Noise measurement in a locating receiver

A method for choosing a frequency to locate an underground object. A locating receiver is provided with a display. The locating receiver scans multiple pre-selected frequencies to determine a noise level, or a signal-to-noise ratio. The locating receiver transmits the chosen frequency to a transmitter, which places the chosen frequency on the underground object to provide an optimal locating frequency.

FIELD

The present invention relates to the location of buried utilities such as pipes or cables, and particularly to the measurement of noise and the signal-to-noise ratio at various locating frequencies to improve location accuracy.

SUMMARY

The present invention is directed to a locating system for locating an underground object. The system comprises a locating receiver, a processor and a display. The locating receiver has a locate mode and a noise mode. The locating receiver is configured to take a noise measurement at a plurality of locating frequencies when in noise mode. The processor is configured to output a plurality of frequency data entries corresponding to the noise measurement. The display displays frequency data entries. The locating receiver detects an electromagnetic field strength at a chosen frequency selected from the displayed frequency data entries when in the locate mode.

The present invention is also directed to a method for detecting an underground object. The method comprises providing a locating receiver capable of detecting a plurality of locate frequencies. A noise measurement is taken at the plurality of locate frequencies with the receiver. Frequency data entries corresponding to the plurality of noise measurements are displayed and a chosen frequency corresponding to a frequency data entry is chosen.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Buried utilities in the form of lines, cables, conduits, pipelines, and other structures are used to supply a variety of utilities. Locating these utilities after installation is necessary in order to repair or replace them. Locating them is also necessary to avoid disturbing or damaging them when excavating for any other purpose.

One method of locating buried utilities is to conductively or inductively apply a signal to the utility so electrical current flows on it. This current produces a magnetic field along the full length of the buried utility. An electromagnetic line locator can then be used above the ground to trace the path of the buried conductor or to precisely determine its location. Nonconductive utilities may also be installed with a conductive tracer wire that runs the length of the utility and can be used to locate the utility.

Locating receivers and transmitters generally contain a predefined set of frequencies. With this limited set of frequencies, it is possible to have more than one locating transmitter in the same area operating at the same frequency. If two transmitters are connected to different utilities in close proximity, both utilities will have a strong magnetic field at the locate frequency. This situation can lead to errors in determining the location of a utility or mistaking another utility for the intended utility. Having two transmitters connected to utilities in reasonably close proximity or connected to the same utility can also cause beat frequency oscillations. This oscillation can confuse the locator operator and/or cause error in locating and depth measurements. This and other issues with locating with two or more transmitters in close proximity demonstrate the need to detect the presence of another transmitter operating at or near a given frequency.

Noise from other sources can be a problem when locating a buried utility. Examples of these sources include utility power grid transmission lines, radio, satellite, radar, telecom transmissions, spurious emissions from electronic equipment, lightning, and solar events. Any of these sources can interfere with proper operation of a locating receiver since the noise adds to the locating signal. These sources may vary in the frequency spectra they affect; some are narrow band while others are wide band. These sources of noise can have varying effects on different locating frequencies. As noise adversely impacts locating performance, there is a need to determine the level of noise at the frequencies used for locating.

With reference now toFIG. 1, shown therein is a general representation of a system10for locating an underground object12. The underground object12, as shown inFIG. 1, comprises a buried cable or utility line13. One of ordinary skill in the art can appreciate that the underground object12may alternatively comprise a buried object such as a downhole beacon located proximate a drilling head of a drill string (not shown), a conductive pipe, a non-conductive pipe having a tracer wire, or other objects.

The system10comprises a locating receiver, or locator14and a transmitter16. The locator14is preferably a portable device capable of detecting magnetic fields generated by currents at certain frequencies. The locator14comprises one or more antennas18for detecting such fields and a processor19for analyzing information. The antenna18may comprise a ferrite or air-cored solenoid antenna, and is preferably oriented horizontally, vertically, or mutually orthogonal (as shown inFIG. 1) to the horizontal and vertical axis, assuming that the locator14is held in a predetermined orientation and that the antenna18moves with the locator. The antenna18may alternatively comprise other antenna types, such as loop antennas.

The locator14generally is adapted to receive signals20generated by the transmitter16as discussed below. However, one of skill in the art will appreciate that locators14may detect signals generated from other sources (not shown), such as power grid components, cathodic protection systems, and communications devices. The locator14is adapted to detect signals20from a plurality of discrete frequencies.

The transmitter16is used to apply a signal20, usually in the form of an alternating electrical current of a specific frequency or frequencies, to the buried utility line13. The transmitter16may apply this current to the utility line13by direct electrical connection17or by electromagnetic induction (not shown). The current flowing along the entirety of the utility line13as a result of the transmitter16radiates the signal20in the form of a magnetic field outward from the line.

The locator14and transmitter16may cooperate in the determination of the signal20frequency to place on a buried utility line13. For example, an operator may choose a preferred frequency at the locator14, which then transmits the frequency information to the transmitter16, allowing the transmitter to apply a signal20at that frequency to the buried line13. The locator14may alternatively automatically choose the frequency prior to transmitting this information to the transmitter16. The transmission of data between the locator14and the transmitter16occurs through communication link22, which may comprise a wireline, wireless communication, Bluetooth, or other known means.

The antenna18detects an electromagnetic field at a given frequency. This field may be due to the signal20placed on the line13by the transmitter16. However, other sources of electromagnetic field may exist at the given frequency. Any electromagnetic field not generated by the signal20may be considered “noise” for locate operations. Noise measurements may be made by the antenna18based upon the vector sum of three mutually orthogonal antenna components without regard for the orientation of the locator14or the noise source. Alternatively, such measurements may be detected by a single-axis antenna18.

With reference now toFIG. 2, a representative display30of the locator14is shown. The display30shows a plurality of frequency data entries32. The frequency data entries32are representative of noise levels when the transmitter16is turned off. The frequency data entries32comprise a frequency indicator34, a scale36, and a quality indicator38. Each of the frequency data entries32represents data about the noise level at the frequencies indicated at each frequency indicator34. As shown, the frequencies listed in the frequency indicators34range from 263 Hz to 83.1 kHz. One of ordinary skill in the art will appreciate that these representative predetermined frequencies are shown herein for illustrative purposes, and alternative frequencies may be utilized by the locator14without departing from the spirit of this invention. Further, each frequency displayed on the frequency indicator34may represent a bandwidth of frequencies containing the displayed frequency. For example, an indicated frequency of 500 Hz may represent a bandwidth range of 2% to 5%. The bandwidth must be broad enough to account for ordinary tuning error between the transmitter16and locator14, but narrow enough to limit interference associated with other electromagnetic noise.

In operation, the locator14(FIG. 1) is placed into a noise measurement mode, as represented by the display30. The locator14detects the ambient electromagnetic signal (or “noise”) at each of the frequencies listed on the frequency indicators34. As shown, there are eight frequency indicators34, but more may be used and a scrolling function may allow an operator to view many frequency indicators on the display30. The locator14may do this automatically or upon prompting from an operator. The detection may be sequential or simultaneous. As shown inFIG. 2, a frequency data entry32is given an active detection indication40when that frequency is being actively detected by the locator14. The active detection indication40may take the form of a highlighted frequency data entry32, and may allow for live updating of the scale36and quality indicator38. During simultaneous measurement, all displayed frequencies may be periodically measured and updated.

The relative strength of the electromagnetic noise is shown in the scale36of the frequency data entry32. The scale36may be represented by a line, a number, or any representation of relative noise. The representation may be tied to a raw noise level, a mathematical reference number, or the like. As shown, noise is represented on the display30by a bar graph42and number44. The bar graph42and number44are preferably logarithmically scaled, though other means of reducing dynamic range or a simple linear scale may also be utilized. The quality indicator38may be utilized to indicate or categorize the relative suitability of a frequency as indicated by the frequency data entry32. As shown, the quality indicator38displays an “X” indicating a high noise level, a “-” representing a moderate noise level, and a check mark representing a low noise level. Colors, faces, and other quality indicators38may be utilized to categorize frequencies or to indicate the best frequency to an operator. Quality indicators38may be based upon the relative levels of detected ambient noise between frequencies, or may be compared to predetermined noise thresholds. Alternatively, the processor19of the locator14may automatically choose the best frequency and communicate this frequency to the transmitter16. It is understood that a low noise level is advantageous when actively locating a buried wire13in what is known in the art as an “active locate operation.” As will be described with reference toFIG. 5below, a “passive locate operation”, or one without the use of a transmitter16to induce or inject a signal, finds a buried wire13through the use of noise. Thus, in passive locate operations, the quality indicator38may favorably indicate the suitability of a frequency when a noise level associated with a frequency data entry32is high.

With reference now toFIG. 3, a method for active location mode operation of the system10is shown. The method starts at100. The locator14is placed in a location where a line13is to be located at102. The locator14is placed in a noise mode at104. The antenna18of the locator14detects the ambient electromagnetic field at a plurality of predetermined frequencies at105, either sequentially or simultaneously. A frequency data entry32is generated for each frequency at106and displayed on the display30at108. A frequency with a favorable quality indicator38is chosen at110, either by the operator or the processor19. The chosen frequency is transmitted to the transmitter16at112. The transmitter16causes the signal20to be placed on the buried line13at the chosen frequency at114. The locator14is placed in a locate mode, either by the operator or automatically, at116. At118, the locator14locates the electromagnetic field generated by the signal20. The operator then utilizes the signal20to locate the buried utility line13at120. The method ends at122.

One of skill will appreciate that the noise level at a locator14may change based upon the location of the locator. For example, when in locate mode, the locator may be moved into a location where the chosen frequency is no longer optimal for the locate operation. In such a situation, the locator14may be placed into noise mode at104periodically and the subsequent steps ofFIG. 3may be repeated to improve the effectiveness of the locate operation.

With reference now toFIG. 4, an alternative method for utilizing system10in active location mode is shown. While the noise level displayed on the frequency data entry32is sufficient for choosing a frequency in most operations, location operations that take place in areas with multiple active buried lines or where precise depth measurements are needed may utilize a “signal-to-noise” ratio which takes into account the relative strength of the signal induced by the transmitter to the ambient noise at the chosen frequency.

The method starts at200. The locator14and transmitter16coordinate their frequencies at202. A first measurement is made at a plurality of predetermined frequencies at204with no signal20on the buried utility13(FIG. 1). A second measurement is made at the plurality of predetermined frequencies at206with the signal20placed on the buried utility13at the plurality of predetermined frequencies by the transmitter16. The signal20may be constant, or may be variable. The processor19calculates the signal-to-noise ratio at208. The signal-to-noise ratio is displayed on the display30for each of the frequencies at210. A frequency with a favorable quality indicator38is chosen at212, either by the operator or the processor19. The chosen frequency is transmitted to the transmitter18at214. The transmitter16causes the signal20to be placed on the buried line13at the chosen frequency at216. The locator14is placed in a locate mode, either by the operator or automatically, at218. At220, the locator14locates the electromagnetic field generated by the signal20. The operator then utilizes the signal20to locate the buried utility line13at222. The method ends at224.

The methods for calculating signal-to-noise ratio at208may differ depending on the application. For simple locates, a quick approximation of signal-to-noise ratio may be made. In this calculation, the first measurement taken, N, is assumed to be uncorrelated additive noise, typically white Gaussian noise. The second measurement (with transmitter16transmitting) represents the quadrature sum of the noise-free signal, S, and the noise, N. The noise-corrupted signal, Sn, is given by Sn=√{square root over (S2+N2)}. The noise-free signal, S, may be estimated using these two measurements and the relationship S=√{square root over (Sn2−N2)}.

The signal-to-noise ratio is calculated using

SNR=SN
where SNR is the signal-to-noise ratio, S is the noise-free signal, and N is the noise. Coordination of the transmitter16and the locator14is required to properly measure SNR. While this could be done by two or more individuals coordinating the transmitter16frequency with the locator14measurements, the transmitter16output and locator14measurements are preferably coordinated by the processor19while operably connected to the transmitter16by a communication link22. The communication link22may also facilitate the changing of transmitter16frequencies necessary to measure SNR at multiple frequencies.

In addition, signal-to-noise ratio may be calculated at208by more precise techniques, when desired for highly precise applications. The transmitter16may be configured to force a higher signal current, I2, to be a known multiple, n, of the normal locating current I1; that is, I2=nI1. This relationship between the higher signal current and normal locating current can be an arbitrary constant n, such that n is greater than one. For the purposes of this disclosure, n=2.

This disclosure assumes the locator14has been properly oriented by conventional procedures, is directly above the line being located with the antenna18oriented for maximum signal (horizontal, above, and normal to the line being located), and is being maintained at a uniform separation from the line13. The frequency and channel gain are assumed to remain constant. The noiseless signals in response to I1and I2will be

S1=(I1d)⁢⁢and⁢⁢S2=(I2d)=(nI2d)
Where d is the distance between the antenna18and the line13. If the signal channel contains noise of amplitude N, the signals actually received and measured in response to the measured signals S1and S2will be given by:
M1=√{square root over (S12+N2)}
and
M2=√{square root over (S22+N2)}=√{square root over ((nS1)2+N2)}

For the purposes of choosing an optimum frequency, the SNR of each signal channel at the normal operating current level I1is S1/N. This may be determined analytically by the following operations:

Form the ratio, R, of the two measured readings M2and M1,

R=M2M1=n2⁢S12+N2S12+N2
Squaring both sides and collecting terms yields
S12(R2−n2)=N2(1−R2)
which leads to the desired result

In practice, the measured reading ratio R will usually be larger than unity unless the measured signals M2and M1are completely dominated by noise. 1≤R≤n. This means both numerator and denominator in the above result will be negative. For computational simplicity, we use the equivalent result

With reference now toFIG. 5, a method for passive location of the buried wire13is disclosed. One of skill in the art will appreciate that while it is preferred to utilize transmitter16to energize a buried wire13, sometimes, the line or lines being located will be inaccessible to apply the transmitter. In such situations, a “passive locate” is utilized to find buried wires13with no transmitter capable of inducing a signal. Buried electric wires will have an electric current on the wire at 60 Hz or 120 Hz. Signals characteristic of power distribution lines are typically located at harmonics of the power line frequency, such as 300 Hz and 420 Hz. When performing a passive locate, the “noise” detected in noise mode should be considered a signal indicative of an underground utility.

A method for passive location begins at300. The locator14is placed in a location where a line13is to be located at302. The locator14is placed in a noise mode at304. The antenna18of the locator14detects the ambient electromagnetic field at a plurality of predetermined frequencies at305, either sequentially or simultaneously. A frequency data entry32is generated for each frequency at306and displayed on the display30at308. A frequency with a favorable quality indicator38is selected at310, either by the operator or the processor19. The locator14is placed in a locate mode, either by the operator or automatically by the processor, at312. At314, the locator14locates the electromagnetic field utilizing the noise information found at306to locate the buried utility line13. The method ends at316.

The method and system herein is used with a buried line13. One of ordinary skill may appreciate that the underground object12may alternatively be a beacon and the locator14may be a walk-over tracking receiver such as that disclosed in U.S. Pat. No. 8,497,684 issued to Cole, et. al., the contents of which are incorporated herein by reference. The system10of the present invention may be used to choose an optimum frequency for a signal transmitter located in a single-location source such as a beacon without departing from the spirit of this disclosure. Further, while the disclosure is directed to the problem of underground objects, wires and utility lines may be in above-ground inaccessible locations such as concrete slab foundations where the system10is advantageous.