Buried Reflector Research Articles

Retrieval of the ground dielectric permittivity by planetary GPR accommodated on a rover: Application to the estimation of the reflectors' depth by the WISDOM/ExoMars radar

Sounding radars, also called Ground Penetrating Radars (GPR), have unlocked a third dimension in planetary exploration, offering the possibility to remotely investigate the subsurface of a planetary object. In the last decade, the increasing developments in the robotic exploration of the Moon and Mars have allowed the accommodation of GPR on rovers. As such, they can perform soundings at the local scale, directly from the surface. This led to the selection of two Lunar and three Martian GPR instruments on board 5 different rovers. The interpretation of GPR soundings in terms of distances between buried interfaces/structures, and the nature of subsurface materials requires the knowledge of the velocity of the electromagnetic wave in the subsurface. This latter depends on the ground dielectric permittivity with is commonly derived from the shape of diffraction curves, if any, in the acquired radargrams. In this work we propose an automatized technique based on the Hough transform designed to detect and characterize diffraction curves. All planetary GPR to this day being “air-coupled” (with antennas a few decimeters above the surface for safety and mobility reasons), the proposed technique accounts for refraction at the surface. It is now included in the data processing and interpretation chain of WISDOM, the GPR of the Rosalind Franklin ExoMars rover. The technique was validated on both synthetic and experimental WISDOM radargrams. • We present a planetary GPR technique to retrieve the ground dielectric permittivity. • Application to buried reflectors' depth retrieval with the WISDOM/ExoMars instrument. • Preliminary validation done with FDTD simulated and experimental WISDOM radargrams.

Semi-automatic determination of layer depth, permittivity and moisture content for unbound granular pavements using multi-offset 3-D GPR

ABSTRACT A semi-automated multi-offset ground penetrating radar (GPR) analysis method called Ray-path Modelling Coherence (RM–C) was developed and field tested. It was used to quasi-continuously predict the depth, permittivity and volumetric moisture content of unbound granular (UBG) layers along a pavement site during two investigations using ground-coupled measurements from a 3-D GPR array. Predictions from the later site visit were compared to physical measurements of layer depth and moisture content at a number of locations along the site, which in general agreed well. RM-C permittivity predictions were compared to results from time domain reflectometery (TDR) sensors and diffraction hyperbola fitting of buried reflectors. The new approach reported higher permittivity values compared to these other methods, although showed similar trends in permittivity change with respect to pavement depth and ⁠variation over time. Reproducibility of the RM–C analysis was confirmed by comparing layer depth and moisture content predictions for repeat runs along the site, although occasional discrepancies were observed.

A time frequency domain feature extraction algorithm for landmine identification from GPR data

ABSTRACTGround‐penetrating radar (GPR) is a promising technique for demining procedures since it is able to detect both plastic and metal cased antipersonnel landmines. Yet, landmine identification using GPR is a challenging task since other buried reflectors such as stones or metallic debris can be detected. In this paper, a target discrimination approach is analysed and tested experimentally. It is based on relevant features extracted from 1D, GPR signals in the time frequency domain using the Wigner‐Ville distribution. Firstly, a filtering algorithm is applied to remove unwanted reflections from the background (e.g., soil surface reflection). Secondly, the preprocessed signal is transformed into a time frequency image using the Wigner‐Ville distribution. Finally, relevant features are extracted based on the singular value decomposition of the time frequency distribution. The algorithm is tested on radar data collected using two different hand‐held systems: (i) an impulse GPR‐based dual‐sensor system and (ii) a stepped‐frequency continuous‐wave GPR. Data were acquired over different types of soil and for different landmines and objects. Results are compared to features extracted using the wavelet transform and the Wilk’s lambda value is used as a criterion for optimal discrimination. Promising results are obtained, which show that time frequency features from Wigner‐Ville distribution could be used for differentiating between landmines and false alarms and could contain more valuable information than the features extracted using wavelet transform.

Photonic-crystal silicon-nanocluster light-emitting device

We report on enhanced light extraction from a light-emitting device based on amorphous silicon nanoclusters, suitable for very-large-scale integration, and operating at room temperature. Standard low-cost optical lithography is employed to fabricate a two-dimensional photonic crystal onto the device. We measured a vertical emission with the extracted radiation enhanced by over a factor of 4, without the aid of any buried reflector. These achievements demonstrate that a cost-effective exploitation of photonic crystals is indeed within the reach of semiconductor industry and open the way to a new generation of nanostructured silicon devices in which photonic and electronic functions are integrated together.

Microwave remote sensing of physically buried objects in the Negev Desert: implications for environmental research

We report remote detections of physically buried specularly reflecting objects using microwave radar at two sites: Ashalim and Tseelim in the northern region of the Negev Desert, Israel. These detections provide confirmation that microwave subsurface remote sensing is a genuine phenomenon. At Ashalim, a scatterometer operating in the P-band (441 MHz, 68 cm) was mounted on a cherry picker truck at a height of 8 m and used to detect two triangular aluminum mesh reflectors (forming a 1-m square area reflector) buried down to a depth of 8 cm in dry sand. At Tseelim, the same scatterometer was mounted on an airplane flying at an altitude of 70 m and used to detect 1-m square aluminum reflectors (each one submerged at a different location along the airplane flight path) buried down to a depth of 20 cm. The experimental results compare favorably with a theoretical model that incorporates radar absorption effects arising in the sandy subsurface layer and radar interference effects arising from phase differences between reflections from the surface and buried reflector. The theoretical modeling also predicts the detection of a subsurface reflector down to a depth of about 4.4 m. This experiment and the associated modeling approach is the first of a series of planned experiments, which we outline for the detection and the theoretical evaluation of buried reflectors using remote microwave and VHF radar. We identify potential subject areas for environmental research.

Microwave remote sensing of physically buried objects in the Negev desert: Implications for subsurface Martian exploration

We report the remote detection of a physically buried specular reflecting object using microwave radar at Ashalim in the northern region of the Negev desert, Israel. Such detection provides an important terrestrial analogy for the potential detection of specularly reflecting subsurfaces under the desiccated regolith on Mars, such as ground‐ice and liquid water. At Ashalim, a scatterometer operating in the P‐band (441 MHz, 68 cm) was mounted on a cherry picker truck at a height of 8 m and used to detect two triangular aluminum mesh reflectors (forming a one meter square area reflector) buried down to a depth of 8 cm in dry sand. The decrease in measured backscattered power with increasing burial depth of the reflector is clearly evident. The experimental results compare well with a theoretical model that incorporates radar absorption effects arising in the sandy subsurface layer and radar interference effects arising from phase differences between reflections from the surface and buried reflector. This experiment and the associated modeling approach is the first of a series of planned experiments, which we outline for the detection and the theoretical evaluation of buried reflectors using remote microwave and VHF radar. We identify many potential subject areas for subsurface remote sensing in the Martian domain, particularly groundwater/ground‐ice.

Subsurface microwave remote sensing of soil-water content: Field studies in the Negev Desert and optical modelling

The population increase in the Middle East and the respective decrease of water resources necessitate innovative methods for utilization and monitoring of water resources. Development of remote sensing tools can pave the way for remote, rapid mapping of soil-water content, control of excessive irrigation, and prevention of water waste. This paper describes a series of experiments conducted in the Negev Desert that were aimed at developing such tools for monitoring soil-water content. The use of visible near infrared and microwave techniques seems suitable. All provide good correlation with soil-water content measured on the ground. However, the microwave techniques presented here using a P-band scatterometer and ERS-2 SAR seem the most promising. Finally the possibility of optical simulation of the microwave processes is presented in an effort to improve the physical basis for empirical studies. A method of fabrication of optical samples that model soils with different water content and different surface roughness is developed, and a system for measuring backscattered signals is designed. It is shown that the reflectivity of a layered medium is a non-monotonic function of the water content. The effect of the surface roughness on the reflection from a strong buried reflector is being studied.

Amplitudes in seismic profiling data: Multilayer interference and attentuation effects at 4 and 6 kHz

Utilizing the Deep Tow instrument of the Marine Physical Labortory at Scripps Institution of Oceanography, quantitative profiling data have been collected for a number of widely separated areas of the sea floor. Data have been collected at both 4 and 6 kHz, along with transponder navigated cores for comparison of core physical properties with acoustic data. Attentuation values have been calculated from the acoustic data which show good agreement with values measured directly. However amplitudes observed for buried reflectors are often too great to be accounted for by expected impedance changes, when attentuation is considered. Thus constructive interference is suggested. This conclusion is supported by 4 and 6 kHz data from a common area which show different reflectors. Detailed physical property analyses have been performed on core samples from a survey area in the equatorial Pacific and used to calculate impedance profiles. Convolution of the 1 ms profiler wave form with these impedance profiles gives r...

NEAR‐BOTTOM OBSERVATIONS OF 4 KHZ ACOUSTIC REFLECTIVITY AND ATTENUATION

A quantitative 4-kHz seismic profiling system has been developed as part of the Deep Tow Instrumentation System of the Marine Physical Laboratory at Scripps Institution of Oceanography. The system utilizes a sea‐going digital computer system aboard ship for real‐time processing and display of the quantitative near‐bottom reflectivity data. Preliminary results from the system are presented for four diverse areas of the Pacific Ocean floor: the Samoan Passage, the Carnegie Ridge, the San Diego Trough, and, in particular, an abyssal hill region several hundred miles west of San Diego where the system was first operated in real time during a survey in May 1974. These results exhibit a number of substantial variations in observed reflectivity on a small horizontal scale. To be more specific, they show (1) several db variations in layer reflectivity for horizontal distances less than 100 m for well‐defined reflectors; (2) abrupt changes of more than 7 db in subsurface reflectivity, which are generally coincident with topographic breaks in slope; (3) several db variations in reflectivity resulting from topographic focusing by buried reflectors, suggesting larger variations for surface observations; and (4) consistently diffuse returns from volcanic basement at relatively low energy levels which are substantially reduced by thin sediment cover, suggesting significant scattering at this frequency. In addition, calculations of acoustic attenuation in the sediment column have been made from the quantitative data by determining reflectivity differences for a well‐defined layer as a function of depth of burial. These calculations give a value of 0.36 db/m for clayey silt in the San Diego Trough, which is consistent with measured values. A value of 0.12 db/m is obtained for calcareous ooze on the Carnegie Ridge. This low value for attentuation suggests that significant lithification may have occurred in these relatively young, high‐porosity sediments. Sediment physical property analyses show relatively high velocities in these calcareous sediments, suggestive of greater rigidity.