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GPR Imaging of the Subsurface in 3D:
Examples from a Coastal Site (USA) and an
Archaeological Site (Israel)
Jenifer A. BodeUniversity of Calgary
Holly J. Johnson and Harry M. JolUniversity of Wisconsin - Eau Claire
[email protected]@uwec.edu
Curt D. PetersonPortland State University
Moshe FischerTel Aviv University
Introduction
Three-dimensional (3D) ground penetrating radar (GPR) surveys can be used to better understand the internal stratigraphy and geometry of the subsurface. A 3D image, or cube, constructed from high resolution GPR datasets provides a unique representation of the subsurface. Such 3D images have been
utilized in interpreting a variety of sites, including coastal geomorphic and archaeologi-cal sites. This imaging technique is an effec-tive and time-efficient method of data collec-tion and stratigraphic interpretation that allows one the flexibility to view and analyze the subsurface from nearly any perspective.
GPR data were collected from two sites. The first survey was in the Pacific Northwest at Long Beach, southwest Washington. The 3D images provided at this coastal site show a scarp that was caused by massive erosion due to coseismic coastal subsidence.
The second site was at the archaeological site of Yavne Yam, located in west-central Israel south of Tel Aviv on the Mediterranean Sea. The 3D GPR survey was carried out over an area that is to be excavated in the near future. The 3D image will assist in artifact location and in constructing a subsurface site map.
To better understand coastal geomorphic sites as well as archaeological sites, ground penetrating radar (GPR) has become a popular and effective method for imaging the subsurface. GPR is based on the propagation of high frequency electromagnetic waves, which are emitted into the ground and are reflected back to the surface in response to differences in the properties of the sediment the waves travel through. Three-dimensional (3D) GPR datasets have been increasingly collected because their results significantly enhance the ability to view, analyze and interpret subsurface stratigraphy. The first site is along the coastline of the Pacific Northwest, USA and as a result of the subduction zone in the area, large earthquakes pose a significant threat. Coseismic subsidence associated with such earthquakes creates erosional scarps into beach face deposits. A GPR grid (225 MHz) was collected over one of these scarps north of Long Beach, WA. The prominent facies that was imaged in the 3D cube is the erosional scarp which was bounded by the associated progradational beach deposits. The 3D views aid in better understanding the extent of erosion due to coseismic subsidence and how the shoreline recovered as the coast rebounded. The second site, Yavne Yam, is in Israel and is an archaeologically significant city, located south of Tel Aviv on the Mediterranean Sea. From the first century AD onward, Yavne Yam served as a sea port for the imperial city of Yavne, which is approximately 24 kilometers to the east. A dense GPR grid was collected adjacent to present excavations with both 225 and 450 MHz antennae. Analyses of the GPR data reveal several linear subsurface anomalies that are interpreted as walls. The interpretation of the collected GPR data will be used by site archaeologists to guide future excavations.
Abstract
GPR is a geophysical tool used to image the subsurface. It is based on the propagation of high frequency electromagnetic (EM) waves, which are emitted into the ground and reflected back to the surface in response to differences in the properties of the sediment the waves travel through. Travel time of the EM waves is used to create a reflection profile or image that can provide the stratigraphy of the subsurface.
Three-dimensional (3D) GPR datasets have been increasingly collected because their results significantly enhance the ability to view, analyze and interpret subsurface stratigraphy and archaeological features.
Methodology
The transmitter antennae (1) sends EM waves into the ground and some of the EM waves are reflected off layers in the subsurface back to the receiver antennae (2). The information travels to the computer equipment via cables (3) and the computer displays the data in real time (4).
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Long Beach, WA
The 3D cubes, viewed from the southwest (left) and southeast (right), utilitzed parallel 2D GPR lines that were collected over the scarp at Long Beach, WA. The cutouts show the internal stratigraphy of the cube. The scarp (1) is indicated by the steeply dipping (~ 6 - 7°) reflections. The shallower dipping (~ 1 - 2°) reflections on both sides of the scarp are interpreted as a prograding beach facies (2).
As the cube is cut away, one can observe individual layers and their trend either upslope or downslope. The end view (left) looks south across the GPR grid, perpendicular to the survey lines, and shows the continuity of the scarp as its depth increases. The view from the top (below) (plan view) shows the scarp move to the west (left) as one cuts down through the cube.
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The Pacific Northwest (British Columbia, Washington, Oregon, and northern California) is situated on the Cascadia subduction zone. Along the coast the Juan de Fuca oceanic plate is being pulled under the North American continental plate. This interaction between the plates causes large earthquakes to regularly occur in the region. The data at Long Beach, WA was collected using a pulseEKKO 1000 GPR system with a 225 MHz antennae frequency. Identical length GPR profiles running parallel to each other along an x-y grid (6 m x 30 m) were collected on a level unpaved road running parallel to the slope of a previously imaged scarp. GPR transects were spaced 0.25 m apart with a step size of 0.10 m. The traces
had a sampling interval of 400 ps and were vertically stacked 8 times. The profiles were processed using pulseEKKO software (dewow, trace-to-trace, and down-trace averaging) and T3D was used for 3D viewing.
The GPR data show reflections to 140 ns, or approximately 7 m. The 3D image can be viewed from any perspective to help interpret the subsurface stratigraphy. These views aid in better understanding the extent of erosion and how the shoreline recovered post-earthquake. Future studies will look at stratigraphic changes that have occurred during shoreline recovery.
The site overlies the Cascadia subduction zone, a very active tectonic setting which results in mega (> 8.0 M) scale earthquakes.
GPR data was collected along Loomis Lake County Park access road, north of Long Beach, WA.
Yavne Yam is an archaeological site with a history that goes back thousands of years. From the first century CE onward, Yavne Yam served as a sea port for the imperial city of Yavne, which is approximately 24 kilometers to the east. Historically, this area served as the border of the Phoenician and Philistine empires in the Bronze and Iron Ages. The Romans used the area as a link between the eastern and western empires and the port was also significant into the Byzantine period. The site is an important location for archaeologists, geographers and historians, as well as for the study of geopolitics.
Tel Aviv University has carried out excavations at Yavne Yam under the direction of Professor Moshe Fischer since 1992. The GPR survey was conducted adjacent to current excavations to collect and interpret datasets so that sites could be suggested for future excavations.
The data at Yavne Yam was collected using a pulseEKKO 1000 GPR system with two antennae frequencies (225 MHz and 450 MHz) and a 200 V transmitter. Identical length GPR profiles running parallel to each other along an x-y grid (6 m x 9 m) were collected. GPR transects were spaced 0.50 m apart with a step size of 0.05 m. The traces had a sampling interval of 200 ps and were vertically stacked 4 times. The profiles were processed and plotted using pulseEKKO software and T3D was used for 3D viewing.
As with the Long Beach data, the data can be viewed from any perspective, giving more detail to the analyses. Analyses of the GPR data reveals several linear subsurface anomalies.
Yavne Yam, Israel
The cutout, from 450 MHz data, shows one of the many hyperbolic reflection patterns within the GPR data. These features are interpreted as possible wall structures, either intact or in various stages of collapse.
The cutout uses 225 MHz data and shows the continuous nature of a reflection pattern across the cube which probably represents an archaeological horizon.
The end view of the 450 MHz cube cuts perpendicular to the survey lines. It was created by making a slice of the cube. The image also shows the continuity of a reflection.
The side view (450 MHz) shows reflections that run parallel to one of the survey lines in the grid. Numerous hyperbolic reflections are visible in this view and are interpreted as probablearchaeological features.
For more efficient data collection, a backpack transport system carried the computer, console and battery while the antennae (225 MHz) were triggered using an automatic step odometer which proved to be significantly more time-efficient than manual data collection.
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The 3D images allow GPR datasets to be interpreted in more detail than possible from widely spaced 2D profiles. The data can be viewed from any perspective to aid in better understanding subsurface features and stratigraphy. The side and end views present the stratigraphy of the site as if there were an outcrop present. The top view allows one to remove layers, cutting down through the cube. Cutouts allow for analyzing the continuity of reflections through the cube, bringing more detail and accuracy to one’sinterpretation.
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Financial and logistical support for the projects was provided by University of Wisconsin - Eau Claire (Office of Research and Sponsored Programs, Foundation, and Department of Geography and Anthropology), Foundation Stone, the Yavneh Yam Archaeo-logical Project (Tel-Aviv University), University of Hartford, Portland State University, Washington Department of Ecology, and Brian Junck.
Acknowledgements
