Original old oil trade magazine vintage article titled HOW THE RADIO FREQUENCY METHOD works as an petroleum exploration tool by William D. Owens, President of Owens Electronic, Inc of Hanover , PA.  Shown is a transmitter truck working in wet Texas.  The vehicle is a four-wheel-drive cab over engine Jeep.  The trailer is a 15kw motor generator set used to power the transmitter and associated equipment.  The boxlike gear is part of an antenna coupling device.  One firm, Raytheon Company succeeded in radio communication through 18 miles of the earth's subsurface.  This 1962 four page article would make a nice addition to your library.