Detecting shallow subsurface anomalies with airborne and spaceborne remote sensing: A review

Advances in air and space sensor technology reveal new opportunities and innovative ways to remotely sense the Earth's subsurface. Considerable spatial coverage, fast and frequent image acquisition and very high radiometric, spectral, spatial and temporal resolution imaging systems can now detect near subsurface anomalies with impressive accuracy. The merits are extensive, with archaeological prospecting, environmental risk mitigation, natural resource exploration, defence and security and speleological research all benefitting from subsurface imaging capabilities over unknown territory, difficult terrain, hazardous environments and inaccessible ground. In this paper, we categorise the ground indicators and potential field characteristics of a general subsurface anomaly before reviewing and documenting over seventy air and space subsurface detection techniques using: photogrammetry, multispectral sensors, thermal infrared, hyperspectral imaging, synthetic aperture radar (SAR), airborne light detection and ranging (LiDAR), airborne gravity and aeromagnetics. The capabilities of each technique are evaluated by reviewing their ability to detect specific characteristics from subsurface anomalies and then they are tabulated by investigable feature and sensor type in seven technique tables. Research trends in motive, sensor type and subsurface anomaly characteristic are discussed and a short review of the major ground-truthing techniques used to verify airborne and spaceborne observations is considered. To close, we take a brief look at future research opportunities with very high resolution (VHR) datasets, multi-branch convolutional neural networks (CNNs) and active remote sensing in variable potential fields.