Fiber optic nuclear magnetic resonance sensor

A nuclear magnetic resonance (NMR) sensor device, comprising:

• a permanent magnet that generates a static magnetic field, the magnet having a north pole and opposing south pole;
• a transmit antenna structure to transmit a time-varying magnetic field in a sensitive volume, the transmit antenna structure comprising an RF magnetic field radiation apparatus;
• a receive sensor comprising a sensitive fiber optic sensor element having a coating to phase modulate or amplitude modulate a light signal in the element in response to amplitude or frequency variations in an echo signal resulting from the time-varying magnetic field, the sensitive fiber optic sensor element having a first sensor element and a second sensor element;
• wherein the first sensor element is located adjacent the north pole of the magnet and the second sensor element is located adjacent the south pole of the magnet, and
• an optical receiver coupled to the receive sensor, the optical receiver configured to provide a modulated electrical signal representative of the phase modulated light signal.

A borehole inspection system, comprising:

• a downhole tool housing including a fiber optic nuclear magnetic resonance (NMR) sensor tool, the sensor tool comprising:
• a magnet that generates a static magnetic field, the magnet having a north pole and opposing south pole;
• a transmit antenna configured to transmit a time-varying magnetic field that is orthogonal to the static magnetic field;
• and
• a sensitive fiber optic sensor element having a coating configured to phase or amplitude modulate a light signal in the element in response to amplitude and/or frequency variations in a reflected signal resulting from the time-varying magnetic field, the sensitive fiber optic sensor element having a first sensor element and a second sensor element, wherein the first sensor element is located adjacent the north pole of the magnet and the second sensor element is located adjacent the south pole of the magnet.

A method for making nuclear magnetic resonance (NMR) measurements, the method comprising:

• generating a static magnetic field using a magnet having a north pole and opposing south pole;
• transmitting a time-variant radio-frequency signal into a geological formation;
• receiving a reflected time-variant signal from the geological formation;
• and
• modulating light in a fiber optic sensor element exposed to the reflected time-variant signal, in response to the reflected time-variant signal, the modulated light signal indicative of characteristics of the geological formation,
• wherein the fiber optic sensor element has a first sensor element and a second sensor element, the first sensor element is located adjacent the north pole of the magnet and the second sensor element is located adjacent the south pole of the magnet.