RE:plane is moving ???I would be surprised this is the gravity survey in action. The elevation changes through the flight line and it's too high.
Generally, surveys of this sort are flown close to the ground, such as 100, 200m elevation. The higher the elevation the weaker the signal, so there's always an effort to do the survey as close to the ground as possible. The plane speed, topography, cultural aspects, etc will control the elevation, but it will be close to the ground.