GB1460186A - Method and apparatus for determining electromagnetic characteristics of earth formations - Google Patents

Abstract

1460186 Well logging systems TEXACO DEVELOPMENT CORP 15 July 1974 [27 Aug 1973 (3)] 31334/74 Heading G1N In order to distinguish a fresh water earth bearing formation from a hydrocarbon bearing earth formation in the vicinity of a well borehole (which formations exhibit similar conductivities), the conductivity and dielectric constant of the earth formation in the vicinity of the borehole is measured simultaneously using a dual high frequency system. Logging sonde 11 is provided with a first electrostatically shielded coil 17 which is supplied with R.F. energy in the 10 to 20 Megahertz range, particularly 16 megahertz, from a crystal controlled oscillator 53 via a gain controllable driver 54 and amplifier 55, details of which are described in Figs. 3A, 3B (not shown), the output being maintained at a constant current by controlling the gain of driver 54 by control stage 60 in accordance with the output picked up by transformer 58. Dual transmitting coils 18, 19 helically wound in opposite directions to produce a reversed polarity pair to focus the energy in the earth formations are supplied with energy in the range 20 to 60 megahertz (particularly 30 megahertz) from crystal controlled oscillator 61 via power amplifier 62, the amplifier 62 and oscillator 61 being controlled by 64, 65, 66 to maintain the output at constant current. Electrostatically shielded receiver coil 21 receives simultaneously signals from currents induced in the earth formation by both the 16 and 30 megahertz transmitters. These signals are split by coil 73 the secondary windings of which are tuned to the respective frequencies. The 16 megahertz signal is mixed at 74 with a local oscillator 75 and the resultant I.F. signal #f, after amplification at 76, 77 is fed to adding stage 82 where it is added to the I.F. signal #f 2 likewise obtained from the 30 Megahertz signal by mixer 78. The summed output of these two I.F. signals is fed to the surface up the sonde cable to processing circuit Fig. 1 (not shown), where after amplification the signals are split again by high Q frequency filters into the respective I.F. signals. Each I.F. signal is applied to an amplitude detector circuit and recorder for recording amplitude as a function of bore hole depth. Alternatively the dual I.F. signal may be applied to a further splitter and the two I.F. signals applied to a respective analogue to digital amplitude detector. The digital signals are then applied to a computer programmed to determine the properties of the earth formation in terms of dielectric and resistivity which are indicated on respective recorders as a function of bore hole depth.