Well logging method and apparatus providing multiple depth of investigation using multiple transmitters and single receiver pair having depth of investigation independent of formation resistivity

摘要

An apparatus and method for determining formation resistivity at a plurality of radial distances from an earth borehole uses a single receiver pair and a plurality of electromagnetic energy transmitters sequentially spaced along the apparatus from the receiver pair, such transmitters being sequentially pulsed at approximately 1-2 MHz each to provide time separation between the pulsing of the plurality of transmitters. The closest spaced transmitter is spaced a predetermined distance from the closer receiver to make the depth of investigation of the closest transmitter essentially independent of formation resistivity. Alternatively, a single transmitter is spaced a predetermined distance from the closer receiver to make the depth of investigation of such transmitter essentially independent of formation resistivity.