Ground-penetrating radars (GPRs) are short pulse, wide-band low-energy radars intended for probing into the earth. The transmitted pulse is usually only one radio-frequency cycle in duration, hence these radars are also called "impulse radars." The transmitted pulse is usually generated by discharging an avalanche transistor into a resistively-loaded dipole antenna which is placed in contact with the earth. The radiation pattern is ordinarily that of a simple dipole radiating into half-space.

Cart-mounted GPRs are limited to a peak power of less than one kilowatt. A typical center-frequency for the wide-band pulse is 100 MHz. The pulse repetition rate is usually of the order of 10,000 pulses per second. Because the antennas are resistively loaded to prevent them from ringing when shock excited, the antenna efficiency is only about 10%.

One cycle of energy at 100 MHz is a pulse approximately 10 nanoseconds (ns) in duration. The energy per pulse at the antenna terminals is thus about 10 (exp-5) joules per pulse. Allowing for antenna inefficiency the radiated energy is about 10 (exp-6) joules per pulse.

More recently developed GPRs designed for portable use (that is, radars that are not cart mounted) achieve a transmitter power of 30 kw. For such radars the radiated energy per pulse is about 3 x 10 (exp-5) joules per pulse.

In terms of average power, the lower-powered GPRs operate at an average power into the antenna terminal of about 10 milliwatts, and the higher powered GPRs about 0.3 watts.

At 100 MHz the wavelength of the signal radiated into the ground is about one meter. In order for an object in the earth to pick up a substantial amount of the radiated signal from a nearby GPR, the size of the object should be of the order of a meter in size. The electric field intensity generated across an object small compared to a wavelength would ordinarily be very small because very small objects are not good receiving antennas.

Electric fields in the earth generated by present-day GPRs are thus very small. These fields fall off very rapidly with distance from the antenna, in fact as the square of the distance outside the "near field." In the very near field of the transmitting dipole, an identical receiving antenna can expect to register a voltage of several volts across its terminals from a 30 kw GPR. This is a very modest electric field strength. The very low energy per pulse, the modest electric fields generated, and the long wavelengths of the radar signal all suggest that GPR's are unlikely to be a hazard as far as unexploded ordinance is concerned.

GPR's produce virtually no radio interference (RFI). Most all of the energy is radiated into the earth and only a tiny fraction goes into the air above the surface of the ground. The energy is distributed over an extremely wide bandwidth and the total radiated energy per pulse is very small. An operating GPR can be detected on a sensitive UHF receiver in the immediate vicinity. The signal resembles automobile ignition noise.

Unexploded ordinance can potentially be detonated either by mechanical pressure or by the appearance of electrical potential, for example a spark or static discharge, across sensitive detonator substances. (For this reason radio transmitters are disallowed in the vicinity of blasting operations). Attempts in the lab at SRI International to set off ordinary standard blasting caps by placing them in the immediate vicinity of an operating GPR have not succeeded in setting off caps in about a dozen attempts. GPRs are in use in coal mines where all explosive hazards regarding methane are stringently guarded against.

One can not conclude of course that operating a GPR in an area containing unexploded ordinance will be free of all risks or 100% safe. However, mechanical pressures and vibration from walking around in the restricted area are probably a much higher risk than electric fields from a GPR operated in the vicinity of unexploded ordinance.

January 1990

Lambert Dolphin
I am now retired from Geophysical Work. Contact International Radar Consulnatnts, Inc. for expert geophysical help.