Geophysical mapping of a classical Greek road network: A case study from the city of Elis, Peloponnese

Many ancient Greek cities are characterised by a regular orthogonal road network. These roads are ideal targets for geophysical investigation mainly due to their extensive geographic extent that makes them challenging to define by excavation. Geophysical mapping of these features will contribute to understanding ancient cities as it can provide considerable information about their geographic extent, spatial arrangement and urban dynamics. Large scale multisensor magnetic and electromagnetic induction methods have been used to map the ancient Greek city of Elis in the Peloponnese (Greece). This work complements other investigations that have been undertaken, employing other methods that include the interpretation of high-resolution satellite imagery (Donati and Sarris forthcoming).

Conference Preview: 2021 Summit on Drone Geophysics set for 2 to 5 November

Postconvention workshops focused on drones, unmanned airborne systems (UASs), and unmanned aerial vehicles (UAVs) have occurred previously at the SEG Annual Meeting. In 2017, the SEG Near-Surface Geophysics Technical Section conducted the first such event called Drones Applied to Geophysical Mapping. This first event ushered in the topic for future Annual Meetings. In 2018, the postconvention event was called Advances in Unmanned Airborne Systems Geophysics. Both events were hugely successful and helped set the stage for the standalone reoccurring virtual Summit on Drone Geophysics.

Submergence of the Western Greco-Roman Archaeological Site at the Eastern Harbor of Alexandria: Emerged from High Resolution Geophysical Mapping

Marine geophysical surveys were carried out at the underwater site in the south-western sector of the Eastern Harbor of Alexandria, opposite to the Egyptian Sea Scout Club. Survey works aimed to detect and study the surface and subsurface geomorphological changes caused by historic sea-level rise and natural hazards, by integrating the results of high-resolution geophysical mapping for the seafloor textures and the subsurface layers with previously published core data and sea-level records, the survey works employed echo-sounder, side scan sonar, and sub-bottom profiler. Acoustic data were ground-truthed using an ROV camera and sediment grab sampler. Results of bathymetric mapping and sonar imaging outlined two breakwaters and quay corresponding to a submerged ancient port; also, sediment types were classified according to variation in the magnitude of the backscattered intensities. Interpretation of sub-bottom profiles illustrated the depositional sequence of the topmost sedimentary layers where the sediment thicknesses were thickened by rates that perfectly matched with the recorded sea-level rise rate during the last two millennia, as indicated by isopach maps. Anthropogenic activities were noticed in particular outcropping areas on the sub-bottom profiles. The results explained the role of natural hazards and sea-level rise in changing the geomorphology of the coastline and seabed features.

Merging tTEM data and borehole lithological information to generate hydrogeologic structural models and redox conditions through Direct Sampling

<p>Detailed 3D structural information of the subsurface is fundamental for the development of both hydrological and geochemical models. This structural information is often derived from geophysical mapping results. Some parts of a catchments areas are however inaccessible for the geophysical mapping or might suffer from low data quality, which results in information gaps. Multipoint statistics can be used to remediate these data gaps and incorporate uncertainty in the construction of the hydrogeological models. This results in an ensemble of plausible 3D hydrogeological models.</p><p>This project focusses on nitrate retention mapping. The approach taken is to start from the resistivity models that are obtained from the tTEM measurement campaign. These resistivity datasets are combined with borehole lithological data from the Danish national well-database in an automated procedure that estimates resistivity-to sand/clay translator functions. This results in a clay fraction – resistivity data pair for every point in the subsurface where resistivity data is collected. These clay fraction – resistivity data pairs are converted to discrete hydrogeological units through clustering. This procedure is performed because the groundwater model that uses the end-product of this workflow, uses hydrogeological units rather than resistivity values or clay fractions to define zones of similar hydrogeological behavior.</p><p>Direct sampling is used to go from the cluster information obtained at the resistivity model location to fill out the full model volume and generate multiple plausible model realizations. This method allows, at the same time, for incorporating uncertainty through separation of data into a hard  data set for the cluster information with higher probability, and a soft data set for the cluster information with lower probability. Since the redox conditions in the subsurface are related to the hydrogeological conditions, we are using this method to co-simulate hydrogeological units and redox conditions by merging the cluster training dataset with a redox condition training dataset that is constructed based on the cluster dataset and hydrogeochemical samples that are collected across the catchment. We combine the three training images: resistivity, cluster and redox condition, to simultaneous simulate the three variables in each grid point as a vector, instead of simulating them as separate variables.  The resulting set of  3D hydrogeologic structural models and redox condition models retains the complex geostatistical spatial relationships that can exists between the different type of datasets within the training image, making them suitable for nitrate retention modeling at catchment scale.</p>