oa Detection and Characterization of Karst Structures Based on Spectral Attributes of Seismic Data

A major hazard in Qatar is the presence of karst, which is ubiquitous throughout the country including depressions, sinkholes, and caves. Causes for the development of karst include faulting and fracturing where fluids find pathways through limestone and dissolve the host rock to form caverns. Of particular concern are the collapse of such caverns in rapidly growing metropolitan areas that expand in heretofore unexplored regions. Because Qatar has seen a recent boom in construction, including the planning and development of complete new metropolitan sub-sections, the development areas need to be investigated for the presence of karst to determine their suitability for the planned project. In this paper, we present the results of a study to demonstrate the use of seismic spectral analysis techniques to detect the presence of a karst analog in form of a vertical water-collection shaft located on the campus of Qatar University, Doha, Qatar. We also present the results of this technique to characterize the overburden over a sinkhole in the Al Duhail Area of 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. In our current paper we build upon previous results (Gritto et al, 2014, Gritto et al. 2016) and employ spectral techniques to demonstrate the detection and characterization of a vertical water collection shaft analyzing the spectral information of ambient seismic noise recordings that have been scattered by the object. The results indicate that ambient noise recordings may generate data with sufficient signal-to-noise ratio to successfully detect and locate subsurface voids. Being able to use ambient noise recordings would eliminate the need to employ active seismic sources that are time consuming and more expensive to operate. In the case of a vertical water-collection shaft on the campus of Qatar University, we deployed a 24 m long geophone line with 48 10 Hz three-components geophones and passively recoded ambient seismic noise data. The geophone line ran across the lid of the water-collated shaft and extended to both sides. Spectral analysis of the ambient noise data revealed an increase in low frequencies directly above the vertical shaft. The low frequencies are attributed to resonance waves that propagate circumferentially in the concrete walls of the vertical shaft. The observation of increased low-frequency energy was accompanied by a simultaneous decrease in high frequencies clearly demarking the location of the shaft. In contrast to the increase in amplitude of the low frequency resonance waves, the high frequencies of ambient noise were attenuated by the presence of the shaft. The clear demarcation of the increase and decrease in amplitudes of recorded waves by the geophones located above the shaft allows to pinpoint its location. In the second part of our paper, we analyze scattered seismic waves to characterize the overburden of a sinkhole. During this experiment at the sinkhole in the Al Duhail Area of Doha an active seismic survey was conducted. Using the same approach described above, spectral analysis of active source seismic data revealed the extent of the sinkhole, while analytical modeling of the recorded data reveals the thickness of the overburden.