Declination compensation for seismic survey

A programmable storage device having program instructions stored thereon for causing a programmable control device to perform a method of ice threat monitoring method for a target marine structure, the method comprising:

• calibrating a declinometer on a vessel in an observed magnetic field of a survey area by compensating for atmospheric magnetic variation in the observed magnetic field of the survey area and determining distortion in geomagnetic calibration measurements of the observed magnetic field caused by soft iron effect of the vessel;
• obtaining one or more seismic signals over a temporal period and over a spatial region with one or more seismic sensors in a seismic survey of the survey area by towing at least one streamer having the one or more seismic sensors behind the vessel in a traverse;
• obtaining one or more local geomagnetic field measurements relative to the one or more seismic sensors, the one or more local geomagnetic field measurements subject to variations in declination over the temporal period and over the spatial region;
• measuring temporal and spatial declination parameters by obtaining vessel measurements, including position measurements and geomagnetic field measurements, at the vessel using the calibrated declinometer and determining, from the vessel measurements, the variations in the declination at the vessel over the temporal period and over the spatial region during the seismic survey;
• compensating, in the determined variations, at least for the soft iron effect of the vessel by correcting for the determined distortion in the geomagnetic field measurements obtained at the vessel;
• correcting, over the temporal period and over the spatial region, the one or more local geomagnetic field measurements based on the temporal and spatial declination parameters compensated at least for the soft iron effect;
• and
• correlating, over the temporal period and over the spatial region, the one or more corrected geomagnetic field measurements with the one or more seismic signals.

A seismic data acquisition apparatus for a vessel towing a seismic streamer in a survey area behind the vessel in a seismic survey, the apparatus comprising:

• at least one seismic sensor towed in a traverse behind the vessel on the seismic streamer and measuring one or more seismic signals in the survey area over a temporal period and over a spatial region;
• at least one geomagnetic heading device associated with the at least one seismic sensor and towed behind the vessel on the seismic streamer, the at least one geomagnetic heading device making one or more local geomagnetic field measurements, the one or more local geomagnetic field measurements subject to variations in declination over the temporal period and over the spatial region;
• a declinometer having a positioning device and at least one magnetometer disposed on the vessel, the positioning device obtaining position measurements at the vessel, the at least one magnetometer making magnetometer measurements at the vessel;
• a controller operatively coupled to the at least one seismic sensor, the at least one geomagnetic heading device, and the declinometer having the positioning device and the at least one magnetometer, the controller configured to:
• compensate for atmospheric magnetic variation in the observed magnetic field of the survey area to calibrate the declinometer in a survey area;
• determine distortion in geomagnetic calibration measurements of an observed magnetic field caused by soft iron effect of the vessel to calibrate the declinometer in the survey area;
• determine temporal and spatial declination parameters from the variations in the declination at the vessel over the temporal period and over the spatial region based on the position measurements and the magnetometer measurements of the calibrated declinometer at the vessel;
• correct for the determined distortion in the geomagnetic field measurements to compensate, in the determined variations, at least for the soft iron effect of the vessel on the geomagnetic field measurements obtained at the vessel;
• correct the one or more local geomagnetic field measurements over the temporal period and over the spatial region based on the declination parameters compensated at least for the soft iron effect;
• and
• correlate the one or more corrected geomagnetic field measurements over the temporal period and over the spatial region with the one or more seismic signals.