Summary

Seismic data recorded during a survey in the vicinity of a sinkhole revealed distinct resonances at 70 Hz. Frequency domain data inversion based on exact modeling produced effective elastic parameters for the location over the sinkhole. The use of near-field terms proved important for accurate estimation of the effective parameters.

Introduction

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.

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