A Ground-Radar View of Japanese Burial Mounds
Goodman, Dean, Nishimura, Yasushi, Antiquity
Recently, high-resolution ground impulse radar has been used to help discover foundations of Roman archaeological sites in Britain (Stove & Addyman 1989), to find buried remains of a 16th-century Basque whaling station on the Labrador coast (Vaughn 1986), and in many other archaeological applications (Goodman & Nishimura 1992a; 1992b; Nishimura & Kamei 1990; McAleer 1988; Imai et al. 1987; Bevan & Kenyon 1975; etc.). As well as broadening advance knowledge of areas to be excavated, it is one of the most valuable remote-sensing tools for areas protected from destructive excavation.
In some instances, the data offered by ground radar are difficult to interpret, even for the trained geophysicist. Factors that significantly influence the radar signatures are:
1 time-varying antenna impulses and instrument responses;
2 'reverberation' of recorded signals due to multiple reflections; and
3 the large field of view inherent in the commercial antenna used to image subsurface structures.
Microwave radar impulses sent into the ground can vary in time due to the instrumentation, as well as due to the dispersive characteristics of the ground in which the waves travel. Since the time-varying nature of ground-probing radar impulses is strongest in the near field region, within about 1 wavelength of the transmitter, the near field region with a low-frequency antenna may encompass important shallow features which are desired to be imaged. For structures beyond 1 wavelength, the far field characteristics remain relatively constant (Duke 1990).
Multiple radar reflections -- 'echoes' or 'reverberations' -- can greatly alter the appearance of measured radargrams, and look significantly different when compared with actual ground structures (Goodman & Nishimura 1992a).
The most significant factor controlling the recorded radar signature is the broad radar beam transmitted into the ground. The field of view of most commercial radar antenna can be quite large, in excess of 120 |degrees~ (e.g. FIGURE 1, above). For this reason, reflections from objects off to the side can be detected. For a cylindrical object buried at depth, a hyperbolic type of reflection pattern can result.
Sometimes cylindrical-shaped objects are the focus of radar surveys (PLATE V, page 225, this number). A survey searched for burial tunnels (and shafts) in what are now cultivated fields at the Nutubaru burial mound #156-7 on the southern island of Kyushu. The burial mounds have been partly or completely cut down for cultivation. A survey near a partly intact region of one revealed a roughly hyperbolic reflection pattern (PLATE V), which should correspond to a cylindrical feature. Excavation later corroborated the ground radar results: a burial tunnel extends into a protected mounded region which may contain a burial chamber.
Here, the ground structure has an easily interpretable radar signature; in structurally complicated areas, a broad-beam radar antenna can sometimes obscure completely a visually meaningful representation of buried structures.
Many geophysical processing techniques (e.g. migration, deconvolution, spatial Fourier transforms, etc.) can alleviate problems with reflection data from a pulse radar. When imaging cylindrical objects with a broad-beam radar, migration can be a useful technique. In complicated areas, many of the processing techniques also have drawbacks; if applied improperly, some can create regions or reverberations of 'pseudo' data in an otherwise 'clean' data-set.
As the raw radar data generally have the most information content, it is only natural to use them.
In this study, three methods of presenting and interpreting the information contained in raw radargrams are examined:
1 Time slices;
2 Synthetic radargram simulation;
3 3-D depth constructions.
In oil exploration studies, time slices of seismic reflection data can reveal the presence of, and depth to, reflecting horizons of oil-producing stratigraphic units. …