Abstract: A source of high energy neutrons and a detector responsive to the thermal neutron population decay rate are utilized in a well logging instrument. The instrument is positioned within a borehole and at least a portion of the fluids within the borehole are irradiated with high energy neutrons. The thermal neutrons emanating from the irradiated fluids are detected and measured. The thermal neutron counts are converted into a time base measurement which is further converted into a measurement of the thermal neutron capture cross-section for the irradiated fluids.

Abstract: Method and apparatus for reducing the dynamic range of measure signals encountered in simultaneously obtaining two focused logs of formation resistivity with different depths of investigation and means for providing the required electrical connections of the electrode system. The apparatus includes an electrode system having a central electrode, a near and far pair of current electrodes, respectively shortcircuited, and aligned symmetrically on both sides of the central electrode, and a near and far pair of measure electrodes, respectively shortcircuited, and aligned symmetrically between the central electrode and the near pair of current electrodes. A signal detected on the near measure electrode pair at a preselected frequency and a first and second signal are combined to produce first and second combined voltages which are used in the production of a first and second frequency, f.sub.1 and f.sub.2, respectively.

Abstract: In a neutron induced gamma ray well logging instrument, a neutron generator is pulsed at a clock frequency of 20 KHz. Each neutron burst produces inelastic scattering gamma rays which are detected during a first time interval coinciding with the neutron source being on. Capture gamma rays are detected during a second interval subsequent to the end of each neutron burst. Upon detection of the scattered gamma rays, a detection pulse is transmitted during each of the two detection intervals. Sync pulses are generated at a scaled down frequency rate of 200 Hz within the well logging instrument for transmission to the earth's surface. The scaled-down sync pulses are applied to a phase-locked loop system for regenerating the sync pulses to the same frequency as that of the clock frequency used to pulse the neutron source and to open the detection gates in the borehole instrument.

Abstract: A pulsed neutron generator in a well logging instrument is pulsed at a clock frequency of 20 KHz. Inelastic scatter gamma rays are detected during a first time interval coinciding with the neutron source being on and capture gamma rays are measured during a second interval subsequent to the end of each neutron burst. Only a single detected pulse, assuming detection occurs, is transmitted during each of the two detection intervals. Sync pulses are generated in the well logging instrument scaled down to a frequency of 200 Hz for transmission to the earth's surface. At the earth's surface, the scaled-down sync pulses are applied to a phase locked loop system for regenerating the sync pulses to the same frequency as that of the clock frequency used to pulse the neutron source and to open the detection gates in the borehole instrument.

Abstract: A pulsed neutron generator in a well logging instrument is pulsed at a clock frequency of 20 KHz. Inelastic scatter gamma rays are detected during a first time interval coinciding with the neutron source being on and capture gamma rays are measured during a second interval subsequent to the end of each neutron burst. Only a single detected pulse, assuming detection occurs, is transmitted during each of the two detection intervals. Sync pulses are generated in the well logging instrument scaled down to a frequency of 200 Hz for transmission to the earth's surface. At the earth's surface, the scaled-down sync pulses are applied to a phase-locked loop system for regenerating the sync pulses to the same frequency as that of the clock frequency used to pulse the neutron source and to open the detection gates in the borehole instrument.

Abstract: The address scaler outputs of a multichannel analyzer are fed to two comparator circuits in an address decoder, one such address decoder being provided for each energy range of interest in the spectrum of detected gamma rays produced, or occurring, in earth formations surrounding a borehole. The address number from the multichannel analyzer is compared with two numbers preset by switches within the decoder, one group of switches and its associated comparator circuit being determinative of the lower boundary number, and a second group of switches and associated comparator circuit being determinative of the upper boundary number. The outputs from the comparators, one of which is inverted, are AND gated with a STORE pulse from the multichannel analyzer. If the address number from the multichannel analyzer falls within the lower and upper boundary numbers in coincidence with the STORE pulse, an output pulse is generated by the AND gate; otherwise, no output results.

Abstract: A pulsed neutron generator for well logging is provided having a resistor connected between the anode and cathode. The resistor provides a direct current path whereby corona current can flow between the cathode and a corona point without the necessity for the ion source to conduct. In an alternative embodiment, the secondary coil of a pulsing transformer is connected in series with a resistor between the anode and cathode. In an alternative embodiment, a corona regulator in series with the collector-emitter of a transistor is connected between the cathode and anode of the neutron source and the base drive to the transistor is provided by a light-responsive solar cell activatable by an external lamp. Circuitry is provided for utilizing the various neutron sources.