Abstract: A disclosed subsurface electromagnetic field monitoring system includes at least one fiberoptic cable to optically communicate measurements from an array of electromagnetic field sensors in a borehole.

Abstract: A disclosed system includes a light source and a nonlinear converter optically coupled to and remote from the light source. The nonlinear light converter converts a light pulse received from the light source to a broadened or spectrum-shifted light pulse. The system also includes a sensor in situ with the nonlinear light converter. The sensor performs a sense operation based on the broadened or spectrum-shifted light pulse and generates an electrical signal corresponding to the sense operation. The system also includes an electro-optical interface in situ with the sensor that transforms the electrical signal to an optical signal for conveyance to a signal collection interface A related method includes generating a light pulse and conveying the light pulse to a remote nonlinear light converter. The method also includes converting the light pulse to a broadened or spectrum-shifted light pulse.

Abstract: A method of measuring an electromagnetic field in a subterranean earth formation can include installing at least one electromagnetic sensor in a well, the sensor including an optical waveguide and a material, the material changing shape in response to exposure to the electromagnetic field, and strain in the optical waveguide changing in response to the material changing shape. A well system can include an optical electromagnetic sensor installed in a well, and a transmitter which induces an electromagnetic field in an earth formation. Strain is induced in an optical waveguide of the sensor in response to the electromagnetic field. A method of monitoring an earth formation can include installing an optical electromagnetic sensor in a wellbore which penetrates the formation, and a strain being induced in an optical waveguide of the sensor in response to the electromagnetic field.

Abstract: A method of measuring an electromagnetic field in a formation can include installing an electromagnetic sensor with improved sensitivity, the sensor including multiple optical waveguides and respective multiple materials, and in response to exposure to the electromagnetic field, the materials changing shape in opposite directions. A well system can include an optical electromagnetic sensor which measures an electromagnetic field in a formation, and wherein optical path lengths or phases in optical waveguides of the sensor change both positively and negatively in response to exposure to the electromagnetic field.