Abstract: Methods and systems are provided to determine a property of an earth formation, comprising a mobile transmitter disposed at a predetermined elevated height above a surface of the earth formation, and one or more receivers moveably disposed in a wellbore penetrating the earth formation. Electromagnetic energy is transmitted from the mobile transmitter into the formation from a plurality of locations; and at the one or more receivers a signal is measured. Using the signal received by the one or more receivers, a property of the formation, such as resistivity, can be determined and mapped.

Abstract: A magnetic position detecting device for detecting a position of a moving body to a fixed body on the basis of a change of a magnetic flux density that a Hall element on the fixed body senses when the moving body provided with a magnet moves relative to the fixed body. An S-pole and an N-pole of the magnet are juxtaposed along a direction of the movement. When the Hall element passes a boundary between the poles of the magnet, the sensed magnetic flux density is zero, the polarities of the magnetic field are inverted, and therefore the Hall element does not sense any variation of the position. Only when the magnet and the Hall element mutually approach, a detect signal based on the sensed magnetic flux density is compared with a reference signal. By detecting a position where both the signals are equal in level, a reference position is detected.