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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to data transmission systems and more
particularly to transmission systems that are used between surface data
acquisition systems and downhole data gathering systems that compensate
for standing wave nodes.
2. Related Prior Art
Prior art transmission systems between surface data acquisition systems and
downhole data gathering equipment are fairly common in the oil industry.
Most transmission systems are "hard wired" or require actual physical
connections between the surface and the downhole equipment. Examples of
relevant prior art systems are as described below.
U.S. Pat. No. 2,505,144, "Signaling System for Use in Mine Shafts",
(Rutherford), relates to a system used to provide an signaling system for
use in mine shafts as a means of communication between the hoist operator
on the cage, or skip, during the movement of the cage. This system uses
the hoisting cable as an autotransformer and is operative irrespective of
its position, or depth, in the shaft, or conditions which may cause
variations in the capacitance between the cage walls of the shaft.
U.S. Pat. No. 4,136,327, "Well Survey System Modulation Technique",
(Flanders, et al.), relates to a communication system that uses a single
pair of conductors in which a drill cable in a borehole is used not only
to transmit power into the hole, and to communicate sensor data back up to
the surface. A frequency for power and communication of 1 kHz is employed.
However, this is frequency modulated by deviation of about five percent to
transmit commands down hole. A different non-interfering phase modulation
is used to send data back to the surface by causing a susceptance
(preferably capacitive) to be connected across the supply cable to signify
a binary pulse. The alternation in phase between current into the cable
and potential across it is observed at the surface and the pulses so
detected are processed by conventional data processing techniques. A
coding method is used for sending commands into the hole. Each of a series
of possible commands is assigned a binary value so that, for example, if
commands two and four are to be sent, a total of two plus eight pulses
(all of equal weight) forming a command word are sent down hole where they
are counted by a binary counter. When the total number has been counted,
outputs two and four of the counter will have signals on them which
constitute the transmitted orders.
U.S. Pat. No. 4,017,845, "Circuitry for Simultaneous Transmission of
Signals and Power", (Kilian, et al.), relates to a transmission line
having a pair of wires enclosed in a shield which is used to
simultaneously transmit high frequency signals and low frequency power
between distant locations. At the sending end of the line a high frequency
source is connected between the first and second wires by circuitry which
prevents the low frequency power from being coupled into the high
frequency source. A low frequency power supply is connected between the
shield and both of the wires by circuitry which prevents the high
frequency signals from being coupled into the low frequency supply. At the
receiving end of the transmission line circuitry is provided which
separates the high frequency signals from the low frequency power.
U.S. Pat. No. 4,156,869, "Conducting Cable", (Schukantz), relates to a
cable for conveying information signals of selected frequencies and for
simultaneously conveying a selected amount of electric power, in which a
central conducting means is surrounded by a dielectric material. An outer
conducting means is positioned around the dielectric, and cooperates with
the central conducting means to provide a first path, through which the
information signals are conveyed. One of the conducting means provides a
second path, through which the electric power is conveyed.
U.S. Pat. No. 3,916,685, "Well Logging System and Method Using an Armored
Coaxial Cable and Compensation Circuit", (Paap, et al.), relates to a well
logging system which includes a transmitter in a borehole, having a sensor
such as a radiation detector, a condition relating to the earth's
formation traversed by the borehole. The transmitter provides data pulses
which correspond in number and peak amplitude to the sensed condition. The
transmitter also includes a reference pulse source and means for combining
the reference pulses and the data pulses. The combined pulses are
conducted to a receiver at the surface by an inner conductor of an armored
coaxial cable. The shield of the armored coaxial cable is insulated from
the inner conductor and from an outer cable armor which surrounds it. The
shield provides a return path for the combined pulses thereby reducing the
skin effect encountered when using the outer armor as a return path. High
voltage for energizing the sensor is also conducted to the sensor by the
inner conductor of the armored coaxial cable from a direct current power
supply on the source. Low voltage is conducted by the shield of the
armored coaxial cable from the power supply to the sensor, to the
reference pulse source and to the combining means, while the outer armor
of the armored coaxial cable provides a common ground connection between
the transmitter, the power supply, and the receiver. The receiver includes
a compensating circuit which monitors the reference pulses and adjusts the
data pulses in accordance with the monitored reference pulses to
compensate for deterioration of the data pulses during their transmission.
The receiver also includes recording means which provides a record of the
sensed nature of the earth formation in accordance with the compensated
data pulses.
SUMMARY OF THE INVENTION
The frequencies listed in the foregoing prior art patents are from 100 kHz
to as high as 20 MHz and contain no teachings to compensate for standing
wave nodes. The present invention provides a data transmission system for
use with a standard seven conductor logging cable that functions to
provide more precise transmission of data signal with reduced interference
from higher powered electrical signals present along the line. The data
signals are modulated on a radio frequency carrier which is then
inductively coupled to the cable jacket or shielding sheath of the logging
cable for conduction along the cable. Thus, as the logging cable supports
a downhole tool in a borehole, transmitter/receiver combinations both
downhole and at the surface can communicate via modulated RF signal
conducted along the standard shielding material of the cable. A primary
advantage to this type of transmission is the fact that no signal must
reverberate through the cable reel as it can be inductively picked off
prior to the cable entering the reel section, resulting in much reduced
attenuation and interference.
BRIEF DESCRIPTION OF THE DRAWINGS
The attached FIGURE is a schematic drawing illustrating a downhole tool
connected to a surface data acquisition vehicle.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention provides a method and apparatus for data transmission
between a downhole tool and a data acquisition system that is located on
the surface. The data transmission system of the present invention is
designed for use with a cable such as a seven conductor logging cable. In
this manner the cable system functions to provide more precise
transmission of data signals with reduced interference from higher powered
electrical signals present along the line.
Referring now to the attached FIGURE, the present invention is illustrated
in schematic form. A logging truck 10 is connected to transmitter/receiver
12 via hard wire connection 14. Logging truck 10 is also connected to
downhole tool 18 through logging cable 20. Transmitter/receiver 12 is
inductively coupled to logging cable 20 through a pickup loop 22 via a
hard wire connection 24. Logging cable 20 preferably includes a conductive
sheathing such as cable jacket 26, so that electromagnetic signals, such
as radio frequency waves, can be easily transmitted along its periphery.
Located in close proximity to downhole tool 18 is a second
transmitter/receiver 28, which is also inductively coupled to logging
cable 20.
In operation, the data signals are modulated by transmitter/receiver 12 on
a radio frequency carrier which is then inductively coupled to cable
jacket or shielding sheath 26 of logging cable 20 for conduction along
logging cable 20. Thus, as logging cable 20 supports downhole tool 18 in a
borehole 30, downhole transmitter/receiver combination 28 and surface
transmitter/receiver 12 can communicate via a modulated radio frequency
(RF) signal conducted along standard cable jacket 26 of logging cable 20.
A primary advantage to this type of transmission is the fact that no
signal must reverberate through the cable reel as it can be inductively
picked off prior to the cable entering the reel section, resulting in much
reduced attenuation and interference.
The purpose of this data transmission system is to increase the amount of
data that can be sent between downhole logging tool 18 and logging truck
10 in real time using a standard seven conductor logging cable 20. A
standard jacketed logging cable is described for convenience but any cable
having conductive material, such as a wench cable may be substituted.
However, an insulated cable provides better isolation from the well
casing.
This can be done by coupling a modulated radio frequency signal to and from
cable jacket 20 as illustrated in FIG. 1. Using pickup loop 22 to couple
signals onto cable 20 at logging truck 10, data signals will not be
attenuated by the full length of cable 20 on truck 10. Also, no slip rings
are required for contact at the cable drum (not shown).
Noisy power signals on the inside conductors will not affect the
transmitted data signals because of the shielding effect of outside cable
jacket 26 and the higher frequency of the transmitted data signal. Logging
cable 20 in conjunction with borehole 30 would act much like a lossy
coaxial transmission line. This is more evident if borehole 30 is cased,
has a steel pipe lining.
By driving radio frequencies in the preferred range of 100 MHz to 150 MHz,
very high data rates could be sent up and down hole. However, it is to be
appreciated that the range of 100 MHz to 150 MHz is only a preferred range
and other suitable frequencies may used. Multiple modulating frequencies
could be applied to the carrier at the same time. Boreholes can be 25,000
ft. or more. The position of logging tool 18 in borehole 30 will always be
changing during logging. This will cause the signals to have standing wave
nodes where signals are weak. If, at a predetermined signal level, the
frequency were shifted to move the node to a higher signal level, data
would not be lost. In other words, if the data signal falls below a
predetermined minimum, the frequency on which the data signals are
modulated is shifted.
While there has been illustrated and described a particular embodiment of
the present invention, it will be appreciated that numerous changes and
modifications will occur to those skilled in the art, and it is intended
in the appended claims to cover all those changes and modifications which
fall within the true spirit and scope of the present invention.
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