Integration of laser level survey, photogrammetry and GPR to examine the deterioration of Roman mosaics: A case study of Venus house, Volubilis, Morocco

Volubilis was founded in the second century B.C. and was one of the largest cities on the fringe of the Roman Empire. In 1997, it was listed as a world cultural heritage site by United Nations Educational, Scientific and Cultural Organization (UNESCO) and is one of the most important archaeological sites of Morocco. Volubilis is renowned for its large number of mosaic floors. Among the major mosaics of Volubilis, and undoubtedly the most remarkable ones given their aesthetic and the variety of themes they hold, are found in the house of Venus. These mosaics have been restored between the 1940s and 1950s, but unfortunately, most of these restorations are past due or nearing their ends. Today, the mosaic floors are becoming unleveled, fractured and separated from their surrounding walls. These deteriorations can be triggered by many factors including inadequate foundation, a clayey soil beneath it sensitive to moisture and the heaving of the thin concrete layer due to the expansion of the soil. The corrosion of the reinforcing bars within these concrete slabs is enabling these deformations as well. The objective of this study is to examine the causes of the mosaics' deterioration by integrating three techniques including a laser level survey, photogrammetry using a small unmanned aircraft vehicles (UAV) and ground-penetrating radar. The methodology adopted herein can generate documentation that can be used during the preparation and decision making prior to any restoration. The restoration should not only be focused on the visible tesserae on the surface but must also consider the entire layer of the mosaics. These techniques were applied to several mosaics in Volubilis, yet only two mosaics within the house of Venus were selected for this study, and the following findings were accomplished: (1) Void spaces were found beneath the mosaic floors, (2) presence of sinking and heaving of concrete slabs and (3) potential corrosion of rebars. The three methods used in this study were in perfect concurrence in showing how the settling of the concrete slab was directly related to the deteriorations visible on the mosaic surface.