Satellite-derived bathymetry for maritime archaeology: Testing its effectiveness at two ancient harbours in the Eastern Mediterranean

Kieran Westley

doi:10.1016/j.jasrep.2021.103030

Abstract

High-resolution bathymetry is an essential tool in maritime archaeology, often used for underwater prospection, imaging, palaeo-landscape reconstruction and change detection. The commonest acquisition method relies on shipborne acoustic systems (e.g. multibeam echosounder). However, most of the world’s seabed has not been surveyed using these methods. Derivation of bathymetry from high-resolution multispectral satellite imagery (Satellite-Derived Bathymetry: SDB) can redress this in shallow (<20–30 m depth) clear water. It can be derived empirically using depth control points, or independent of control data through physics-based approaches. Given global satellite coverage, it is applicable almost anywhere with suitable water conditions and is becoming increasingly widespread in the hydrographic survey community. However, archaeological application of SDB is almost totally un-investigated. This paper examines the utility of SDB for maritime archaeology through 2 case studies: the ancient harbours of Tyre (Lebanon) and Ras Ibn Hani (Syria). Tested data comprise 2 m-resolution physics-based commercial SDB and medium-resolution (10–30 m) SDB empirically derived from open access multispectral imagery (Landsat 8, Sentinel-2). SDB was assessed for its ability to detect submerged cultural features and landforms and as input into palaeo-geographic reconstructions. Tested SDB performed poorly for feature detection, but was better-suited to landform identification and palaeo-geographic reconstruction. SDB also has clear advantages: low cost, wide area coverage, speed and data availability. Disadvantages include variable accuracy, technical limitations (e.g. need for clear water) and approach-specific requirements (e.g. depth control points for empirical and complex modelling for physics-based approaches). For maritime archaeology, its current limitations means that it will not replace acoustics for prospection and imaging but it does have utility for submerged landscape investigation and reconstruction, particularly in areas which now only have no/poor bathymetric data.

Highlights

Aerial and satellite remote sensing play a key role in archaeological research and heritage management
To image the seabed and any archaeological material lying on it, maritime archaeologists instead rely heavily on shipborne acoustic systems like sidescan sonar (SSS) or swath bathymetry generated from multibeam echosounder (MBES) or interferometric sonar
The application and effectiveness of Satellite-Derived Bathymetry (SDB) to maritime archaeology has been tested by reference to case studies in the Eastern Mediterranean (Lebanon, Syria)

Summary

Introduction

Aerial and satellite remote sensing play a key role in archaeological research and heritage management They enable rapid cost-effective site prospection, landscape characterization, change detection and risk assessment (Cuca and Hadjimitsis, 2017; Hritz, 2014; Parcak, 2007; Philip et al, 2002; Rayne et al, 2020, 2017). The term Satellite-Derived Bathymetry (SDB) arguably covers a range of techniques which derive water depths from space-based sen sors. This includes approaches that rely on gravity measurements (Watts et al, 2020), nearshore wave characteristics (Almar et al, 2020), stereo photos (Hodúl et al, 2020), space-based laser (Parrish et al, 2019), and multispectral imagery (Salameh et al, 2019). Once water column and atmospheric effects are accounted for, the energy reflected back to a satellite should be inversely proportional to water

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