A relatively inexpensive, lightweight, durable, easy-to-operate radio-frequency (RF) instrument has been developed, along with special electromagnetic tags, for use in detecting buried objects to which the tags are attached. Each tag comprises a dipole antenna (basically, two collinear straight wires) with a passive, electrically nonlinear load between them. By virtue of the nonlinearity of the load, the antenna reradiates harmonics and/or mixer product(s) of one or more RF signal( s) transmitted by the instrument. The instrument detects one or more of the harmonics and/or mixer products and executes a time-of-arrival measurement procedure to locate the tag. The instrument is especially well suited for locating buried pipelines marked with such tags.
The instrument is expected to make it possible to overcome deficiencies in the means used heretofore to locate buried objects. These means have included various electromagnetic objects, tracer wires, acoustic detection, holography, ground-probing radar, and record-keeping.
- Inaccurate making and keeping of records are the bane of the natural-gas industry. Because landmarks can disappear and human errors occur, records are often imprecise.
- Prior electromagnetic and magnetic metal detectors can detect only objects that are metallic, large enough, and buried at depths of
- Tracer wires, which enhance the detectability of nonmetallic objects, must be carefully handled to protect them from corrosion and disintegration over the years.
- Active and passive acoustic detection techniques are promising but can be complicated by soil moisture and a variety of soil types. If a target is close, active acoustic imaging and acoustic holography are also inaccurate.
- Ground-probing radar (GPR), the most recent addition to the buried-object-location arsenal, has afforded limited success, in situations in which moisture is low and homogeneity is high. Moreover, GPR units must be operated by only trained, experienced professionals. For these reasons, GPRs frequently do not satisfy the needs of the natural-gas industry.
The design of the present instrument and of the associated antennas and loads for marking buried objects involves, among other things, the choice of transmitting and receiving radio frequencies to obtain efficient matches for a variety of burial depths and types and conditions of soil. In the case of a pipeline, a typical antenna is about 1.5 ft (≈0.5 m) long and the nonlinear load at the middle of the antenna has a volume of
The instrument, which is carried above the ground, includes one or more transmitters operating at a frequency or frequencies in the range from 10 to 100 MHz. The frequency or frequencies are chosen low enough to enable penetration of a wide range of soil types and high enough to enable the use of a conveniently short transmitting antenna or antennas. The harmonic and/or mixer- product frequency or frequencies to be monitored are chosen low enough that sufficient reradiated signal power reaches a receiver that is part of the instrument.
In one example, a 50-MHz transmitter with a power of 1 W would suffice for detecting pipelines buried as deep as 4 ft (1.2 m). In this case, the receiver could monitor the 100-MHz (second harmonic) reradiated signal with the help of band-pass filters and a diplexer that would suppress any second-harmonic signal component radiated by the transmitter. Provided that the location of a buried object is initially known or guessed to within a few meters (so that the instrument can be brought close enough to obtain a detectable reradiated signal), the instrument configured as described above could locate a buried marker to within a few centimeters.
This work was done by G. D. Arndt of Johnson Space Center and J. R. Carl of Lockheed Martin.
This invention has been patented by NASA (U.S. Patent No. 6,097,189). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Johnson Space Center
(281) 483-0837.
Refer to MSC-22743.
