Physical Parameters and Contrasts of Wooden Objects in Lacustrine Environment: Ground Penetrating Radar and Geoelectrics

Abstract

We investigate how suitable ground penetrating radar (GPR) and geoelectrics are to prospect the remains of submerged wooden archaeological constructions in the water column. For this purpose, we determined the contrasts of electric resistivity and dielectric permittivity from measurements on present-day wood samples, serving as simplified approximations of water saturated and undegraded archaeological wood. As common substitutes of hard and soft construction wood, we investigated oak and spruce wood. The electric resistivity and dielectric permittivity were determined with increasing moisture content from small-scale electric and GPR measurements using a Wenner alpha array and a 2 GHz Palm antenna in a watering experiment. In a water-saturated state, resistivity values of <270 Ωm and relative dielectric permittivity values of >20 were observed. The anisotropy effects and deviations of the wood species were seen to be up to 30%. On the basis of this, the relative material contrasts of wood with respect to fresh water, sand, and clay were calculatedand compared to values found in the literature for seismic contrasts. Geoelectric, GPR, and seismic measurements show contrasts of 0.3 to 0.8, −0.4 to 0.2, and −0.24 to 0.35, depending on the surrounding material and structural orientation of the wood. The highest contrasts were found for wood in fresh water, followed by clayey and sandy subsoils. On the basis of the determined contrasts, analytical calculations were performed showing that an object of 0.5 m diameter can be detected at depths between 0.5 m and 1.5 m with geoelectrics (Schlumberger) and at depths between 0.5 m and 3 m with ground penetrating radar measurements (400 MHz).

From Its Core to the Niche: Insights from GPR Applications

“…Waterborne GPR mapping of lakes has several objectives, including studies of sedimentation environments and stratigraphic boundaries [267,268], the estimation of a sedimentation rate and volume of lacustrine sediments [269], and the tracing of tectonic faults and displacements beneath the lake [270], improving the understanding of the evolution of water bodies at a local scale. GPR has also been applied to delineate reference horizons and local geophysical features [271].…”

The Fortress Beneath: Ground Penetrating Radar Imaging of the Citadel at Alcatraz: 1. A Guide for Interpretation

“…Note that the area of fine-scale lineations on the depth slices extends to the region x~45-47 m; these "extra wood planks" occur because the dry moat beneath this radar profile projects around the SE bastion (see Figure 4). GPR non-destructive imaging of wood structures is becoming important in cultural heritage investigations [28] as well as forestry and agriculture applications [29,30].…”

Multi-Coil FD-EMI in Tidal Flat Areas: Prospection and Ground Truthing at a 17th Century Wooden Ship Wreckage

“…Magnetic gradiometry allows fast measurement progress but does not provide any depth information if not combined with other methods. DC geoelectric methods, such as electrical resistivity tomography (ERT), are of course applicable to tidal flat and shallow water areas ( [6,7]) but not feasible for area-wise prospection because of the laborious measurement setup. Furthermore, large changes in conductivity over the cycle of a day due to tidal water level change can be expected in the tidal flat region.…”