Manuel Pereira

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.

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.

Imaging prestige fuel layers below sand using in situ radar sensors

Prestige fuel oil tanker was damaged during a storm in November 13rd, 2002, close to the coast of Galicia (Spain). After some days the Prestige broke in half and sank, leaking about 40.000 tons of oil which affected more than 1.000 km of the coast in Spain, Portugal and France. Some months later, layers of fuel contamination still appear at different depths in the sand of the beaches. The tidal process is that the first tide brings fuel over the sand but, if it is not removed, following tides place clean layer of sand on the top of fuel, and the beaches appear to be clean. Layers of fuel appears at different depths in the sand, from some cm to 1-2 meters. The lateral extent of the contamination also varies from some cm to more than 1 m. Radar sensors could be used in-situ to detect and imaging fuel layers below sand in some inland areas, which are under the influence of high winter tides but remain out of the influence of salt water from the sea during spring and summer time. This study show some tests carried out on the beaches with a ground-penetrating radar system operating with 500 & 800 MHz nominal frequency antennas, and a study case made in the beach of Carnota (Galicia) where it was possible to detect an imaging a buried fuel layer 6 months later.