Patent GB-784641-A (GB)
Title: Neutron well logging apparatus
IPC: G01V5/10
Filed: 1954-12-20
Abstract: 784,641. Well logging by neutrons. SCHLUMBERGER WELL SURVEYING CORPORATION. Dec. 20, 1954 [March 8, 1954], No. 36847/54. Class 40 (3). [Also in Group XL (a)] Neutron well logging apparatus comprises a pressure-resistant housing 13 enclosing a neutron generator 14, Fig. 1B, a radiation responsive device 15, Fig. 1c and associated electronic equipment. A shield plate 16, Fig. 1C, disposed above detector 15, shields the detector from the generator and may be of lead, for neutrongamma-ray logs, or of boron carbide embedded in paraffin for neutron-neutron logs. The housing 13 is suspended by an armoured cable 17, Fig. 1A, which supplies power to the housing and transmits information from the housing to surface equipment. The main power supply comes from a centre-tapped single-phase source 100, the centre tap being connected to the shield 108 of the cable and to earth; this supply is connected to transformer 111 within the housing, and a centre tap of the transformer is connected to the shield 108 of the cable and to the wall of the housing. In order to control the neutron flux produced by generator 14 a monitor detector 127, Fig. 1B, is disposed adjacent the generator and its output is connected to a conventional pulse amplifier and integrator unit 131, Fig. 1A; an output from unit 131 is used to control the generator 14 and a further output is connected through one of the leads of cable 17 to an indicator 135, for example an integrator and peak-reading voltmeter, located at the surface. The output of the detector 15, Fig. 1C, is connected through an amplifier 159 and pulse transformer to one of the leads of cable 17 to a pulse transformer 161, Fig. 1A, at the surface; this feeds an integrator and recorder unit 164 which may be of a continuously recording type. If desired, the neutron source 14 may be pulsed and the output of detector 15 gated to obtain an activation log. Additional information may be supplied to indicators at the surface by conventional multiplexing techniques; for example a detector of slow neutrons may be provided so that simultaneous neutron-gamma and neutron-neutron logs may be obtained. Fig. 5 shows a circuit for controlling the neutron generator. Pulses from unit 131 are fed to an integrator 171 which develops a negative voltage which increases as the neutron count increases ; an increase in the output of integrator 171 reduces the current through valve 173 and thus reduces the potential across resistance 176 which is applied between a cathode and deflecting plates of the neutron generator and so reduces the output of neutrons from the generator. The initial neutron flux may be controlled by a potentiometer 179. Fig. 6 shows a circuit for regulating the gas pressure in the neutron detector. The ion source of the generator is in series with a diode 185 which is operated at reduced filament current so as to comprise a constant current device. Thus, if the pressure in the ion source rises the voltage across it decreases and vice versa, so that the ion concentration remains substantially constant. If the pressure rises excessively, the voltage across diode 185 rises sufficiently to operate a relay 193 which energizes a getter filament 85. If the pressure falls excessively the current through the diode is insufficient to hold open the contacts of a relay 191 so the contacts close and energize a filament 83 which releases deuterium into the ion source.