Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. This nondestructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum and detects the reflected signals from subsurface structures. GPR can have applications in a variety of media, including rock, soil, ice, fresh water, pavements and structures. GPR can be used to detect subsurface objects, changes in material properties, and voids and cracks.
A variety of considerations are important in the design and operation of a GPR device, some of which are discussed below.
Sampling-Circuits.
In the prior art, an Analog to Digital Converter (ADC) is used for time-equivalent sampling. A periodic signal is transmitted. One sample of the received signal is taken on each repetition of the periodic signal. The point where the sample is taken is advanced by a fraction of the period at each repetition such that, over time, samples are taken over the full period.
In the prior art, this progressive advancement of the sampling point is accomplished through analog techniques. For example, in a typical prior-art implementation, a first circuit generates a so-called saw-tooth waveform with the desired period, and a second circuit generates a second saw-tooth waveform having the same voltage range, but with a much longer period. The two waveforms are fed to the two inputs of a comparator which generates a trigger signal when the two waveforms have the same voltage value. The trigger signal can then be used, for example, to trigger the generation of the transmitted signal, while the faster saw-tooth waveform can be used to cause the ADC to take a sample at a fixed point in the faster saw-tooth waveform.
Timing of Sampling.
In the prior art, analog saw-tooth waveforms are used. Timing accuracy depends on the linearity of the waveforms. Any distortion of the waveforms results in timing inaccuracies which, in turn, lead to errors in estimating the depth in the ground of detected objects.
System Configuration.
In the prior art, analog circuits offer only a limited range of configurations that are the hardware configurations that were envisioned when the system was designed.
Reference Pulse Timing.
In the prior art, the signal transmitted form the antenna can be monitored by coupling an attenuated version of it out of the transmission line that feeds the transmitting antenna. This attenuated version is referred to as a “reference pulse”. In the prior art, a dedicated circuit is used to measure the exact timing of the reference pulse.
Amplifier Gain.
As already mentioned in a previous paragraph, in a typical prior-art implementation, a first circuit generates a so-called saw-tooth waveform with the desired period, and a second circuit generates a second saw-tooth waveform having the same voltage range, but with a much longer period. The second saw-tooth waveform is also used, in the prior art, to control the gain of an amplifier that amplifies the received signal (equivalently, gain control can be accomplished via a fixed-gain amplifier in conjunction with a variable attenuator). When reconstructing the actual received signal, it is important to know, with good accuracy, what the gain of the amplifier was at the time when each portion of the received signal was sampled. In the prior art, it takes a careful calibration of the analog waveform and of the relationship between waveform and amplifier gain to achieve the desired accuracy.
Multi-Frequency Operation.
In some applications, it is desirable to transmit multiple signals that, together, span a wide range of frequencies. Generally, antennas that are optimized for one frequency are not particularly good at different frequencies. In the prior art, different antennas are used for different signals at different frequencies, such that a system with multi-frequency capability needs to have multiple antennas.
Multi-Channel Operation.
Prior-art GPR technology involves using Step Recovery Diodes, one for each signal generator, to create ultra-wide band pulses. Each signal generator is connected to its own separate transmitting antenna, such that each antenna receives a signal from only one signal generator.
EM Sensor.
The prior art provides a single EM sensor integrated inside a GPR antenna housing.
GPS Accuracy.
In the prior-art, a GPS receiver is used to estimate the position of the GPR system while performing measurements.
Path Reconstruction.
A GPR system is usually mounted on a wheeled cart, with a shaft encoder attached to one of the wheels for measuring the distance traveled by the cart through a measurement of the number of turns experienced by the wheel.
Mechanical.
The mechanical housing and mount of prior-art GPR systems offer limited features for ease of use.