Analytical Approach for RASCAN Radar Images of Dinosaur Footprints through Basic Experiments

Transcription

1 1586 PIERS Proceedings, Stockholm, Sweden, Aug , 2013 Analytical Approach for RASCAN Radar Images of Dinosaur Footprints through Basic Experiments M. Inagaki 1, T. Bechtel 2, L. Capineri 3, S. Ivashov 4, and C. Windsor 5 1 Walnut Ltd., , Saiwaicho, Tachikawa, Tokyo , Japan 2 Department of Earth and Environment, Franklin & Marshal College, Lancaster, PA, USA 3 Department of Information Engineering, University of Florence, Florence 50139, Italy 4 Remote Sensing Laboratory, Bauman Moscow State Technical University, Moscow, Russia 5 116, New Road, East Hagbourne, OX11 9LD, UK Abstract The existence of a footprint fossil indicates that some discontinuity occurred at the footprint plane during sedimentation. RASCAN holographic radar reveals the plan shape of shallow buried objects. A simple experiment showed that a clod of loam soil buried in powdered loam soil was recognizable in a radar image. Another experiment showed that a thin film inserted between dielectrically similar materials also provides recognizable reflection. Finally, blind tests were performed for realistic surrogates for dinosaur footprints. The radar image has no recognizable pattern for the case of microscopic air gap at the footprint parting plane. However, when the air gap was replaced by very thin clay film, the shape of the toes became recognizable. Thus, there may be some cases in which some discontinuity during sedimentation makes the shape of a dinosaur footprint fossil recognizable by holographic radar. 1. INTRODUCTION A dinosaur footprint, which is not the creature itself, but a mold of markings made by the activites of the creature, represents a trace fossil. A RASCAN holographic radar image taken at Parco dei Lavani site (Italy) shows that some reflection related to dinosaur footprints is produced inside rock body [1] (Figure 1). Generally speaking, the reflection circumstance inside sedimentary rock is not necessarily cooperative to GPR. Nevertheless, a small blow of a hammer may unveil a footprint with amazingly fine shape. This indicates that some discontinuity occurred at the footprint plane during sedimentation even if the rock appears homogeneous. The possibility of revealing the shape of trace fossils by radar is analytically investigated using simple experiments. Figure 1: Partially exposed dinosaur footprint and its holographic radar image. 2. RASCAN HOLOGRAPHIC RADAR RASCAN holographic radar provides plan subsurface images which can reveal the shape of buried objects. Although an impulse radar records reflected wave strengths and travel times as direct values, holographic radar does indirect values determined by the degree of phase coincidence between incident and reflected waves. As a result effect from strong surface reflections can be avoided. Therefore RASCAN can produce clear images especially at shallower depths. RASCAN operates at 5 discrete continuous wave frequencies from 3.6 to 4.0 GHz with two receiving antennae in parallel and cross polarizations relative to the transmitter. As a result, two sets of five images are obtained. At least one image among ten is expected to provide a fine view by realizing the optimum combination of the permittivity of the medium and the distance from antenna to the target.

2 Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug , SMALL DIFFERENCE IN PERMITTIVITY Electromagnetic waves reflect at the boundary between layers with different permittivity. The intensity of reflection decreases with smaller difference of permittivity. Reflected waves become unrecognizable below some intensity threshold, and reflections cease at zero difference Obviously, identical materials have identical specific permittivity. Permittivity of soils varies strongly with water content. However, slowly dried loam forms clods and instantly dried loam forms powder. These two soils then have identical mineralogy and water content, but different texture due to different history, and are expected to have slightly different permittivity. Loam soil was left to dry naturally for about four years after full saturation, finally forming cohesive lumps or clods. Some of the clods were smashed to make granular powder. Three clods were placed in a container and buried in the powder. Figure 2 shows the clods before buried in the powder. RASCAN holographic radar was scanned over the test bed. Figure 3 is a radar image of the scan. Next the powdered loam soil was replaced by dry sand, and RASCAN holographic radar was scanned again. Figure 4 is a radar image of the clods-in-sand scan. Figure 2: Clods of dried loam soil in the test bed. Figure 3: RASCAN holographic radar 3.8 GHz parallel polarization image of the clods buried in powdered loam soil. Figure 4: RASCAN holographic radar 3.8 GHz parallel polarization image of the clods buried in sand. In Figure 3 the right clod is very hard to recognize. However, the upper and the left clods are both subtly recognizable. In Figure 4 it is obvious that all the clods are fully recognizable. This fine sensitivity of holographic radar for even small difference in permittivity might be helpful to determine the presence and shape of hidden dinosaur footprints. 4. EFFECT OF THIN LAYERS A base material in which a thin layer of some impurity is inserted produces reflections at this thin layer. If the layer is too thin, the reflection will not be recognizable. However, the reflection becomes stronger with increasing impurity layer thickness and finally reaches a recognizable level [2]. Experiments were carried out for some materials with different impurity layers. Rubber plates and vinyl chloride plates were used as the base materials. An air gap was used as the impurity layer. The permittivity is 8.3 for rubber, 2.7 for vinyl chloride and 1 for air. Figure 5 shows a test body using rubber plates. The left body has no gap and the right body has a small air gap. The air gap is set by lifting the uppermost rubber plate (10 mm thickness) with paper slips piled as spacers placed under the both sides of the plate. Each paper slip has 100 µm thickness, so the gap can be increased in this increment. This method was applied in the same manner using vinyl chloride plates as the base material, but with the thickness of upper part 12 mm instead of 10 mm. The

3 1588 PIERS Proceedings, Stockholm, Sweden, Aug , 2013 (a) (b) Figure 5: Test body for contrast experiment. Figure 6: Holographic radar images. Air gap is (a) 200 µm and (b) 1 mm. test body was scanned by RASCAN holographic radar with air gap varying from 200 µm to 9 mm. The left and right bodies become sufficiently identical when the gap is more than 1 mm for rubber, and 3 mm for vinyl chloride. Figure 6 shows holographic radar images with 200 µm gap (top) and 1 mm gap (bottom). The top image indicates no contrast difference and the bottom image clearly shows the difference. The intensity contrast between the right body and left body were calculated and plotted on Figure 7 for rubber and Figure 8 for vinyl chloride. This means that the contrast of permittivity and the thickness of impurity layer are both important factors. In Figure 7 (rubber) 4 GHz data seem to represent the best distance from antenna to the target among five frequencies. The contrast is very small at less than 1 mm thickness and it almost linearly increase from 1 mm through thicker gaps. In Figure 8 (vinyl chloride) 3.6 GHz data represent the best distance. The contrast linearly increases from 3 mm to thicker gap. These results indicate that the impurity layer may help to reveal the shape of buried target by holographic radar. Figure 7: Image contrast to basic material with the Figure 8: Image contrast to basic material with the = Vinyl chloride, Impurity material = Void). 5. HOLOGRAPHIC RADAR IMAGES BASED ON ACTUAL DINOSAUR FOOTPRINTS Artificial footprints were carved on water saturated loam soil in a test bed. After leaving for three weeks to dry, the test bed was covered with dry powdered loam soil. The depths of the footprint planes are approximately 30 mm. Figure 9 shows the footprints before covering. The holographic radar image is shown in Figure 10. Despite of small permittivity contrast the three footprints are all recognizable. Separation of three toes is imperfectly attained at the leftmost footprint. No separation is observed at the other footprints. The radar scan was performed again after changing the covering material from loam soil to sand above the center and rightmost footprints. The result is shown in Figure 11. The reflection was intensified for footprints covered with sand. The toes are also separately recognized. The increase of contrast is important not only for better recognition but also for precision of the shapes.

4 Progress In Electromagnetics Research Symposium Proceedings, Stockholm, Sweden, Aug , Figure 9: Image contrast to basic material with the = Rubber, Impurity material = Void). Figure 10: Image contrast to basic material with the Figure 11: Image contrast to basic material with the Figure 12: Anamoepus cast with clay film coating. Figure 13: RASCAN 3.8 GHz cross polarization image for clay film on the cast-mold interface. Finally a test piece was produced by a cast taken of an actual dinosaur footprint (Anamoepus) from the Early Jurassic rocks of Dinosaur State Park in Connecticut, USA [3]. The cast was done with a quick dry lime plaster consisting of calcium carbonate and silica sand and oven-dried. The cast was then covered with a layer of plaster to make a tight-fitting mold. A hidden test was carried out. The hidden footprint is located about 3 cm depth. The mold and cast were scanned first with nothing but the microscopic air gap separating them, and this produced no recognizable contrast pattern. However, when the mating surface was painted with a very thin clay-water mixture (See Figure 12), and allowed to dry overnight (air dry, not oven dry), a shape strongly resembling the track appears (See Figure 13). Note that the long middle toe is clear, and the shape of the heel is obvious, but the outer toes are subtle at best. 6. CONCLUSIONS RASCAN holographic radar showed fine sensitivity to small differences in permittivity and to thin layers of impurity material. Dinosaur footprints are sometimes found as trace fossils preserving realistic shapes. This usually means that the rock layer containing the trace fossils has experienced a special history of sedimentation that created a subtle permittivity difference suitable to the application of the holographic radar. Low noise imaging in the shallower subsurface is a unique

5 1590 PIERS Proceedings, Stockholm, Sweden, Aug , 2013 feature of RASCAN holographic radar [4], and provides a way to detect and image the shape of dinosaur hidden footprints. This capability of RASCAN radar may add to the already-demonstrated applications in archaeology and inspections of civil structure such as delamination of pavement. REFERENCES 1. Leonardi, G. and P. Mietto, eds., Dinosauri in Italia: le orme giurassiche dei Lavini di Marco (Trentino) e gli altri resti fossili italiani, 494, Accademia editoriale, Pisa, Italy, Inagaki, M., Quantitative proof of interference, IWAGPR 2009, Granada, Spain, Capineri, L., V. Razevig, S. Ivashov, F. Zandonai, C. Windsor, M. Inagaki, and T. Bechtel, RASCAN holographic radar for detecting and characterizing dinosaur tracks, IWAGPR 2013, Nantes, France, July 2 4, Ivashov, S., V. Razevig, V. Vasiliev, A. Zhuravlev, T. Bechtel, and L. Capineri, Holographic subsurface radar of RASCAN type: Development and applications, IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, Vol. 4, , 2011, ISSN: , doi: /jstars