Seismic detection of a sinkhole using spectral-based analysis

Abstract

Karst is ubiquitous on the peninsula of Qatar, including depressions, sinkholes, and caves. Aerial reconnaissance indicates that the widespread depressions, sinkholes, and caves reveal NE-SW and NW-SE orientations, similar to the joint and fracture systems (Sadiq and Nasir, 2002). Faulting and fractures play a major role in the development of karst, where fluids find pathways through limestone and dissolve the host rock. In many temperate and tropical areas where limestone is exposed, environmental water may combine with CO2 in the atmosphere or in the soil above the rock to produce a weak solution of carbonic acid, which slowly dissolves the limestone. The low concentration of the acid and the resistance of limestone to mechanical weathering results in the slow dissolution of rock. The resulting fissures may gradually open in the limestone and may spread throughout the subsurface. These fissures may grow larger as more surface water is funneled through to form cavities or karst. The geometries of karst pits, identified in Qatar, include cylindrical, bottle-shaped, compound, and bowl-shaped structures, as indicated in Figure 1. Sinkholes may also form, when cavern roofs collapse, and it is this last characteristic that is of concern to rapidly growing metropolitan areas, that expand in heretofore unexplored regions. Qatar has seen a recent boom in construction, including the planning and development of complete new sub-sections of metropolitan areas. Before planning and construction can commence, the development areas need to be investigated to determine their suitability for the planned project. Of particular concern to construction projects are ubiquitous karst features that are prone to collapse, particularly when surface loading is increased due to construction. In this study, we present a spectral-based analysis to seismically detect the presence of karst below a sinkhole in Doha, Qatar. Seismic waves are well suited for karst detection and characterization. Voids represent high-contrast seismic objects that exhibit strong responses due to incident seismic waves. However, the complex geometry of karst, including shape and size, makes their imaging nontrivial. While karst detection can be reduced to the simple problem of detecting an anomaly, karst characterization can be complicated by the 3D nature of the problem of unknown scale, where irregular surfaces can generate diffracted waves of different kind. An additional complication generated by shallow high-contrast objects is their possible location within the near field, which typically extends to a distance of one wavelength from the seismic source or a sensor. Near field terms of the seismic wavefield dominate this zone and need to be taken into account when inverting the data. The application of near-field terms is not part of typical seismic exploration techniques.