This is a ground penetrating radar transect of a portion of the outer ditch surrounding the Navan Fort mound. The most interesting aspect of this transect is that a subsurface feature can be traced across from right to left of the slide. Note that it has a different shape than the overlying topography of the ditch -- the subsurface feature dips down to the right before the topography changes. This may mean that the ancient ditch had a different profile than the current ditch.
This is a 3-dimensional rendering of a series of ground penetrating radar profiles. These profiles were collected on the eastern flank of the large mound at Navan Fort. Shown are horizontal slices (same depth horizons) through the ground penetrating radar data at four separate depths, roughly corresponding to 30, 60, 80, and 100 centimeters below ground surface. The bright blue colors show where strong reflectors are located. The perspective is such that we are looking towards the north from the foreground rightmost corner of the block. The designation N1020, E1020 is an internal location referencepoint. Several observations can be made about this diagram. First, differences in strong reflector patterns (bright blues) with depth are observed. To us, this means that distinct layers of archaeological structures occur, with the youngest structures at the top of the stack, and the older structures near the base of the stack. Second, the strongest areas of reflectors appear to be in the 50-80 cm depth interval. This may mean that the major evidences for human habitation are present within this stratigraphic interval. This depth range also corresponds to the approximate depths at which human occupation evidence is found from excavations carried out at Navan in the 1960's and 1970's in areas to the west of this block of ground penetrating radar profiles.
One tempting question to ask is whether or not individual slices through the GPR data truly represent consistent patterns in subsurface structures, or, instead, are particularly felicitous, but random, arrangements of radar amplitudes. One way to address this concern is to superimpose adjacent depth slices in a stack. In other words, instead of looking at a single slice at a certain depth horizon, what will result if we examine 5-10 slices between 35-45 cm depth horizon? If structures are indeed random, than stacking 5-10 slices should result in a "white noise" display, in which no consistent patterns in reflector strength are observed. Conversely, if consistent patterns do emerge, then we might argue that the radar amplitude patterns are indeed images of subsurface features. In this slide, we see stacks of 5 slices each centered around the 40 cm and 85 cm depth intervals. Note that only the highest amplitudes are displayed -- lower amplitude ranges are left transparent for viewing clarity. Now we can see the distinctly different nature of shallow and deep structure more clearly. The shallow structures include the "bull's eye" anomaly at the southern margin of the display, as well as the paired east-trending bands of anomalies. The deeper structures includes several patterns of curving, and intersecting east- trending anomalies. Notice the virtual absence of the "bull's eye" anomaly along the southern margin.
Again, to support our contention that the anomaly patterns in the shallow region are quite a bit different from those in the deeper region, and that fewer anomalies are present at shallow depths, we display a stack of 5 slices centered around 25 cm depth. Note the lower amplitudes present, the existence of bull's eye feature in the southern margin, and northeast trending band of anomalies on the eastern edge of the block.
Click your Mouse on "Back to the Beach!" below if you want to return to the California State University at Long Beach (CSULB) home page
Or click here to return to the Department of Geological Sciences home page
Or click here to return to the Remote Sensing at Navan Fort home page
Written by Elizabeth L. Ambos (bambos@beach.csulb.edu). Last update: 8/18/95.
