A variety of devices are used to determine the location and orientation of concealed underground objects. Determining the location of such concealed objects as underground gas, sewer and water pipes, power cables, and telephone and CATV cables or conduits is a necessary prerequisite to excavation and/or the laying of new lines, pipes or cables. For simplicity, these underground objects are hereinafter referred to as underground "lines."
In some applications, an underground steerable boring tool is utilized to form an underground tunnel through which underground lines are subsequently routed. While utilizing a boring tool, it is important for an operator to trace or keep track of the relative location of the existing underground lines with respect to the tool, in order to avoid contacting the existing lines with the tool. In other applications, a trench is excavated and lines are subsequently placed in the open trench. While excavating these trenches, it is equally important for an operator to know the locations of any existing lines in order to avoid contacting them with the excavating equipment.
Special-purpose electromagnetic signal detector systems, which are commonly called "locator systems," have been used for many years to locate and/or trace the path of a boring tool or concealed underground lines. Various types of locator systems exist, but receivers that detect electromagnetic signals radiating from either the underground lines or a small transmitter located within the boring tool are by far the most widely used. Such radiated signals are generally produced in two ways: (1) an alternating current signal from an aboveground transmitting source is induced into an underground conductive line which generates an electromagnetic field around the line, or (2) a signal is radiated from a small transmitter either mounted inside a boring tool or positioned within a non-conductive line.
Generally, two types of signal sources will induce a current in a conductive line which, in turn, will generate an electromagnetic field around the line: active signal sources and passive signal sources. An example of a passive signal source in a locating environment is the signal radiated from a radio broadcast station. When such a signal encounters a portion of a buried conductive line, the signal induces a current in the line, which generates an electromagnetic field around the line. Such a source is called a passive signal source, because it requires no operator intervention to generate the electromagnetic field. The problem with many passive signal sources is that the same signal may be induced into many different lines, which complicates the operator's task of distinguishing between the different lines. Certain passive signal sources, on the other hand, have characteristic radiations that can be used to identify them. For example, CATV cables radiate signals at television synchronization frequencies and harmonics thereof. The present invention permits a locator receiver to be tuned to these characteristic frequencies for passive locating, and tuned to different frequencies for active locating.
Conversely, an active signal source is intentionally utilized by an operator to generate an electromagnetic field directly associated with the line to be traced. For example, an operator may couple a signal having a known frequency to an underground cable, for the purpose of generating a distinct electromagnetic field around the cable. The presence of the distinct electromagnetic field allows the operator to locate the cable and distinguish it from other cables with an aboveground receiver. This ability to distinguish between the cables is a key advantage of an active signal source over a passive source.
As discussed above, a key advantage of an active signal source is the capability of coupling a distinctive frequency signal onto one conductive line, and distinguishing that particular line from adjacent or nearby lines. Consequently, the conductive line of interest can be traced with less confusion or interference from the adjacent lines. Since the frequency of the coupled signal can be controlled precisely, a narrow bandwidth can be used for greater selectivity in the receiver. Also, the use of a narrow bandwidth improves the signal-to-noise ratio and increases the sensitivity of the receiver. The use of a narrow bandwidth in a locator receiver can be especially important for locating conductive lines in the vicinity of a strong radio transmitter, where the airborne signal can swamp out an underground signal unless the airborne signal is filtered out by the receiver's selective, narrow-band circuitry. Another advantage of an active source is that locator calculations of the position and depth of underground conductive lines are not affected by electromagnetic field distortions from multiple signal sources to the same degree as they are with a passive signal source. Thus, a need has developed for multiple transmitter frequencies and narrow-band receiver tuning to avoid interfering signals and improve signal-to-noise ratios. With previous transmitters and receivers, only a few frequencies were provided for these purposes (typically one or two and a maximum of four).
The most practical way to couple a signal to an underground conductive line is simply to attach a wire directly from the transmitter to the line. Such a technique is illustrated in U.S. Pat. No. 4,387,340 to Peterman. If this approach is not feasible, it may be possible to attach the transmitter wire to a toroidal clamp, which is placed around the circumference of the line in order to induce a signal current into the line. The signal induced into the line is then radiated from the line and detected with a locator receiver.
As shown in Peterman, an active signal source is commonly used when an operator desires to locate and trace a specific underground line that is near numerous other lines. A distinctive frequency signal is coupled from a locator transmitter to the line to be traced. For example, the transmitter generates a signal at a specific frequency. A locator receiver is manually set to the specific frequency. A locator receiver is manually set to the frequency of the transmitted signal, and the receiver operator can thereby distinguish the particular line which is radiating the transmitted signal from the other, nearby lines.
More specifically, FIG. 1 illustrates a perspective view of a conventional aboveground locator system utilizing an active signal source. Transmitter 10 is positioned on the surface of earth 15 above buried line 20, which is the concealed object to be traced. Transmitter output connector 12 is connected to wire 18, which is in turn connected to conductive line 20. The connection of wire 18 to line 20 may be accomplished by directly attaching wire 18 to line 20, thus providing an electrical connection therebetween, or by connecting a toroidal clamp (not shown) to wire 18 and placing the clamp around line 20 to thereby induce a current. Thus, the output signal of transmitter 10, which is an AC continuous wave (CW) signal, is induced into line 20. Ground stake 16 is placed deep into the earth and connected to transmitter 10 by ground lead 17, in order to provide a ground return path for the signal current induced into line 20. Consequently, the output signal from transmitter 10, which is at a unique frequency, generates an electromagnetic field that radiates from line 20 with a field pattern such as that illustrated in FIG. 2. Receiver 30 is positioned on the surface of earth 15 in the general vicinity of line 20 and manually set by an operator to the frequency of the transmitted signal. By sensing and processing the signal radiated from line 20, and using conventional locating techniques, the receiver operator locates the position of line 20 and traces the signal along the line's path.
Nevertheless, conventional locator systems are relatively inflexible and inefficient from an operational standpoint, because they are constrained to locating and tracing only one line at a time. Because locating, tracing and distinguishing between multiple, concealed underground lines has become increasingly time consuming and costly, a need has existed in the art of locator system design to provide more flexible and less costly locating techniques and equipment. For example, at a relatively large excavation site, it may be necessary to trace many different lines. Utilizing a conventional active signal source locating system, an operator selects a particular line to be traced, attaches a coupling wire from an aboveground transmitter to the line and a lead from the transmitter to a ground stake, and manually selects a transmitter frequency. Either the same or another operator sets up the locator receiver to receive the transmitted signal (i.e., selects the appropriate receiver frequency) and then proceeds to locate and trace the line of interest. Since the conventional locator transmitter is configured to output only one signal frequency at a time, the operator is limited to tracing one line at a time with that transmitter. Consequently, if more than one line is to be traced simultaneously, then a separate transmitter must be attached to each line to be traced. However, the present invention is not so limited by such constraints. For example, in order to reduce the time required to trace a number of separate lines, without incurring the cost of utilizing a multiple number of separate transmitters, the present invention utilizes one locator transmitter capable of simultaneously coupling a separate frequency signal to each of the lines to be traced. Consequently, each line radiates a unique signal that can be identified by an operator by tuning a receiver to the different frequency on each line of interest.
Interference from extraneous signals creates another problem for conventional locator systems. For example, power lines in the United States radiate signals having a fundamental frequency of 60 Hz. Some other countries' power lines radiate signals at 50 Hz. Although these 50 and 60 Hz signals are not in themselves bothersome in a locating environment (i.e., they are well below the locating frequency band of 1 kHz-100 kHz), signals at their harmonic frequencies are generated that fall within the locating frequency bands. These power line harmonics are of appreciable signal strength and interfere significantly with the reception of the radiated signals from the underground lines. As another example, very low frequency (VLF) radio transmitters radiate signals of enormous power that are used for communicating with submarines. The VLF signals radiated from these transmitters penetrate the earth to a considerable depth and are induced into all conductive lines in the signal path. These signals are then reradiated from the lines. The frequencies of these VLF signals fall within the locating signal bands and also interfere with the radiated signals from the underground lines to be traced. Because of the limited number of frequencies available with conventional locator systems (e.g., one or two), an operator may not have the flexibility needed to avoid receiving a significant amount of interference along with the signals of interest. However, the present invention provides a novel arrangement for transmitting and receiving essentially an unlimited number of different frequency signals within the locating band, which provides the operational flexibility required not only to avoid any extraneous, interfering signals, but also to provide a greater operating range in the presence of interfering signals. The present invention also provides a novel capability of preselecting an operating frequency, in order to minimize the amount of signal interference that likely would be encountered.