at least one of the energy sources or one of the detectors being located on or above the ground surface

Full Title

Survey of boreholes or wells (monitoring pressure or flow of drilling fluid E21B21/08) > Determining slope or direction > of the borehole, e.g. using geomagnetism > using electromagnetic energy or detectors therefor > at least one of the energy sources or one of the detectors being located on or above the ground surface