Marine Magnetic Surveying and Disturbance Field Monitoring by Autonomous Marine Vehicles

Abstract

Abstract Directional drilling uses measurement while drilling (MWD) to infer the inclination and azimuth of the drill bit. The accuracy of the measured azimuth critically depends on the local direction and strength of the geomagnetic field. Lateral uncertainties in the bottomhole location can typically be reduced by 50% if the local geomagnetic field is known accurately. This requires both the mapping of spatial crustal anomalies as well as the continuous monitoring of "space weather" disturbance field variations during drilling operations. Traditionally, these tasks are carried out by aircraft, ships and onshore geomagnetic observatories. A novel study employed autonomous marine vehicles for geomagnetic surveying. Two autonomous marine vehicles were equipped with towed magnetometers and were deployed for 2 months offshore the Hawaiian Islands. To investigate the accuracy of the measurements, the two vehicles surveyed exact-repeat profiles. The measurements of the two vehicles agreed to within 2nT, exceeding even the stringent 5nT standard for geomagnetic observatories. To avoid entangling the towed magnetometer, the autonomous vehicles had to be in continuous motion. They demonstrated their ability to circulate around a given target. This "base station" mode allows them to monitor the time variations of the disturbance field. After correcting for spatial gradients of the crustal field, the measured disturbance field variations were found to be in excellent agreement with the Honolulu observatory. In the final phase of the study, one of the vehicles acted as a base station, while the other surveyed a regular pattern. A total of 700 line kilometers were flawlessly surveyed by one vehicle within 14 days. The resulting crustal magnetic anomaly map agrees to within 10nT with previous, lower-quality ship-borne magnetic surveys. This study demonstrates the utility of autonomous marine vehicles to carry out crustal magnetic surveys and monitor disturbance fields in support of offshore directional drilling operations. These new capabilities improve well placement for increased oil and gas recovery. Accurate geomagnetic reference values further enable tighter quality control of MWD equipment, enhancing safety and reducing nonproductive time.