Note: Descriptions are shown in the official language in which they were submitted.
2~333~ll
LOCAT~BLE OBJECT SUITABLE FOR UNDERGROUND USE
AND METHODS OF LOCATING SAME
Technical Field
This invention relates to a locatable object suitable for
5 underground use and to methods of locating same.
Background of the Invention
Copper and optical fiber communications cables, while differing
substantially in the mode in which they transmit information and even the
form of energy they transfer, are nonetheless used in somewhat similar ways
10 using similar installation methods. For instance, either cable may be used
in terrestrial applications or either may be buried directly in the ground.
Because copper cable is inherently metallic, unlike optical fiber
cable which need not contain any metallic materials, that is all materials
may be dielectric, special shielding is often necessary to protect that cable
15 from the hagard of lightning strikes. Portions of suspended cable as well as
surface exposed portions of buried cable may attract lightning. The
problem with lightning strikes has been overcome partially by incorporating
into the cable a metallic shield that encloses the circumference of the cable
and extends longitudinally throughout the cable. The metallic shield is
20 often disposed between an inner jacket and an outer jacket of the cable and
is caused to be grounded at different points along the length of the cable. If
lightning strikes a portion of the cable, the energy couples into the metallic
shield and travels to ground, hopefully preventing damage to the
transmission portions of the cables disposed within the metallic shield.
~5 Optical fber cables are often strengthened by incorporating
metallic strength members into the sheath system thereof . Typically, a
plurality of strengthening wires are disposed helically along the length of
the cable before the cable receives a final outermost plastic jacket. Because
these wires are metallic, they may attract lightning strikes to the cable.
30 Lightning protection is provided for some lightguide cables in a method
similar to that used to protect copper cable, that is, a metallic shield is
disposed about an inner member of the cable sheath system during cabling.
A final plastic jacket is disposed about the metallic shield.
It is desirable to be able to locate, for example, buried cables, for
35 the purposes of maintenance and rearrangement and marking the path of
the cable so as to avoid cutting the cable during future cable placement and
2`~3~
digging operations. Buried cables, wires, pipes and other objects whose
structures comprise continuous, longitudinally extending metal portions
such as shields or strength members may be located by use of equipment
referred to as cable locators. These so-called cable locators comprise two
5 components, a signal transmitter and a signal receiver. The signal
transmitter which is connected directly or inductively to a metallic portion
of the buried structure and left in a stationary position is caused to
transmit a so-called tracing tone into the structure. The tracing tone is an
electric signal which causes the metallic portions of the buried object to
10 radiate a characteristic electromagnetic field. An operator holds the
receiver close to the ground and causes it to swing in a side-to-side motion
above an area where the operator thinks the cable is located. The receiver
is fitted with an electromagnetically inductive pick-up coil transducer which
when excited by the electromagnetic field produced by the tracing tone
15 produces a signal which can be interpreted to indicate the relative location
of the buried object.
Recently, optical fber cable sheath systems have been developed
which provide the strength necessary for optical eable integrity without the
use of metallie strength members. In U.S. patent 4,874,219, there is
20 disclosed an optical fiber cable system comprising an optical fiber core, a
tube in whieh the core is disposed, a longitudinally extending water
blocking tape, a plurality of helieally wrapped longitudinally extending
non-metallie strength members and an outer jaeket. One of the benefits of
this so ealled all-dieleetrie design is that, sinee it includes no metallic sheath
25 system members, it does not attraet lightning strikes nor will it transmit
hazardous voltage if inadvertently crossed with power cables.
One of the drawbaeks of the aforementioned all-dieleetric design
is that conventional buried cable locating teehniques may not be used. If
the buried cables do not contain any longitudinally extending metallie
30 portions, then obviously the eonventional means for loeating buried eables
described hereinbefore may not be used. What is needed and what
seemingly is nowhere shown in the prior art are apparatus and methods for
locating all-dielectric buried cables.
Summary of the Invention
The foregoing problem of locating buried all-dielectric cables has been
solved by the methods and system of this invention.
In accordance with one aspect of the invention there is provided a method
5 of locating a concealed cable, said method including the steps of: causing at least one
of a plurality of electronically resonant markers which are disposed along a length of
cable which is concealed to become energized by inducing into the electronicallyresonant markers associated with the concealed cable an alternating electric current
comprising at least one frequency, the source of which is a remote radiating
10 transmitter; and detecting a radiated electromagnetic field, having a frequency, which
is different from said at least one frequency, the source of which is at least one of the
energized electronically resonant markers, wjth a remote detecting device to locate the
cable.
Jn accordance with another aspect of the invention there is provided an
15 optical fiber cable, comprising: at least one optical fiber; and a sheath system with a
plurality of [said] resonant markers spaced therealong, each said marker comprising an
electronically resonant marker associated with said cable, said marker comprising a
self-contained resonant loop and being capable of receiving energy comprising at least
one frequency and of generating and radiating a signal of at least one other frequency
20 which differs from said at least one frequency.
In accordance with yet another aspect of the invention there is provided a
system for locating a concealed cable, said system comprising: a concealed cable with
a plurality of electronically resonant markers being associated with and spaced along
said concealed cable, said at least one marker comprising a self-contained resonant
25 loop, said at least at one marker being capable of receiving energy from energy being
transmitted at least at one frequency and of generating a signal of another frequency
which differs from said at least one &equency and of radiating a signal at said another
frequency; means for transmitting electromagnetic energy comprising said at least one
frequency and for receiving signals which are radiated at said another &equency;30 means for discriminating between the transmitted energy and the radiated signal; and
indicating means responsive to receipt of a valid signal at said another frequency for
indicating the presence of the concealed cable.
` A
- 3a -
An electronically resonant marker comprising, for example, a printed
circuit which includes an inductive portion and a capacitive portion and a sur~ace
mounted diode is caused to be disposed in association with a concealed object, such as
an all-dielectric optical cable, for example. The reactive components of the marker
S and the diode are caused to be of such values that the marker characteristically
resonates when excited by an external electromagnetic stimulus.
In one embodiment, a transceiver comprising a transmitting portion is
provided to produce and transmit through a high gain antenna electromagnetic energy
comprising energy of at least two frequencies, a first frequency, fa~ of the transmitted
energy, and a second frequency, fb, which is within 5% of the value of the firstfrequency, f~. A narrow bandwidth receiver, which has its input connected to a high
gain antenna which may be the same antenna to which the transmitting portion is
attached, is provided and is effective to receive an intermodulation product frequency
of the two transmitted frequencies.
An optical flber cable which includes electronically resonant markers may
be buried and thus obscured from vision. In the case of an optical cable, the
electronically resonant markers may be included within or between coaxially disposed
dielectric sheath members of the cable. The electronically resonant markers are
spaced along the length of the cable and are circumferentially staggered.
The cable may be located with the transceiver device described above. To
do so, a tradesperson causes the transceiver to be scanned or waved in the general
area where the cable is believed to be buried. The transmitting portion is caused to
transmit electromagnetic energy comprising energy of at least one frequency and
which in one embodiment may comprise two frequencies. As this is done, the
electromagnetic energy generated by the transmitting portion penetrates the earth
and in so doing causes the electronically resonant markers to become energized.
Because the electronically resonant markers have a non-linear response, they
radiate an electromagnetic field having a frequency which is different from said at
least one frequency and which for transmitted energy comprising two frequencies
. ,9~
2~3~4
may be an intermodulation product of the two transmitted frequencies.
The receiver receives the intermodulation signal and converts the signal into
an output signal useful for determining the location of the buried cable.
Brief Description of the Drawin~
FIG. 1 is a pictorial representation of the practice of a cable
locating method of this invention;
FIG. 2 is a perspective view of an all-dielectric, communications
cable having a sheath system which includes a plurality of electronically
resonant markers;
FIG. 3 is a pictorial representation of the practice of a prior art
cable locating method for locating cables which include continuous metallic
elements;
FIG. 4 is a perspective view of a carrier tape upon which a
plurality of electronically resonant markers are disposed;
FIG. 5 is a plan view of the electronically resonant marker shown
in FIG. 2;
FIG. 6 is an equivalent circuit of the electronically resonant
marker shown in FIG. 5;
FIG. 7 is a block diagram of a transceiver used in carrying out
20 the locating method of this invention; and
FIG. 8 is a graphical representation of one-way soil attenuation
of an electromagnetic signal.
Detailed Descr~ption
Looking now at Fig. 1 there is shown a pictorial representation of
25 the practice of a cable locatlng method of this invention. As can be seen in
Fig. 1 a cable 20 is shown buried beneath a ground surface 21. As shown in
Fig. 2, the cable 20 comprises a core 22 which comprises at least one optical
fiber 23 which is disposed within a longitudinally extending core tube 24. In
a preferred embodiment, a plurality of strength members 26-26 are disposed
30 longitudinally or helically along the length of the core tube 24. Further, a
water blocking material 27 may be disposed about the core tube. The water
blocking material may be a tape as shown or, alternatively, may be a water
blocking yarn (not shown). It should be understood that the cable 20 is an
all-dielectric cable, that is, all of the materials that comprise the cable, for35 example, the strength members, are non-metallic. For reasons which should
be apparent, it is important for the cable 20 to be locatable notwithstanding
2~333~
its underground concealment.
When a buried non-dielectric cable, that is, a cable which
contains continuous metallic elements, is desired to be located, a prior art
cable finding method may be utilized. Looking at Fig. 3, there is shown a
5 pictorial representation of a practice of a prior art cable locating method.
As can be seen in Fig. 3 a cable 30, comprising a core 31 of copper wires
32-32 is shown buried beneath a ground surface 33. A signal transmitter 34
is shown inducing a so called trace tone 36 into a portion 37 of the cable 30.
It should be understood that in an alternative embodiment, the signal
10 transmitter 34 could be connected conductively to the cable 30 by a wire 38.
The trace tone 36 may be within a frequency range of 50 hertz (Hz) to a few
hundred kilohertz (KHz). In this frequency range the trace tone 36 is in the
form of an A.C. current and will travel along the length of the cable 30 and
in so doing generate a magnetic field 35 around the cable.
A tradesperson is shown with a hand carried detector 39
^ comprising a sensor portion 40 which comprises a multi-turn wire loop, a
handle portion 41 which comprises a signal strength indicator and a stalk
portion 42. During a cable locating episode, the hand carried detector 39 is
held such that the sensor portion 40 is positioned just above but not
20 touching the ground surface 33. As the tradesperson sweeps the sensor
portion 40 above the general location of the cable 30, the magnetic field 35
induces an electric current into the multi-turn wire loop of the sensor 40.
The induced electric current in the wire loop is converted so that an
indication of it's strength may be displayed-on a meter or be heard as a
25 varying volume tone produced by a tone transducer either of which may be
located in the handle portion of the hand-carried detector 39. As the hand
carried detector is swept in a general area above the cable a relatively high
meter reading or tone volume would indicate for example that the sensor
portion 40 is directly above the buried cable 30. As the sensor portion is
30 moved laterally the meter reading or tone volume would decrease.
Consequently, a relatively precise location of the cable may be determined
using a homing process such as that just-described.
The just-described prior art method cannot be used to locate the
all-dielectric cable 20 of the FIG. 2. In order to locate a concealed all-
35 dielectric cable ~0, the cable must be modifled in accordance with thisinvention.
2~3~
- 6 -
In order to be able to locate a concealed cable such as the all-
dielectric cable, the cable is provided with an electronically resonant system.
To this end, a plurality of electronically resonant tags or markers 44-44 (see
FIG. 2) is caused to be disposed between the core tube 24 and an outer
5 surface 46 of an external sheath member 48. In one embodiment, the
electronically resonant markers 44-44 are caused to be embedded in a plastic
material which comprises the sheath member 48 adjacent to an inner
surface 50 of the sheath member. In a preferred embodiment, the
electronically resonant markers 44-44 are spaced not only longitudinally
10 along a carrier tape 49 (see FIG. 4) but also laterally so that when the tape is wrapped about the core tube 24 during a sheathing operation, for
example, each successive electronically resonant marker is displaced in a
rotational sense, such as for example, 120 degrees from the preceeding
electronically resonant marker. The electronically resonant markers 44-44
15 which are incorporated into the otherwise all-dielectric sheath comprise
metallic portions; however, these portions are non-continuous with respect
to the length of the cable and in fact are spaced about every 60 cm within
the cable. Nor do the electronically resonant markers 44-44 provide any
kind of path to electrically ground any portion of the cable. The
20 electronically resonant markers 44-44 then do not cause an increased
potential for lightning striking the cable.
An illustration of one of the electronically resonant markers
44-44 and an accompanying equivalent circuit are shown in FIGS. 5 and 6.
As will be recalled, one method of incorporating the electronically resonant
25 marker into the all-dielectric cable structure is to cause the electronicallyresonant marker to be disposed between or embedded in members of a cable
sheath structure. It is desirable, then, that the electronically resonant
marker comprise substantially planar surfaces and be relatively thin.
A typical electronically resonant marker comprises a foil circuit
30 51 disposed on a planar surface of a relatively thin dielectric substrate 52
and a surface mounted diode 53. The shape of the foil pattern and the
arrangement of portions thereof on the planar surface of the dielectric
substrate provides capacitive and inductive elements which are represented
in the equivalent circuit, FIG. 6, as a capacitor 54 and inductor 55,
35 respectively.
3 ~ i~
- 7 --
In a preferred embodiment, the inductive foil loop 51 is an open
loop with ends 56-56 and is approximately 12 mm wide and 420 mm long
which is equal to an electrical 1/2 wavelength of an electromagnetic signal.
A pair of leads 57-57 of the diode 53 are caused to be welded to the ends
5 56-56 of the inductive foil loop and thus electrically closes the conductive
foil loop. Another inductive foil loop, one designated 58, is disposed
electrically parallel to the diode 53. The inductive foil loop 58 has an
inductance value which exactly resonates with a junction capacitance of the
diode 53 at the operating frequency of the electronically resonant marker.
10 This inductance is necessary to prevent the junction capacitance of the
diode 53 from shunting the diode which would otherwise reduce the diode's
effectiveness as a signal mixer.
The cable 20 to be located in accordance with the methods of
this invention would normally be buried to a depth in a range of 0.5 - 1.5
15 meters. A tradesperson is shown with a hand held transceiver 60 (see FIG.
1) which comprises a sensor portion 62 which comprises a transmit and
receive antenna 63, a transmitter portion 64 and a receiver portion 74 (see
FIG. 7). A handle portion 66 comprises a meter transducer 67 and an aural
transducer 68 the functions of which are to convert a signal propagated by
20 the sensor portion to a metered signal and an audible signal, respectively. A stalk portion 69 connects the handle portion with the sensor portion.
The tradesperson locates the buried cable 20 by first estimating
a general location of a portion of the cable, evidence of which may be the
known location of an associated closure in which the cable terminates or is
25 joined to other cables or a general location dictated by custom, practice or
map. The tradesperson holds the transceiver 60 in that general location and
in such a way that the sensor portion 62 is held close to but not in
engagement with the ground surface 21. The sensor portion is also caused
to sweep over a portion of the ground surface which includes a portion
30 which is directly above the buried cable 20.
As the tradesperson sweeps the sensor portion 62 above the
ground surface, the sensor portion is caused to transmit electromagnetic
energy which comprises at least one frequency and which in one
embodiment comprises energy of at least two frequencies, fa and fb, which
35 are within approximately 5~ of each other and which are generated by the
transmitter portion 64. The electromagnetic energy comprising at least two
~33~
- 8 -
frequencies is referred to hereinafter as an electromagnetic signal 71. The
electromagnetic signal 71 penetrates soil 72 and energizes the electronically
resonant markers 44-44 by inducing alternating electric current into those
electronically resonant markers which are within range of the
5 electromagnetic signal 71.
The diode 53 of the electronically resonant circuit of each marker
44 acts as a mixer of the two frequencies fa and fb of the electromagnetic
signal 71 transmitted by the transmitter of the sensor portion 62. When
two frequencies are mixed in a non-linear device, such as a diode, harmonic
10 and intermodulation frequency components of the combination of the two
frequencies are produced. Therefore, if two signals, represented by the
expressions Acos (27rfat) and Bcos(27rfbt), are allowed to be mixed in the
diode 53, one particularly useful intermodulation component is produced
and is represented by the expression kA2Bcosl27r(2fa--fb)tl in which k is a
15 coefficient related to a conversion efficiency of the circuit. If fa and fb are
fairly close in frequency, the intermodulation frequency component (2fa -
fb) resulting from the frequencies being mixed in the diode 53 will be
relatively close to the frequencies fa and fb. If fa, fb and the intermodulationfrequency (2fa - fb ) are relatively close, it is possible that the antenna 63
20 may be a common antenna to transmit frequencies fa and fb and to receive
a return signal 73 comprising the intermodulation frequency (2fa - fb )
which is radiated by the electronically resonant marker. Since the power of
the return signal 73 is proportional to the square of the power of the signal
fa it should be realized that the return signal may be maximized by
25 providing extra power into just the fa signal.
The magnitude of the radiated signal 73 received by the sensor
portion 62 must be of sufficient strength to provide an acceptable signal-to
noise ratio for the receiver portion 74. The magnitude of the energy
absorbed and radiated by the electronically resonant marker and then
30 received by the receiver portion 74 of the sensor portion 62 is, among other
things, a function of the frequency of the electromagnetic signal 71, the
magnitude of the electromagnetic signal incident on the electronically
resonant marker, an effective exposed surface area of the electronically
resonant marker and local soil conditions.
2~33~
g
The frequency and magnitude of the radiated signal 73 are to a
great extent determined by the characteristics of the electronically resonant
marker chosen, the range of depths over which the cable locating method
and equipment are to be effective, and the local soil conditions. The
5 frequency and magnitude are optimized as necessary to satisfy performance
requiremenss.
The effective exposed surface area of the electronically resonant
marker is the actual surface area of the electronically resonant marker
multiplied by the sine of the angle of incidence of the transmitted signal
10 with the electronically resonant marker. Therefore if the electromagnetic
signal 71 is incident normal to the planar surface of the electronically
resonant marker, then the effective exposed surface area of the
electronically resonant marker is the product of 1 and the actual surface
area which is equal to the actual surface area and thus the magnitude of the
15 energy absorbed and radiated would be maximized. If the electromagnetic
^^ signal is parallel to the planar surface of the electronically resonant marker
then the effective exposed surface area is the product of 0 and the actual
surface area and therefore the absorbed energy would be zero. As can be
seen, the effective exposed surface area of the electronically resonant marker
20 varies depending on the orientation of the electronically resonant markers
with respect to the transmitted signal.
Soil attenuation is a variable that influences not only the
electromagnetic signal on its trip to the electronically resonant marker but
also influences the signal radiated by the electronically resonant marker and
25 received by the receiver portion 74. The attenuation of an electromagnetic
signal as it passes through soil is a function of the condition of the soil
through which the signal passes and the signal frequency. Looking at FIG.
8 it may be seen that one way soil attenuation rises as transmitted signal
frequency rises for all types of soil conditions. Further, one way soil
30 attenuation is also a function of the moisture content of the soil. For
instance, soil attenuation (dB/meter of depth) of an electromagnetic signal
for dry soil rises from approximately 0.5 dB to 2.5 dB/meter of depth for
signal frequencies from 1 MHz to 1 GHz, respectively. For damp soil, the
soil attenuation rises from 1.5 dB to 7.5 dB/meter of depth of soil for
35 electromagnetic signal frequencies from 1 MHz to 1 GHz, respectively. Also
included on the graph of FIG. 8 is a plot 77 showing the attenuation of an
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- 10-
electromagnetic frequency through water as a function of frequency. An
electromagnetic signal traveling through water attenuates at the rate of 4
dB/meter of depth and 17.5 dB/meter of depth for electromagnetic signal
frequencies from 1 MHz to 1 GHz, respectively.
For a signal which propagates from a point source, which is the
case of the signal propagated by the transmitter portion 64 of this
invention, the signal strength at a distance d from the source is proportional
to l/d2. In the case of a signal returning through the distance d (total
distance being 2d), the strength of the return signal is proportional to 1/d4.
10 The 1/d4 is a familiar relationship of the strength of a return radar signal
and applies for distances which are large compared to a wavelength. For
the purposes of buried cable, the distance is on the order of one meter or
less. A frequency with a wavelength of the order of one meter or less
corresponds to a frequency of 300 ~Iz or higher. For frequencies at or
15 below this frequency, the relationship of returned signal strength to d
changes from 1/d4 to 1/d6. It therefore is preferable to operate at a system
frequency of at least about 300 MHz.
A choice ~f a relatively high operating frequency limits detection
techniques used to separate the return signal from the transmitted signal.
20 For instance, pulse gating techniques cannot be used. Assuming a resonant
circuit "Q" of about 10 (wherein the Q of a system relates to a slope of a
decay curve of a de-energizing resonant circuit), the energy in the
electronically resonant marker is such that ringing could be expected to last
for only about 30 nanoseconds. This would not be enough time to transmit
25 a signal, to turn off the transmitter and then to turn on a receiver to look
for a return signal. Therefore, in an alternative method, a signal comprising
two frequencies to generate a signal having a third frequency, is used. The
third signal is the intermodulation component as described hereinbefore,
and the detection thereof may be used for distinguishing a transmitted
30 signal from a received signal.
In FIG. 7 there is shown a block diagram of the transceiver 60.
The transmitter portion 64 comprises a pair of stable oscillators 81 and 82
for generating signals fa and fb, respectively. The signals fa and fb are
amplified by amplifiers 83 and 84, respectively. The signal fa is amplified to
35 a higher level than the signal fb for the reason discussed previously.
Typically, the amplitude of signal fa is caused to be 20 dB higher than the
2~33~
11 - . .
amplitude of signal fb. For the purpose of enhancing the signal detection
ability of the receiver portion 74, the transmitted signal fb may be
modulated by a mixer 86 at the output of the oscillator 82.
After sufficient amplication in amplifiers 81 and 82~ respectively,
5 the signals fa and fb are caused to be combined in a power combiner 88 and
the combined signal comprising two frequencies is sent to the high gain
antenna 63 via a directional coupler 92. The power combiner 88 also serves
to isolate the amplifiers 81 and 82 from each other so that spurious
intermodulation products are not generated within the amplifiers. It should
10 be understood that the transmitted electromagnetic energy may comprise
two signals, each at its own frequency, with the signals being combined at a
target marker 44. The directional coupler 92 allows a common antenna to
function as both a transmitting and receiving antenna. The directional
coupler prevents the direct entry of the transmitted signal into the receiving
15 portion 74 of the transceiver 60.
- The return signal 73 is received by the antenna 63 and is fed to a
notch filter 93 via the directional coupler 92. The notch filter 93 is effectiveto prevent portions of the transmitted signals fa and fb from reaching other
portions of the receiver portion 74 of the transceiver 60. Otherwise, portions
20 of the signals fa and fb could overload those portions of the receiver portion
74 which would possibly result in the generation of extraneous
intermodulation products identical in frequency to those intermodulation
components being radiated by the electronically resonant marker.
The return signal 73 leaves the notch filter 93 and is caused to
5 be amplified and filtered in at least one band pass filter 94 SO that only a
signal comprising a very narrow frequency spectrum in which primarily the
desired intermodulation frequency (2fa--fb) is included, is allowed to pass to
a detector 96. The detector ~6 produces a D.C.voltage or low frequency
audio signal as a result of the modulation of signal fb in the transmitter
30 portion. The D.C. voltage may be used to drive a meter or other signal level
indicating device and the audio signal may be sent to a speaker for the
purpose of providing an audible output as an indicator of the strength of
the radiated signal.
The receiver portion 74 ~see again FIG.7) is tuned to receive
35 only the intermodulation component of the two transmitted frequencies.
The return signal 73 varies in strength depending on, among other things,
- 12-
the depth to which the cable 20 is buried and the conditions of the soil 70.
As the sensor portion of the transceiver 60 is caused to sweep above the
ground and transmit and receive signals, the return signal is caused to be
converted into a human sensory stimulating signal, for example, an audible
5 signal or a meter signal which varies depending on the strength of the
return signal. For example, when the sensor portion 62 is positioned directly
above the cable a highest relative volume of the audible signal is heard.
The audible signal will decrease in volume as the sensor portion 62 is moved
from this position. A homing process wherein the sensor portion is caused
10 to be moved from side-to-side with progressively shorter sweeps until the
highest indicated signal strength is achieved will allow the tradesperson to
locate accurately the position of the cable 20.
It is to be understood that the above-described arrangements are
simply illustrative of the invention. Other arrangements may be devised by
15 those skilled in the art which will embody the principles of the invention
and fall within the spirit and scope thereof.
