Abstract:
A well logging system and method in which a transmitter in a borehole has at least two radiation detectors sensing either the same condition or two different conditions relating to the earth&#39;s formation traversed by the borehole and providing data pulses corresponding in number and peak amplitude to the sensed condition. The transmitter also includes a reference pulse source and circuitry for combining the reference pulses with each set of data pulses to provide combined pulse signals. Each combined pulse signal is sampled at difference times by a sampling circuit which provides pulses of opposite polarity whose amplitudes correspond to the maximum amplitude pulses occurring during sampling periods. The pulses from the sampling means are conducted to surface electronics by a single conductive path such as the inner conductor and the shield of an armored coaxial cable. The surface electronics includes a pulse separation circuit which separates the pulses by polarity. Processing circuits process the separated pulses to provide records of at least two spectra. The invention is of particular utility as embodied in a nuclear logging system.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part as to all subject matter common to U.S. application Ser. No. 379,033, filed July 13, 1973 now abandoned by Robert W. Pitts, Jr. and Houston A. Whatley, Jr. and assigned to Texaco Inc., assignee of the present invention, and a continuation-in-part for additional subject matter. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to well logging systems in general and, more particularly, to the transmission and processing of signals in a well logging system. 
     2. Description of the Prior Art 
     Heretofore, well logging systems, such as the type described in U.S. application No. 192,883, now U.S. Pat. No. 3,916,685 assigned to Texaco Inc., assignee of the present invention, provided for a single sensor for sensing one condition. The present invention allows two sensors to provide information by way of a single conductive path to surface equipment. Both sensors may sense the same condition; usually they are spaced a predetermined distance from each other. The sensors may also sense different conditions. 
     The desirability of using two sensors sensing two different conditions and providing separate information is evident when one considers that under the reference application two separate logging runs must be made which consume considerable time and are quite expensive. Another point is that when different conditions are being sensed they are being sensed simultaneously so that factors affecting the sensing of one condition also affect the sensing of the second condition. 
     SUMMARY OF THE INVENTION 
     A well logging system provides at least two outputs corresponding to at least one condition sensed in a borehole. The system includes a logging instrument adapted to be passed through a borehole which has at least two sensors for sensing at least one condition in the borehole. Each sensor provides a signal representative of the sensed condition. A sampling circuit periodically samples the signals from the sensing means and provides output pulses in a manner so that output pulses of one polarity correspond to the samples of the signal from one sensor and are provided during first time intervals and output pulses of another polarity correspond to the samples of the signal from the other sensor are provided during second time intervals. The output pulses are conducted to the surface where surface electronics include a circuit for receiving and separating the output pulses in accordance with their polarity. The output pulses of one polarity are processed to provide one output corresponding to the sensed condition while the output pulses of the other polarity are processed to provide a second output corresponding to the sensed condition. 
     The objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawings wherein one embodiment of the invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration purposes only and are not to be construed as defining the limits of the invention. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are simplified block diagrams of a well logging system, constructed in accordance with the present invention, for providing records of at least one condition relative to the earth formation traversed by a borehole. 
     FIGS. 3A through 3F are diagrammatic representations of pulses occurring during the operation of the well logging system shown in FIGS. 1 and 2. 
     FIG. 4 is a detailed drawing of the cable shown in FIGS. 1 &amp; 2. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, there is shown a logging instrument 1 which may be moved through a borehole and which includes a 100 KC oscillator 3 providing pulses E 1  to a divider 4 and to a flip-flop 5. Divider 4 provides a pulse E 3  for every 1000 E 2  pulses from oscillator 3. The pulses E 3  are applied to a conventional type pulse shaper 9. Pulse shaper 9 provides shaped reference pulses E 4 . 
     Flip-flop 5 provides two pulse trains E 5  and E 5A , as shown in FIGS. 3A and 3B, respectively, in response to the E 1  pulses. The pulse repetition rate of pulse trains E 5 , E 5A  is one half that of the pulse repetition rate of pulses E 1 . 
     A radiation detector 14 senses a condition relative to the earth formation and provides negative going data pulses E 6  corresponding in number and amplitude to detected gamma radiation. The condition may be, by way of example, the chlorine content of the earth formation. Detector 14 may be a conventional type sodium iodide (thallium activated) crystal detector detecting gamma radiation resulting from the natural isotopes of the earth formation or from neutron bombardment of the earth formation. Neutron bombardment is well known in the art and it is not necessary for one skilled in the art to know the details of neutron bombardment in order to understand the present invention. Detector 14 may also include a photo multiplier tube optically coupled to a crystal detector portion which receives light pulses from the crystal detection and provides pulses E 6 . 
     A preamplifier 20 receives data pulses E 6  and reference pulses E 4  to provide a pulse train E 7  which includes amplified data pulses E 6  and amplified reference pulses E 4  to another amplifier 23. It should be noted that the amplitude of reference pulses E 4  is substantially larger than the amplitude of the expected largest amplitude data pulse E 6  or E 6A . Amplifier 23 provides positive pulses E 8  through a diode 25 to a voltage follower type amplifier 27, to one plate of a capacitor 28 and to a collector 30 of an NPN transistor 32 having an emitter 33 and a base 34. The other plate of capacitor 28 and emitter 33 of transistor 32 are connected to ground 39. 
     The output provided by voltage follower 27 is applied to a collector 50 of an NPN transistor 51 through a resistor 55. Transistor 51 has an emitter 58 connected to ground 39 and a base 60. The collector 50 of transistor 51 is connected to a non-inverting input of an amplifier 68 whose output is provided to a conductor 70 of an armored coaxial cable 72 through a blocking capacitor 73. Cable 72 will be described in detail hereinafter. 
     A filtering capacitor 74, connected to ground and to a resistor 75, allows pulses E 9  to be applied to conductor 70 while a direct current voltage present on conductor 70 may be picked off for use as an energizing voltage for detectors 14, 14A. 
     Base 60 of transistor 51 receives a positive biasing direct current voltage through a resistor 71 from a DC to DC converter 75 which receives a DC voltage V 2  from a shield 80 of cable 72. DC to DC converter 75 may be of a conventional type well known in the art. In this instance it takes a direct current voltage of one polarity and provides two direct current voltages of equal amplitude but opposite polarity. A filtering capacitor 83 is connected between shield 80 and ground 39 to filter out any noise that may appear on shield 80 and to maintain shield 80 at a signal ground. An outer armor 84 of cable 72 is connected to ground 39. 
     Radiation detector 14A, preamplifier 20A, amplifier 23A, diode 25A, capacitor 28A, voltage follower 27A, transistors 32A and 51A and resistors 40A, 55A and 75A are connected in a similar manner. Pulses E 10A  from voltage follower 27A are applied to an inverting input of amplifier 68 through resistor 55A. 
     Pulses E 8  from amplifier 23 are provided to a high impedence sample and hold circuit comprising diode 25, capacitor 28, transistor 32 and voltage follower 27. Capacitor 28 charges to the peak value of the largest amplitude pulse E 8  occurring during the sampling period. 
     Each pulse E 5  from flip-flop 5 triggers a one-shot multivibrator 44 to provide a negative going pulse E 11 , as shown in FIG. 3C, to base 60 of transistor 51 and to another one-shot multivibrator 45. Transistor 51 is turned on during the absence of a pulse E 11  and turned off by occurrence of a pulse E 11 . The trailing edge of pulse E 11  triggers one-shot 45 causing it to provide a positive pulse E 12 , as shown in FIG. 3D, to base 34 of transistor 32. Transistor 32 is cut off during the absence of a pulse E 12  and is turned on during the occurrence of a pulse E 12 . 
     Referring to FIGS. 1 and 3C through 3F, just prior to time t o , transistors 32 and 51 are turned off and on, respectively. At the time t o  transistor 51 is turned off by pulse E 11  to effectively disconnect the non-inverting input of amplifier 68 from ground 39 so that voltage across capacitor 28 is provided to amplifier 68 as a pulse E 10 . Pulse E 10  is a positive pulse whose amplitude corresponds to the amplitude of the largest amplitude pulse E 8  occurring during the sampling period as hereinafter explained, and whose pulse width is equal to the of pulse E 11 . At time t 1 , transistors 32 and 51 are both turned on by the occurrance of pulse E 12  and the termination of pulse E 11 , respectively. Transistor 51 again shorts out the non-inverting input of amplifier 68. Transistor 32 effectively discharges capacitor 28 to remove any voltage across it. 
     From time t 2  when pulse E 12  is terminated to the time when pulse E 12  again appears, capacitor 28 is effectively sampling pulses E 8  passed by diode 25. Due to the nature of being a capacitor, capacitor 28 develops a voltage across it whose amplitude is the amplitude of the largest pulse E 8  appearing during the sampling period between pulses E 12 . 
     Similarly transistors 32A and 51A are controlled by pulses E 12A  and E 11A , respectively, from one shot multivibrators 44A and 45A, to sample pulses E 8A , passed by diode 25A, and to cause voltage follower 27A to provide positive pulses E 10A . As can be seen in FIGS. 3C through 3F, pulses E 8A  are sampled for the same elapsed time as pulses E 8  but at different times while pulses E 10A  are provided at different times from the occurrance of pulses E 10 . Pulses E 10A  are provided to an inverting input of amplifier 68. 
     Amplifier 68 provides each pulse E 10  as a positive pulse E 9  and each pulse E 10A  as a negative pulse E 9 . 
     As mentioned hereinbefore, reference pulses E 4  are substantially larger than data pulses E 6 , E 6A . When, during a sampling period, pulses E 8  or E 8A  include a reference pulse, the voltage across capacitor 28 or 28A charges to the amplitude of the reference pulse of pulses E 8  or E 8A . The next subsequent pulse E 10  or E 10A  corresponds to a reference pulse. Thus pulse E 9  will include positive and negative reference pulses necessary to the surface processing of pulses E 9 . 
     Cable 72 may be of the type manufactured by the Victor Cable Company under their part number A-4029 and has an inner conductor 70, shield 80 and an outer armor 84. Armored coaxial cable 72 is shown in greater detail in FIG. 4. Conductor 70 is No. 16 AWG, 19 strands of 0.0117 inches tinned copper wires. Conductor 70 is separated from shield 80 by a coaxial insulator 90 made of a propylene copolymer dielectric having a wall thickness of 0.062 inches. Shield 80 is No. 36 AWG tinned copper wire, 9 ends, 16 carriers, 10 ppi with 90% coverage. A mylar tape 91 is wrapped around the outer side of the shield 80 with a 45% overlap. Another propylene copolymer dielectric coaxial insulator 90A has a wall thickness of 0.015 inches and separates tape 91 from armor 84. Armor 84 is divided into two sections 84A and 84B. Armor 84A is composed of 18 strands of 0.042 inches galvanized steel wires preformed right lay and has a coating of anti-corrosion compound. Armor 84B is composed of 18 strands of 0.059 inches galvanized steel wires, preformed left lay, and has a coating of an anti-corrosion compound. The opposite lays of the inner and outer armor is to prevent unravelling while in use. 
     Thus, cable 72 has pulse E 9  applied across inner conductor 70, coaxial insulator 90 and shield 80, direct current V 2  is applied across shield 80, coaxial insulator 90A and outer armor 84, and direct current voltage V 3  on inner conductor 70 with outer armor 84 as the return path. 
     Referring to FIG. 2, the pulses E 9  and conductor 70 of cable 72 are provided by slip-rings 105 to capacitor 110 while the return path for pulses E 9  is provided by shield 80 of cable 72 through slip-rings 105 and capacitor 112. A high voltage power supply 115 provides voltage V 3  through a current limiting resistor 107 and slip-rings 105 to conductor 70 of cable 72. Capacitor 110 blocks voltage V 3  while passing pulses E 9 . 
     A low voltage power supply 116 provides voltage E 2  to shield 80 of cable 72 through slip-rings 105. Capacitor 112 blocks voltage V 2  while providing a return path for pulses E 9 . A resistor 113 connects capacitor 110 to capacitor 112 and pulses E 9  are developed across resistor 113. 
     Amplifiers 120, 121 and 122, resistors 125, 126 and 127 and diodes 130, 131 form a pulse separation circuit which separate the positive pulses E 9  from the negative pulses E 9  appearing across resistor 113. Resistors 125, 126 and 127 have the same resistance value. 
     When a positive pulse E 9  is applied through resistor 125 to amplifier 120, a positive pulse E 15  appears at the output of amplifier 120 causing diode 131 to have a very low forward impedance so that the effective gain of amplifier 120 is the ratio of the resistance value of resistor 127 to the resistance value of resistor 125, which is unity. Positive pulse E 15  is blocked by diode 130 so that no positive pulses E 15  are applied to amplifier 
     Amplifier 121 provides a corresponding positive pulse E 20  which may correspond to a reference pulse or to the largest amplitude sampled data pulse. 
     Pulses E 20  are provided to signal processing apparatus 150. Signal processing apparatus may be of the type described in aformentioned U.S. application, which provides a spectral record represented by the sensed condition. When the apparatus is of the type described in aforementioned U.S. application, signal processing apparatus 150 encompasses those elements numbered 50 through 94 and as such includes a spectrum stabilizer, pulse high analyzing means, the signal processing network and recording means. 
     The spectral record provided by apparatus 150 is correlated to the depth at which the logging instrument 1 is passing through by means of an alternating current synchro 155 receiving a voltage V A  applied to one end of a rotor winding 156 having its opposite end connected to ground 39. Cable 72 is wound on a reel 170 and as it leaves reel 170 it passes over a roller 172. Roller 172 is mechanically connected to rotor winding 156 of synchro 155. As cable 72 moves roller 172, rotor winding 156 rotates accordingly. The voltage across rotor winding 156 is inductively coupled to stator windings 180, 181 &amp; 182 of synchro 155 having a common connection to ground 39. As rotor winding 156 rotates, voltages across stator windings 180, 181, 182 vary as a function of the angular displacement of rotor winding 156. The voltages across 180, 181 and 182 drive the recording means in signal processing apparatus 150, 150A so that the recordings are correlated sensed condition to the depth at which the condition was sensed. 
     A negative pulse E 9  results in amplifier 120 providing a negative pulse E 15  which affects amplifier 120, diode 130 and resistors 125, 126 in the same manner that a positive pulse E 15  affected amplifier 120, diode 131 and resistors 125, 127. Negative pulse E 15  passes through diode 131 and is provided to amplifier 122. Amplifier 122 provides pulses E 20A  to processing apparatus 150A which in turn provides a spectral record in the same manner that processing apparatus 150 simultaneously provides the record of the first spectra. 
     In another embodiment, sodium iodide crystals in detectors 14, 14A may be doped with an alpha emitting isotope such as Americium 241 or other transuranic isotopes having high energy alpha emission, low intensity and low energy gamma emission. When so doped, the crystals periodically provide a pulse which causes photomultiplier tubes in detectors 14, 14A to provide corresponding reference pulses of sufficient amplitude. Divider 4 and pulse shaper 9 may be eliminated. Amplifiers 20, 20A would be retained, but they would be used for amplification of the pulses from detectors 14, 14A and not for combining signals. 
     The present invention as heretofore described is a dual spectra well logging system. Two detecting means may either detect one condition, such as chlorine, or two different conditions such as carbon and oxygen, in a bore-hole. The dual spectra information is transmitted to surface electronics by a single conductive path. The surface electronics provide two spectral recordings of the sensed condition or conditions. The dual spectra well logging systems are not restricted to nuclear well logging since they are applicable to any well logging method whereby one or two conditions are sensed. 
     Although the well logging system of the present invention has been shown using an armored coaxial cable, it is not restricted in use to an armored coaxial cable. The number of dual spectra logs that may run simultaneously is equal to the number of conductive paths from the downhole instrument to the surface in any logging cable for the providing of such information to the surface.