Análisis de causas

The soil composition and the varying groundwater table have a major impact on the signal response and the penetration depth of the GPR, as the combination of minerals and water content enhance or reduce the radar signal propagation significantly. The topsoil at Angkor mainly consists of clayey sand that is several meters deep.564 _{When excavated and dry, it can}

become very hard and impenetrable to water,565 _{as found on the earthen embankments and so}

called “occupation mounds,” and in the base of clay beds in canals.

The calculation of the two-way travel time (TWTT) GPR signal and its conversion to actual depth in meters was compared to the measured depth of structural foundations in associated excavations, mainly laterite in hardened clayey sand.566 _{At those sites the approximated}

standard ground velocity of 0.1m/ns567 _{was reasonably close to the results measured, especially}

if the depth of the features was rather shallow. This average is close to the velocity of loamy soil of 0.09m/ns568 _{and dry sand 0.11m/ns.}569

To have approximate depths for water management features the standard ground velocity of 0.1m/ns were then applied. The fluvial plains provide similar near surface ground composition all over Angkor; the standard ground velocity was adopted to display approximate depth for all archaeological sites in the region. Since the surveys mainly concentrated on gathering information to ascertain if something existed and not how deep it was located, the exact depth of a feature was not of crucial importance for the outcome of the survey. As the surveys were generally conducted in the dry season, the seasonal weather condition would have influenced the signal velocity only to some extent.

The signal depth or TWTT range was set up according to the survey focus in Cambodia as the penetration depth varied significantly. The maximum depth depended on the antenna type used, which required for the landscape surveys the use of the larger 250MHz antenna. The principal factor for penetration depth was the local ground composition. Measurements ranged from less than one meter depth to approximately four to five meters, considering the limited knowledge about the soil velocity in the area to calculate the travel time to depth conversion.570

The worst signal penetration was recorded over the compact clay of roads, large embankments and some habitation mounds where the signal depth sometimes reached less than one meter. This contravenes Angkorian archaeology: as the main construction took place on raised mounds they provide the highest probability to discover buried features. The subsequent construction and destruction periods are detectable in several areas where archaeological evidence was found several meters under the ground, such as the areas of the royal palaces at Angkor Thom571

and Prei Monti.572 _{For most archaeological places of interest at Angkor though, signal}

penetration depth of one or two meters was usually sufficient to detect structural remains, such as buried walls or foundations. Smaller features below this depth would be difficult to interpret anyway, since the resolution of the radar signal degrades with depth. For grids, a depth of about

50ns TWTT was set up, as the survey was mainly done on dry ground and concentrated on masonry remains close to the surface; this travel time corresponds to a depth of about 250cm when assuming the average velocity.

The best GPR penetration depth was achieved mapping channel features on sandy soil in saturated but not flooded areas, for example in the plain around Hariharalaya with its vast rice fields, which were reachable in the late dry season (see Chapter 8). A signal depth of 400- 500cm573 _{was measured in the desiccated ponds and saturated ground of former river beds. The}

increased penetration depth from dry to saturated sand has been known as “radar lensing.”574

Nobes et al. explain this effect: “the lower velocity (due to high dielectric permittivity) causes the

radar signal to be more focussed into the subsurface, and more energy goes into the area beneath the stream.” 575 _{Therefore: “The apparent increased depth of penetration is partly due to the}

increased time required for echoes to return at the slower velocity, but the larger two-way travel time does yield a greater depth of penetration.”576 _{For this reason a depth of 80ns-100ns TTWT}

(corresponding to about 450-500cm) was set up for the long GPR surveys.

FIG.[17]:CORING AT SELECTED SITES FOR GPR ANALYSIS (BACKGROUND:POTTIER).

Coring was applied to several sites in order to understand the nature of the clay where the GPR was not able to penetrate the surface, in comparison to deep penetration, see Fig. [17]. The results from the hand coring and probing showed that penetration depth was directly related to the density of the earth, the tougher the ground the higher the resistance toward signal and corer. Once extracted from the ground the hard clay turned into very fine grained loose soil.

The ground composition of mainly fluvial deposits in the Angkor plain differed significantly from other Cambodian archaeological sites. Comparing the surveys with results from the Kulen Hills, Koh Ker, Banteay Chhmar and Preah Khan of Kompong Svay was helpful to correlate the reflection of the GPR signal with different types of surface geology and environmental properties in Cambodia.

Koh Ker

Lateritic ground was covered by a thin layer of soil and there were no fluvial deposits close to the temple site. Using the 250MHz Antenna, the GPR did not get a reading below one meter in rice fields. It was not clear if the non-detection of canals was based on the ground composition, or if canals that were suspected to run on the west side of Prasat Thom simply did not exist.

Preah Khan of Kompong Svay

Similar to Koh Ker, laterite was detected very close to the surface, covered by approximately 30- 40cm of soil.577 _{Penetration depth of the GPR survey was thus shallow and the noise factor very}

high. The mapping of the canal, running through the centre of the baray, showed however that for a number of purposes GPR proved useful at PKKS.

Banteay Chhmar

The region provided less penetration depth than the floodplain of Angkor, but it was significantly better than in the previously described areas.GPR results from within the reservoir did not provide sufficient information to support or negate a proposed natural laterite layer close to the surface.578

Kulen

A survey in the Kulen Hills, that was initiated to investigate the interior of a recently discovered enclosure, did not produce readily interpretable results. Although information was missing from excavation about the consistency of the subsurface, preliminary results showed the soil was soft and signal penetration depth reasonably deep to about two meters.