Ž .
Journal of Applied Geophysics 43 2000 259–269 www.elsevier.nlrlocaterjappgeo
Gated stepped-frequency ground penetrating radar
G.F. Stickley , D.A. Noon
1
, M. Cherniakov
2
, I.D. Longstaff
3
Department of Computer Science and Electrical Engineering, The UniÕersity of Queensland, St. Lucia, Qld 4072, Australia CooperatiÕe Research Centre for Sensor Signal and Information Processing CSSIP , Australia
Received 14 September 1998; received in revised form 26 February 1999; accepted 16 March 1999
Abstract
Ž .
This paper describes a newly developed stepped-frequency ground penetrating radar SFGPR prototype operating across the 10–620 MHz frequency band. The SFGPR system has been developed to improve the maximum penetration depth
Ž .
capability of ground penetrating radar GPR applications without degradation of resolving power. The radar has the unique capability to employ a short gate at each frequency step that can suppress unwanted strong signals relative to weak
reflections from deep targets. The system performance is approximately 175 dB in ungated mode and 170 dB in gated mode, with reference to a 10 ms integration time and a 10 dB signal-to-noise ratio. The current system is suitable for operation in a
vehicle and incorporates a real time display similar to conventional impulse GPR. It is capable of recording 50 depth profiles per second. Subsurface images obtained with the gated SFGPR of a perched water table on a sand island, a culvert in
high-loss ground, and a 500 lb bomb buried at 1.7 m deep are presented. A frequency compensation procedure is used to improve image clarity. The weakest confirmed underground target has a signal power 100 dB down on the transmitted
power. Such a signal would be near the noise floor of a commercial impulse GPR. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Ground penetrating radar; GPR; Stepped frequency radar; Gating
1. Introduction
Ž .
Commercial ground penetrating radar GPR systems dominantly use impulse transmitters
Ž .
with sampling head receivers Daniels, 1996 . Such receivers are inefficient because many
Corresponding author. Tel.: q61-7-3365-3693; fax: q61-7-3365-3684; e-mail: stickleycsee.uq.edu.au
1
E-mail: nooncsee.uq.edu.au.
2
E-mail: mccsee.uq.edu.au.
3
E-mail: idlcsee.uq.edu.au.
transmitted pulses are required to sample the entire waveform. In addition, the mean power
available from the impulse transmitter is limited by the extremely low duty cycle.
The stepped-frequency radar technique offers substantial benefits over current impulse radar
systems. The main advantage of the stepped- frequency technique is that it is relatively easy
with current technologies to efficiently sample ultra-wideband signals with low speed analog-
to-digital converters. Also, due to the transmis- sion of long duration waveforms, a high average
transmitted power is much easier to obtain than
0926-9851r00r - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž
. PII: S 0 9 2 6 - 9 8 5 1 9 9 0 0 0 6 3 - 4
for short-pulse and impulse waveforms. Another advantage of stepped-frequency radar is its
agility to skip over frequencies that could inter- Ž
fere with commercial broadcast stations Poirier, .
1993 . Ž
. Robinson et al.
1972 first proposed the
synthesized pulse or stepped-frequency radar method as a means of improving the penetration
performance of GPR without compromising res- Ž
. olution. Noon et al. 1994 have given a histori-
cal review and technical description of the Stepped-Frequency Ground Penetrating Radar
Ž .
SFGPR technique. SFGPR consists of measur- ing the complex reflection coefficient of the
ground at a number of discrete continuous-wave frequencies, and then transforming to the time
domain via a discrete Fourier transform. The main disadvantage with continuous-wave trans-
mission with SFGPR is that a strong signal either from leakage between the transmitter and
receiver antennas or from shallow reflectors can mask weaker signals from deep reflectors.
Ž A gated SFGPR Hamran et al., 1995; Stick-
. ley et al., 1996
combines the high average power and efficient sampling of stepped-
frequency radar with the ability of impulse GPR Ž
. to image deep weak targets in the presence of
Ž .
shallow strong reflectors. In such a system, the stepped-frequency transmissions are pulsed, and
a receiver ‘‘gate’’ is used to pass the reflections of targets from desirable depths and ‘‘blank’’
the unwanted reflections. Unlike the sampling head used by most impulse GPR systems, the
gated stepped-frequency receiver coherently samples the gated reflections from multiple
transmitter pulses, thereby improving the sys- tem performance. It is possible for the stepped-
frequency radar to coherently sample reflections from multiple receiver gate depths, although the
system described in Section 2 does not have this implemented.
Section 2 of this paper describes the newly developed SFGPR system that operates across
the 10–620 MHz frequency band. The gating concept for SFGPR is described in Section 3.
Results of the gated SFGPR system arising from field tests are presented and discussed in Sec-
tion 4, with conclusions stated in Section 5.
2. Stepped-frequency GPR system
