This paper describes a study conducted to develop new capabilities for offshore geomagnetic surveying. To conduct the research we equipped two (2) new generation autonomous marine vehicles (AMVs) with towed marine magnetometers. This paper describes the validation study of the measurement system offshore between 2013 and 2015 that compares a regular pattern with main, tie and perimeter lines.

It was investigated whether the new AMV was suitable for use as a "base station" to monitor time variations of the disturbance field. The geomagnetic data measured by the vehicles from May 11 - 14, 2015 was analyzed and compared against data obtained from the USGS Honolulu Geomagnetic Observatory (HON). Highly sensitive sensors were used to establish the minimum required separation between the AMV and the magnetic sensor payload.

Directional drilling requires accurate knowledge of the geomagnetic field direction and strength in the wellbore. To compute wellbore azimuth, the measurement while drilling (MWD) tool takes a measurement of the magnetic field and then the directional driller compares it with the geomagnetic field reference in order to position the wellbore in real-time while drilling. By utilizing the AMV we were able to accurately map the crustal field direction and strength in the area of interest where the drilling activity for oil and gas will take place.

This study shows that the AMV is ideally suited to carry out geomagnetic surveys in offshore remote locations. Correction for the disturbance field is essential for crustal field mapping that requires a base station recording the variations of the disturbance field. A second AMV circulating at a fixed location provides a more accurate base station than a land-based station. This is due to two factors: The land- based station is further away from the measurement site and the disturbance fields are different on land than in the ocean, due to the different electrical conductivity of the subsurface.

The new generation AMV offers distinct advantages for data collection in offshore environments. Their low cost compared to seaborne or airborne vehicle allow them to be deployed in tandem and collect data repeatedly over predefined areas yielding accurate measurements to determine the disturbance and crustal fields. It solves the problem of accurately mapping the geomagnetic reference field in offshore locations that previously unknown of.

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