitted by the light source and/or the index of refraction of the optical medium, (a) a reference displacement of the first fixed reflecting surface with respect to the second fixed reflecting surface in the reference cavity and (b) a sense displacement of the fixed reflecting surface with respect to the non-fixed reflecting surface in the sense cavity;
• a synchronous processor configured to (a) synchronize with the master time signal, (b) calculate the acceleration using each of the first displacements and the second displacements, the properties sensed by the reference sensors, and the calibration response;
• memory configured to store the calculated acceleration;
• and
• a communication device configured to transmit the calculated acceleration stored in the memory uphole towards the surface of the earth.

A method for measuring acceleration, the method comprising:

• using a reference cavity comprising an optical medium and a first fixed reflecting surface and a second fixed reflecting surface disposed a distance dREF from the first fixed reflecting surface;
• using a sense cavity comprising the optical medium and a fixed reflecting surface and a non-fixed reflecting surface disposed a distance dSENSE from the fixed reflecting surface, the non-fixed reflecting surface being configured to be displaced when subject to an acceleration force, wherein the reference cavity and the sense cavity are disposed in a same environment;
• illuminating the reference cavity and the sense cavity with a light source that is configured to emit light at one or more wavelengths;
• varying a wavelength of light emitted by the light source and/or an index of refraction of the optical medium with a controller;
• detecting light emitted by the reference cavity and the sense cavity using a photodetector;
• measuring with an interferometer sensor using the light detected by the photodetector, for each variation of the wavelength of light emitted by the light source and/or the index of refraction of the optical medium, (a) a reference displacement of the first fixed reflecting surface with respect to the second fixed reflecting surface in the reference cavity and (b) a sense displacement of the fixed reflecting surface with respect to the non-fixed reflecting surface in the sense cavity;
• and
• calculating with a processor the acceleration using each of the reference displacements and the sense displacements.