Dean Goodman

GROUND-PENETRATING RADAR SIMULATION IN ENGINEERING AND ARCHAEOLOGY

Forward modeling of ground penetration radar is developed using exact ray‐tracing techniques. Structural boundaries for a ground model are incorporated via a discrete grid with interfaces described by splines, polynomials, and in the case of special structures such as circular objects, the boundaries are given in terms of their functional formula. In the synthetic radargram method, the waveform contributions of many different wave types are computed. Using a finely digitized antenna directional response function, the radar crosssection of buried targets and the effective area of the receiving antenna can be statistically modeled. Attenuation along the raypaths is also monitored. The forward models are used: “1” as a learning tool to avoid pitfalls in radargram interpretation, (2) to understand radar signatures measured across various engineering structures, and (3) to predict the response of cultural structures buried beneath important archaeological sites in Japan.

A ground-radar view of Japanese burial mounds

Ground radar investigates shallow archaeological features in burial mounds of the Kofun Period (300-700 AD) in Japan. A variety of analytical methods – time slices, synthetic radargrams simulated from model structures, and 3-D depth constructions – are used to interpret the profiles, to reveal the location, depth and shapes of buried features.

Discovery of a 1st Century AD Roman Amphitheater and Other Structures at the Forum Novum by GPR

A Roman marketplace and town called the Forum Novum, initially began construction in the 1st century BC and flourished well into the 4th century AD. At present most of the town remains is below ground. The site is situated next to a completely restored 1st century AD church and a partially reconstructed marketplace. Ground penetrating radar surveys have been conducted at the Forum Novum during 1998 through 2001 field seasons to remotely detect buildings of this ancient town. Using 300 and 500MHz radar antennas and closely spaced radar profiles, time slice and 3-D volume analysis of the reflection data were computed. GPR surveys in the area detected a large oval-shaped structure 45meter along its major axis. This structure is from a 1st century AD Roman amphitheater. Subsequent excavations from one of the eight entrances reveal that the internal amphitheater oval is nearly a meter thick and is composed of a cemented stone wall buried 60cm below the ground surface. The time slices revealed completely differ...

Ground-penetrating radar survey at Wroxeter

Two seasons of ground-penetrating radar were undertaken at selected locations within the Roman town of Wroxeter. Both surveys used a GSSI system incorporating a 300 MHz antenna. A strategy has been developed to collect large data sets in an efficient manner. The results have been processed and displayed as time-slice images to facilitate interpretation of complex data collected over part of this Roman urban landscape. Copyright

Very low frequency Ocean Bottom ambient seismic noise and coupling on the shallow continental shelf

Sources of very low frequency (0.01 to 1.0 Hz) ambient seismic noise in the shallow (<100 m) water continental margin sediments are investigated using Ocean Bottom Seismometers (OBS). The predominant seismic motions are found to be due to surface gravity (water) waves and water-sediment interface waves. Actual experimental measurements of seabed acceleration and hydrodynamic pressure are given, including side by side comparisons between buried and plate-mounted OBS units. OBS-sediment resonant effects are found to be negligible at the low frequencies under investigation. Wherever there exists relative motion between the seabed and the water, however, an exposed OBS is subject to ‘added mass’ forces that cause it to move with the water rather than the sediments. Calculations based on measured seabed motions show that a neutral density, buried seismometer has superior sediment coupling charactersitics to any exposed OBS design.

Directional spectra observations of seafloor microseisms from an ocean‐bottom seismometer array

Observations of the directional spectra of seabed motion in shallow water were conducted off the New Jersey coast during the summer of 1987. Using a six‐point ocean‐bottom seismometer array, each instrument supporting a pressure transducer, and two horizontal and vertical accelerometers, measurements of gravity and seismic waves across the ULF/VLF band were collected in 12.5 m of water. Array dimensions were tuned particularly for directional spectra observations of short‐period seafloor microseisms. Directional spectra analysis indicates that in the short‐period microseismic band, 1.5–2.5 s, motion of the seafloor is primarily a result of slow seismic waves traveling at apparent velocities near 200 m/s. These propagation velocities for the microseismic band in shallow water are an order of magnitude less than microseismic velocities from similar studies on land. Contemporaneous measurements of the directional spectra of long‐period ocean gravity waves, 15–85 s, show an eastern direction of origin; short‐...

Measurements of ambient seabed seismic levels below 1.0 Hz on the shallow eastern U.S. continental shelf

Measurements of ambient seismic noise levels in the range 0.03–1.0 Hz were made using ocean‐bottom seismometers (OBS) at four shallow‐water (<100 m) locations on the New Jersey Shelf and George’s Bank. Surface gravity‐wave‐induced seabed motion (single‐frequency microseism) was found to be dominant in the frequency range 0.03–0.3 Hz, with the high‐frequency cutoff strongly dependent on water depth. The peak seismic level in the water wave band was measured at 2.0×10−8 (m/s2)2/Hz in 12 m of water. This level was observed to decrease rapidly with greater water depth. Seismic interface waves (microseisms) of power level approximately 5×10−10 (m/s2)2/Hz were observed in the range 0.25–1.0 Hz. This microseism power level was found to be roughly constant in water depths from 12 to 70 m. A quiet ‘‘notch’’ between the two wave bands, in the range 0.15–0.3 Hz, was observed. The background seismic level in this notch was determined to be less than 5×10−12 (m/s2)2/Hz. Extrapolations of the observed pressures and sea...

Forum Novum–Vescovio: Studying urbanism in the Tiber valley

The Roman town of Forum Novum lies in the Sabine hills to the northeast of Rome. Its study forms part of the British School at Romes Tiber Valley Project, a collaborative research initiative which studies the Tiber valley as the hinterland of Rome, tracing the impact of Romes development on the history of its settlement, economy, and cultural identity from 1000 B.C. to A.D. 1300 (Patterson and Millett 1999; Patterson et al. 2000) (fig. 1). The project draws on the extensive work carried out in this area to produce a new, material-based history of the valley. While the project seeks to re-evaluate past survey material, a vital contrast is provided by the development of new field projects to fill the gaps in settlement knowledge. Three main lacunae have been identified: the study of urban centres; the dearth of data from the E bank of the Tiber; and the poor understanding of the late-antique and early Mediaeval landscape. Forum Novum offers an opportunity to address all these lacunae. Urbanism forms a key research theme for the Tiber Valley Project. In marked contrast to the intensity of archaeological work on rural settlement in this area, there has been little systematic research on towns. Study has tended to concentrate on the excavation of monumental structures or, more rarely, the investigation of single and exceptional towns such as Ostia and Rome itself. Surprisingly little is known of the organization of the smaller towns and knowledge of their history is based largely on the epigraphic and documentary evidence.

GPR time slice images of the Villa of Emperor Trajanus, Arcinazzo, Italy (AD 52-117)

The villa of the Roman Emperor Marcus Ulpius Trajanus (AD52-117) was built in Arcinazzo Italy, approximately 55 km northeast of Rome. Today, the only remains left standing at the site are public building entrances comprising a small portion of the entire site. As part of an ongoing study to rescue this national archaeological treasure, an extensive grid system was laid out and high-resolution GPR surveys using sub-meter profile spacings were conducted. Amplitude time slice analysis indicates that many structural foundations of the villa are still well preserved below the ground surface. Time slices below 40 ns in one area reveal several large mushroom shaped structures enclosed within a large building over 100 meters in length. These rounded structures are believed to be dipping pools within the bathhouse to the villa. At a location west of the bathhouse, a large oval shaped anomaly 45 meters along its major axis was discovered. Several intermediate time slices show a very faint overlapping oval reflection with a different orientation. The fainter anomaly may indicate the initial construction geometry was adjusted soon after construction began on the site for a garden pond or outdoor pooi area. The data are also examined using fast animation of the radar time slices . In this dynamic display of the datasets, several other information such as overlying topsoil depths across the site, and the relationship of structural anomalies at different levels, can be easily visualized when compared to normal static displays of radar data.

GPR Image Construction and Image Processing

Often the most valuable information contained in GPR radargrams is not from examination of individual radargrams and their vertical profile of the ground, but from the generation of images which connect anomalies from closely spaced profiles. Images that look across horizontal slices in the ground and changes in recorded reflection amplitudes are referred to as time slices (Goodman et al. 1995; Malagodi et al. 1996). Time slicing is an essential image process which can often contain the most important information on the subsurface archaeology at a site. They are called time slices since the vertical axis of the radargrams – before they are converted to depth with knowledge of the ground microwave velocities – is the travel time of reflections. In 1994, the authors had seen unpublished reports with images and photos that date back from as early as 1981 of a digital GPR system at Battelle National Laboratories in the US, with the antenna and electronic equipment being deployed by a tractor. In these reports, there are also the first versions of a time slice maps made at an archaeological site and the authors believe that these are probably the earliest time slices ever generated from GPR (?).