GB1310141A - Methods and apparatus for investigating earth formations - Google Patents

Abstract

1310141 Borehole logging SCHLUMBERGER INLAND SERVICES Inc; 6 April 1970 [7 April 1969] 16240/70 Heading G1N The invention relates to resistivity borehole logging apparatus of the kind described in Specification 696535, i.e. a survey current is passed between a remote point and a central electrode A 0 on a sonde and the potential gradient between pairs of surrounding measuring electrodes M is automatically maintained substantially at zero by their automatic control of the "focusing" current from auxiliary electrodes A 1 . In this case the focusing current also passes the central electrode A 0 instead of through an earth return. In Fig.4 (not shown) this arrangement is used with a phase-sensitive detector to determine the resistive component of the signal. In Fig.6 (not shown) two pairs of measuring electrodes (M 3 ..M 6 ) are used to control the current through two sets of focusing electrodes (A 1 , A 2 ) In Fig. 8 a CONSTANT VOLTAGE Vref (instead of a constant current) from oscillator 78 is applied between electrode sets M 0 and M 1 , 2 and feedback through amplifier 85 supplies auxiliary-current through electrodes A 0 , A 1 , to reduce e 5 to zero, i.e. to make V (M0#M1,2) equal to Vref. Survey current I 0 is supplied by amplifier 87 between the sonde cap and electrode A 0 to make V M1 -V M2 substantially zero this current is measured by a series dropping resistor 89. (The sonde electrode construction is similar to that described in Specification 1146394 and also includes an induction logging system not used in this embodiment.) Fig.10 provides two measurements of resistivity:- R x0 locally around the sonde, and R 4 the more remote non-invaded zone. The sonde has additional outer measuring electrodes M 3 , M 4 , maintaining themselves at zero potential gradient by controlling the focusing-current between electrodes A 2 , A 3 . The inner measuring electrode M 1 M 2 control focusing current between A 0 A 1 . The local measurement R x0 is made between the innermost measuring electrodes M 0 and the second pair M 1,2 . R 4 is measured between M 1,2 and the remote electrode N (as in the embodiment of Fig. 4 (not shown)). Fig. 16. Three constant voltage electrode sets (upper, middle, and lower) are provided each with its own central (A 0 ) electrode and surrounding measuring and focusing electrodes. Central System operates similarly to the embodiment of Fig.8 but provides measurements for R x0 = V (M0#M5 ,6) and Rt = V (N#M5,6) and V (M5#M6) controls the voltage opposing the constant reference-voltage, and hence the focusing current to electrodes A 1 to maintain the gradient between M 5 , M 6 zero. Upper System. V (M1#M2) automatically controls the focusing current between its central electrode A of and auxiliary pair A if to maintain the gradient at M 1 , M 2 zero. Lower System operates similarly V (M9#M10) controlling current between its A of and A 2f electrode pair. Auxiliary Plugging. Voltage between the electrodes M o1 of the upper set and M o2 of the middle set applied to amplifier 164 provides further focusing current between the cable armour and the central upper electrode A of . Similarly voltage between the middle and lowerset M o2 #M o3 is applied to amplifier 161 to provide focusing current between the lower A of and the cable armour. Fig.17 (not shown) provides a further constant voltage system with reciprocal electrode arrangement viz, volts A 0 #A 1 ) controls the focusing current through M 1 #M 2 . Since the oscillator is not stabilized a signal proportional to the survey current as well as the signal voltage is measured. Electrode arrangements on a contact pad are illustrated in Figs.12, 14 (not shown).