The AT1M Sensor

The AT1M sensor is a scalar gravity /specific force sensor mounted in a 2-axis gyro stable platform that
keeps the sensitive axis of the gravity meter aligned with the local gravity vector. Only the magnitude of
this vector is recorded on the data file.
The main sensor basic principle is the suspension of a “Gravitational sensing mass” by a very stable and
low temperature sensitivity metal spring. The small variation of local gravity is measured by measuring
the current of a Lorentz force actuator which together with position detector keeps the sensing and
spring at a constant length.
The instrument measures specific force acting on the sensitive axis of the instrument. During post
processing all the kinematic effects need to be removed from the measurement. In the most Marine
surveys, the short vertical accelerations can be neglected because the average vertical position tends to
be constant after gravity data is filtered with relativity long filter times, since the sea level tends to be
constant.
Therefore, the only kinematic correction applied during the processing is the Eotvos correction. The
instrument has an arbitrary offset with respect to the full field gravity or the value that we will measure
with an absolute gravity meter in the same location. This offset can be easily determined by occupation
of location with known absolute gravity values.
The sensor has a relatively low drift rate, in the order of 2 mGal a month and below of 0.2 mGal / month
after aging. This drift is very lineal and can be removed by re occupation of gravity locations during the
survey.
The gravity meter is positioned perfectly aligned to local level to guaranty that the sensor measures the
local full gravity vector. This continuous alignment is performed by a 2 axis the gyro stabilized platform.
The platform uses only the accelerometers outputs. The mechanical gimbal together with gyros, torque
motor and accelerometers behave as if the gravity meter were hanging from a pendulum with long
period. This long period platform attenuates the tilt response of the gimbal and gravity meter when the
ship changes velocity or heading. If the ship is traveling at constant speed in straight line or turning with
a big radius, the gravity meter will be aligned with the local gravity level vector and no level correction
will be necessary. Only when the ship is experiencing long period velocity or heading variations, the gyro
stable platform will experience meaningful deviations from the local vertical, and in this situation the
measurement axis of the gravity meter will be miss aligned with the local gravity vector. A number of
corrections can be used to compensate for the unmeasured fraction of the local gravity vector.