Patent GB-437937-A (GB)
Title: Method and apparatus for differentiating geological formations penetrated by depth bores
IPC: G01V3/26
Filed: 1935-06-17
Abstract: 437,937. Prospecting for minerals &c. GES. FUR NAUTISCHE UND TIEFBOHRTECHNISCHE INSTRUMENTE, (formerly GES. FUR NAUTISCHE INSTRUMENTE GES.), and MARTIENSSEN, O., Werk Ravensberg, Kiel, Germany. June 17, 1935, Nos. 17442 and 17443. Convention dates, July 9, 1934 and Feb. 1, 1935. [Class 37] In a method of investigating the geological formations penetrated by depth bores, which may be filled with water, a high-frequency current is generated within the bore and a lowfrequency or direct current is conducted above ground, the frequency or intensity of the latter current being modified according to variations in the high-frequency current, which variations are dependent on the electrical or magnetic characteristics of the formation surrounding the part of the bore in which the high-frequency current is generated. In the arrangement shown in Fig. 3, a casing G is lowered into the bore by means of an insulated steel cable K which can be connected through a switch key U, transformer Tr3 and a two-valve amplifier to telephones T. The valve 3 in the casing generates a high-frequency current in an oscillatory circuit I comprising an inductance 20 and condenser 11. An insulated antenna A, loaded by a weight 10, depends vertically from the apparatus and constitutes a capacity in parallel with the condenser 11. Variations in the capacity of the antenna due to changes in the adjacent rock formation vary the frequency of the oscillatory circuit I. A second oscillatory circuit II with valve 5 is loosely coupled with the circuit I by means of coils 4, and the beat frequency between the circuits I and II is amplified by a valve 7 and transmitted by a transformer 8 to the cable K and telephone. Variations in the beat frequency produced by variations in the geological formations penetrated are detected by sound variations in the telephones T. The frequency of the circuit I may be adjusted by means of a variable condenser 12, the capacity of which is adjusted by supplying current impulses to a trip gear 14 by means of the key U. If the variations are to be recorded, the beat frequency produced by the circuits I and II is arranged to be above the audible limit, and an additional oscillatory circuit III, Fig. 4, is provided above ground to produce an audible frequency. The circuit III includes a variable condenser 19 which is continuously adjusted by hand in such a manner that the note in the telephone T remains constant when the casing G is being lowered through the bore. The adjustments of the condenser 19 are recorded on a moving strip which is advanced at a speed proportional to the rate of movement of the apparatus within the bore. The antenna A may be replaced by a coil, the inductance of which varies with changes in the formations penetrated. In another arrangement, Fig. 5 (not shown), the casing contains a single highfrequency circuit with a depending coil, and changes in current intensity produced by variations in the damping effect of the surrounding formations on the coil are measured by means of a thermo-couple, which is excited directly or indirectly according to the intensity of the high-frequency current, the direct current generated by the thermo-couple being led through the cable to a sensitive ammeter above the ground.