Different types of GPR equipment emit different forms of EM signal. The most commonly-used system types are Impulse GPR, which emit brief pulsed signals, and Stepped-Frequency GPR, which emit narrowband sinusoids. Both types typically use a high-fidelity receiver (such as a 16-bit Analog to Digital Converter, ADC) to measure returning reflections.
These receivers, however, are unable to measure the entire return signal in one operation. So instead, an incremental sampling approach is used.
Impulse GPR systems do this by measuring the magnitude of the return signal for one or more brief instants after each emitted pulse, while adjusting the delay between pulse generation and sampling. Measurements from many pulses at different delays are then combined to create one equivalent A-scan. While simple, this approach is also highly inefficient because the ADC only “listens” to a very small fraction of each return signal.
Step frequency GPR systems, on the other hand, emit a series of narrowband signals at discrete steps over the frequency range of interest. The amplitude and phase of the response is measured at each frequency step, before moving to the next. An Inverse Fourier Transform (IFT) then combines and converts the measurements at all frequency steps into the time-domain, producing the GPR A-scan. While far more efficient in terms of receiver design (i.e. the ADC is continuously “listening” at each step), the hardware is also more complex and difficult to scale. Furthermore, the narrowband signals used are problematic in terms of regulatory compliance.