System and method for monitoring a field

A method for monitoring a field under a body of water, using a system comprising:

• a reference station that does not move significantly horizontally or vertically with respect to a seafloor and having a known position and/or gravity;
• a plurality of permanent seafloor sensors, wherein each permanent seafloor sensor is fixed relative to a seafloor on or at the field, and each permanent seafloor sensor is associated with a unique drift function d(t) at least comprising a drift rate (a);
• and
• a nearby survey station associated with the permanent seafloor sensors, the nearby survey station does not move significantly horizontally or vertically with respect to the seafloor, and the nearby survey station is sufficiently distant from the permanent seafloor sensors to ensure that a movable sensor visiting the nearby survey station does not disturb measurements from the permanent seafloor sensors and sufficiently close to the permanent seafloor sensors to ensure that the offset (Δp, Δq) from a value provided by the permanent seafloor sensors is constant or can be modeled to account for changes in the pressure/depth relation due to changes in water density, wherein the reference station and nearby survey station are utilized in calibrating the permanent seafloor sensors, wherein the method comprises the steps of:
• conducting a calibration survey using a movable sensor to obtain a calibration value d for each of the permanent seafloor sensors on nearby survey station;
• computing a calibration difference for each of the permanent seafloor sensors, by subtracting a previous calibration value dT-1 from the calibration value d;
• updating the drift function for each of the permanent seafloor sensors;
• and
• adjusting each measurement value from each of the permanent seafloor sensors by a value d(to) provided by the associated drift function computed at the time to of measurement.