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 2
nT, 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 10 nT 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.
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