Mapping The Underworld (MTU):Mapping The Underworld (MTU):Mapping The Underworld (MTU):Mapping The Underworld (MTU):Ground Penetrating Radar Design and OptimisationGround Penetrating Radar Design and OptimisationGround Penetrating Radar Design and OptimisationGround Penetrating Radar Design and Optimisation

Dr. Adham Naji, Dr. Steve Pennock and Dr. Miles Redfern

Focus of this Work PackageFocus of this Work PackageFocus of this Work PackageFocus of this Work PackageThis work package is concerned with the design of the hardware of Ground Penetrating Radar (GPR), while introducing novel measurement techniques and operation modes.We design the system’s microwave hardware as to operate using 3 modes of operation, each using a different modulation scheme, to scan the frequency band (approximately from 150 MHz up to 3 or 4 GHz). The three different schemes are:

• Frequency-Modulation Continuous Waves, FMCW.• Stepped-Frequency Continuous Waves, SFCW.• Orthogonal-Frequency Division Multiplexing, OFDM.

The design also uses novel modes of operation; namely: • The conventional mode of operation, where the transmitter and receiver both are on the ground surface sending signals into thesoil and capturing their echoes. This is called that ‘Look-Down’mode. • The ‘Look-Through’ mode, where we insert a transmitter into a buried pipe to transmit the signals from below the surface, and receiving them from surface (here, the synchronisation between the transmitter and the receiver is secured using a fibre optic link). • The ‘Look-Out’ mode, where an inserted ‘mole’ in a buried pipe both transmits and receives the signals and their echoes from below the surface of the ground.

The hardware design also aims at maximising the Dynamic Range(DR) of the system, optimising wideband antennas and signal processing.

Contact DetailsContact DetailsContact DetailsContact DetailsProject Coordinator: Rosie Phenix-Walker. Telephone: 0121 414 3544. Address: University of Birmingham, School of Civil Engineering, Edgbaston, Birmingham, B15 2TT. Email: r.phenixwalker@bham.ac.ukWebsiteWebsiteWebsiteWebsiteMapping the Underworld has a new website which was launched at the end of 2009, including a News feed and Blog to furnish you with regular web-based updates. This can be found at www.mappingtheunderworld.ac.ukwww.mappingtheunderworld.ac.ukwww.mappingtheunderworld.ac.ukwww.mappingtheunderworld.ac.uk

Operation Using Three Different Modulation SchemesOperation Using Three Different Modulation SchemesOperation Using Three Different Modulation SchemesOperation Using Three Different Modulation SchemesLike any radio system, GPR emits electromagnetic waves into its vicinity. The electromagnetic waves used in GPR applications usually fall in the sub-band of microwaves, within the electromagnetic spectrum. Currently the regulatory limits for the frequency band of GPR operation lies within the range: 150MHz to 4GHz.When processing the signal in GPR, we either deal with very sharp time pulses (in the time-domain) that cover a wide spectrum, or we deal with frequency scans and sweeps (in the frequency-domain) to cover that spectrum. Each of these methods have its pros and cons. However, in this work, we advocate the use of the frequency-domain method due to its larger flexibility to signal processing, its cheaper implementation, and its higher ability to control the spectral-contents of the signal (e.g. tailored spectral contents to avoid interference in a specific atmosphere, to meet a specific regulatory mask on emissions on a specific scenario, or even to relax the signal processing in a specific sub-band).Within the frequency-domain signalling, we design our system to operate using 3 different schemes of modulation, which are basically different approaches to scan the same stretch of spectrum, but using different calculations and speeds. In the FMCW scheme, the frequency is swept linearly with time. When the reflected echo is compared to the transmitted signal, a difference in frequency will be measured, and that will correspond to a difference in time, which gives us the distance.In SFCM, we increase the frequency in steps, and at each step we record the measured channel’s (the soil medium’s) response in amplitude and phase, which then gives us a picture of what that medium contains of objects.OFDM, which is a technique inspired from modern communication systems, uses chunks of frequency sub-bands to scan at each instance, rather than a single frequency tone per step. This speeds up the operation of SFCM, with a little toll in signal processing. It promises to be of very desirable features, mainly due to its high speed and good accuracy.

Integration of GPR with Other Sensors Integration of GPR with Other Sensors Integration of GPR with Other Sensors Integration of GPR with Other Sensors and the Measurement Cycleand the Measurement Cycleand the Measurement Cycleand the Measurement Cycle

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Transmitter

Receiver

Time delay (result),

which gives the distance

Frequency

Time

Frequency difference (measured)

Amplitude

Frequency

Spectral amplitudes control

Amplitude

Frequency

e.g., 100MHz chunks of bandwidth

FMCW

SFCW

OFDM

Spectral amplitudes control

The different modes of operation to find buried assets using GPR.

The general operation of the system, and the three modulation schemes it utilises.

The different modulation schemes used, as seen in the time or frequency domains.

GPR uses microwave signals, which propagate best when the medium of propagation is of similar dielectric characteristics assumed in the system’s antennas. Microwave signals suffer strong reflection from conductive material, and do not penetrate metals and good conductors for more than a few microns. Therefore, the addition of other sensors that could operate effectively to detect objects with conditions difficult for GPR-detection makes sense to circumvent such a problem. Acoustic detection, low-frequency electromagnetic detection, and magnetic coils for current detection are being combined with GPR in this MTU project, to optimise ourchances of successfully detecting a wider range of buried assets, and in different soil conditions.The combined sensors will co-exist on the same trolley, which will physically scan the field or road of interest to the operator. To minimise any possible mutual effects among the different sensors’electronics (and not necessarily their emitted signals), we opted to have staggered operation times, with one sensor operating at a time, at every given (x,y,z) location of measurement.The trolley structure would contain a minimum of metal, to assist the magnetic coils operation, and would allow for automated movementof parts, where needs.A microcontroller system will exist on-board to govern the timing or operation among the different sensors and to control the trafficbetween the sensors and the PC that will store the measured data. Such a power microcontroller will also be able to offer brief assistance in signal processing and conditioning, before it is stored for further post-processing by software.

Sensors

Micro-controllerTiming control + storage control

+ fast DSP processing

PCStorage + Post-processing

GPR measurement points

GPR storage instances

GPR Post-prossingOther sensors’

storage

Other sensorsmeasuring

Permission to start granted?

Cycle starts

End of GPR measurements,start of data post-

processing and start of other sensors

operation

GPR finished post-processingwhile the full cycle has not; other

sensors still in operation.

One Measurement Cycle(at a given location)

The GPR is expected to be the fastest devise to operate, and therefore would finish scanning the different frequency points first (according to its modulation scheme) and then wait for the other sensors to finish. In the meantime, GPR can also benefit from some fast signal processing from the microcontroller before storage via the USB port of a PC.At a given location, and on the order of the controlling software, each measurement cycle would look like that illustrated below.

Realisation and ProgressRealisation and ProgressRealisation and ProgressRealisation and ProgressThe hardware realisation at this stage of the project focuses on having the hardware in a compact PCB format, especially when considering using the ‘mole’ for in-pipe detection. We subdivide the system into sub-blocks, test them separately, and then combine them together. Weare currently finalising our last sub-blocks to start putting the whole system together. The theoretical DR for the system is 90dB, and the system block diagram is illustrated below. The chosen microcontroller to control this

timing among the different sensors, and to control the data traffic to the USB port for PC storage, is the Atmel AVR-32 microcontroller. This can also offer fast signal processing and conditioning for the data before storage, to ease the task of software in post-processing and to speed the operation from the moment of measurement to the moment the result are viewed on the user’s screen.

Photographs showing an example of systems sub-blocks prototyping (left) and the AVR-32bits

Microcontroller’s starter-kit (right).