Systems and Methods for Downhole Magnetic Field Measurement

A downhole electromagnetic field measurement system that includes:

• a first downhole micro-opto-electro-mechanical system (MOEMS) electromagnetic field sensor that comprises:
• a first surface that is at least partially reflective;
• a second surface that is at least partially reflective, said second surface suspended by one or more flexible members to define an optical cavity having a variable distance between the first and second surfaces;
• and
• a magnetically polarized layer attached to the second surface, wherein a magnetic field passing through the optical cavity interacts with the magnetically polarized layer and a gradient in the magnetic field produces a displacement of the second surface that alters the variable distance and causes a spectrum variation in light exiting the sensor.
• a transmitter that induces an electromagnetic field within a formation surrounding the first sensor, wherein variations in the magnetic field gradient correspond to variations in the electromagnetic field;
• and
• a computer that processes optical signals received from the first sensor to measure said electromagnetic field and to derive a formation property based at least in part on said electromagnetic field.

A method for measuring an electromagnetic field using one or more downhole micro-opto-electro-mechanical system (MOEMS) electromagnetic field sensor, the method including:

• inducing an electromagnetic field into a formation surrounding a borehole;
• exposing a magnetically polarized layer to a magnetic field, the magnetically polarized layer being attached to one of two surfaces that are at least partially reflective, and the two surfaces defining an optical cavity within a first sensor having a variable distance between the two surfaces;
• directing to the borehole surface the beam of light exiting the first sensor;
• detecting a spectrum variation in the light exiting the first sensor, the magnetic field interacting with the magnetically polarized layer and a gradient in the magnetic field causing a displacement of the one of two surfaces to alter the variable distance and cause the spectrum variation;
• and
• processing a first signal representing the detected spectrum variation to measure said electromagnetic field and to derive a formation property based at least in part on said electromagnetic field.