Fernando I. Rial

3D GPR in forensics: Finding a clandestine grave in a mountainous environment

In the present work we show a forensic case study carried out in a mountainous environment. Main objective was to locate a clandestine grave which is around 10-20 years old and contains human remains of one individual and a metallic tool, probably a pick. Survey design started with an experimental burial of a pick at the expected depth (1m) as well as the calculation of synthetic radargrams in order to know if the 250MHz antenna was suitable for its detection and to have a record of the reflection of the pick. Conclusions extracted from the experiments together with rough terrain conditions suggested the use of the 250MHz antenna which allowed a good compromise between target detection and dense grid acquisition of an extensive survey area.

Waveform Analysis of UWB GPR Antennas

Ground Penetrating Radar (GPR) systems fall into the category of ultra-wideband (UWB) devices. Most GPR equipment covers a frequency range between an octave and a decade by using short-time pulses. Each signal recorded by a GPR gathers a temporal log of attenuated and distorted versions of these pulses (due to the effect of the propagation medium) plus possible electromagnetic interferences and noise. In order to make a good interpretation of this data and extract the most possible information during processing, a deep knowledge of the wavelet emitted by the antennas is essential. Moreover, some advanced processing techniques require specific knowledge of this signal to obtain satisfactory results. In this work, we carried out a series of tests in order to determine the source wavelet emitted by a ground-coupled antenna with a 500 MHz central frequency.

Analysis of the Emitted Wavelet of High-Resolution Bowtie GPR Antennas.

Most Ground Penetrating Radars (GPR) cover a wide frequency range by emitting very short time wavelets. In this work, we study in detail the wavelet emitted by two bowtie GPR antennas with nominal frequencies of 800 MHz and 1 GHz. Knowledge of this emitted wavelet allows us to extract as much information as possible from recorded signals, using advanced processing techniques and computer simulations. Following previously published methodology used by Rial et al. [1], which ensures system stability and reliability in data acquisition, a thorough analysis of the wavelet in both time and frequency domain is performed. Most of tests were carried out with air as propagation medium, allowing a proper analysis of the geometrical attenuation factor. Furthermore, we attempt to determine, for each antenna, a time zero in the records to allow us to correctly assign a position to the reflectors detected by the radar. Obtained results indicate that the time zero is not a constant value for the evaluated antennas, but instead depends on the characteristics of the material in contact with the antenna.

Characterization of a Romanesque Bridge in Galicia (Spain)

This article presents the characterization of a mediaeval bridge located in Fillaboa, Galicia (northern Spain). The study of this bridge involves data acquisition about the structure (geometry, visual inspection of damages, and nondestructive testing), the evaluation of the possible damage mechanisms compatible with the observed cracks and fissures, and the dynamic evaluation of the structure. This bridge is a masonry four-lancet arches bridge, with damage on the piers and abutments. Two non-invasive methodologies are applied to obtain information about the bridge: ground-penetrating radar (GPR) survey and ambient vibration noise measurements. The drawings of the structure were created using close-range photogrammetry (CRP). A finite elements model of the structure was obtained prior to the vibration field measurements, as a preliminary evaluation. Data obtained from GPR and the geometry determined with CRP were the information used in this preliminary model of the bridge. This model was improved using the dynamic field test to compare model behavior and to validate the numerical results. A second and more accurate model was then obtained by using finite elements according to the experimentally measured modal frequencies (the possible first three transversal vibration modes of the bridge).

Masonry arch bridges evaluation by means of GPR

Some masonry arch bridges in Galicia (NW Spain) were surveyed with GPR using 250 and 500 MHz antennas. The main goal of this work was to perform an analysis of historical bridges, obtaining information about filling material homogeneity, detecting inclusions of different materials, defining structural faults -such as internal voids or cracks- and detecting ancient features -as hidden arches or previous profiles of the bridge-. Geometric 3D models of the surveyed bridges obtained with Laser Scanning methods, were used as inputs to create synthetic radargrams through FDTD simulations in order to help GPR data interpretation.

Checking the Signal Stability in GPR Systems and Antennas

The different components of GPR equipment have particular characteristics that, when taken together as a whole, give the system the stability required for correct usage. Besides the information provided by manufacturers about parameters affecting the stability of GPR equipments, their evolution with use and ageing suggest that each GPR system employed in detailed qualitative studies should be subject to routine analysis. This type of analysis is especially important in novel systems and antennas to understand their real capabilities and limitations. In this work, several tests are carried out in order to evaluate the stability of a GPR system working with three different antennas with nominal frequencies of 500, 800, and 1000 MHz. Some tests published by other authors, together with other tests proposed here, are adapted to be the starting point to develop a methodology for calibrating GPR devices and to verify proper operation.

Vertical and Horizontal Resolution of GPR bow-tie antennas

Since the characteristics of the detected reflections depend on the issued signal properties, a key factor for carrying out a successful GPR survey is to know as much as possible about the transmission features of the antennas. This information is essential when deciding the antenna and which is the most appropriate parameter configuration setting for a specific study. These characteristics vary for the different available GPR equipments. Numerous experimental tests have been developed in this way. In this paper we present the first results of set of experiments about the resolution capabilities of two commercially bow-tie antennas (1GHz and 800 MHz). The propagation media was air in this first study and the experimental results are compared with the theoretical estimations. The obtained conclusions are the first step in order to establish the real bounds for the detection capability of these antennas.

Analysis and calibration of gpr shielded antennas

Abslruct-A key factor for the accurate interpretation of surface-penetrating radar records is to know as much as possible about the transmission features of our antennas. The characteristics o f the detected reflections (trace time zero, duration and shape of the reflected pulse, minimum overlap distance between direct signal and first reflection, etc) depend on the issued signal properties. Since these characteristics can vary for the different GPR equipments available, in this paper we present the results of various experiments to analyze and calibrate 500,800 and 1000 MHz shielded antennas

Acquisition and synchronism of GPR and GPS data. Application on road evaluation

At the moment of carry out a study with ground penetrating radar (GPR) it is interesting to count with the support provided by other information sources. All the available information relative to the study area will be valuable in the subsequent phases of processing and interpretation of the obtained GPR records. Nowadays there is a logical trend to the integration of GPS devices. The decrease in size of these equipment, the increase of their accuracy and new wireless communication technologies (802.11, Bluetooth,...) encourage this incorporation. GPR/GPS integration allows an accuracy positioning of the radar data under favourable conditions. Furthermore it brings the possibility to import this data into a geographic information system (GIS). This study deepens the process of integration of both technologies applied to road evaluation. To the accomplishment of this study, a dual frequency (L1+L2) RTK GPS, two Bluetooth GPS receivers (with SiRF chip) admitting both real time differential corrections (SBAS), and a GPS receiver with post-processed sub-meter accuracy were used. As regards GPR equipment, shielded 500, 800 and 1000 MHz antennas were used in different configurations.

From Pseudo-3D to full-resolution GPR imaging in archaeology: A complex Roman site in Lugo, Spain

This case study demonstrates how extra effort on data acquisition can beneficiate advanced interpretation of 3D GPR data over a complex Roman site situated in a semi-urban area. Two 250 MHz surveys during May 2007 and July 2008 were accomplished after Roman wall remains had been found by chance during soil remove works at Agro da Ponte (Lugo, NW Spain). First campaign covered the whole area by using a pseudo-3D strategy which was only enough for defining the areas of interest and some archaeological features. According to those results, next campaign was focused on a smaller area and based on an ultra-dense grid strategy which eventually revealed full-resolution images of walls, apses and chambers of a Roman villa.