oa Detection and Location of Subsurface Voids Using Seismic Transmission Data

Karst is ubiquitous on the peninsula of Qatar, including depressions, sinkholes, and caves. Faulting and fractures play a major role in the development of karst, where fluids find pathways through limestone and dissolve the host rock. The resulting fissures may grow larger as more surface water is funneled through to form cavities or karst. 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 are ubiquitous karst features that are prone to collapse, particularly when surface loading is increased due to the construction of new buildings. In this paper, we present the results of a study to demonstrate a variety of seismic 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. 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. In our current paper we build upon previous results (Gritto et al, 2015) and employ a variety of seismic techniques to demonstrate the detection and characterization of a vertical water collection shaft analyzing the phase, amplitude and spectral information of seismic waves that have been scattered by the object. The experiment consisted of seismic transmission and reflection surveys, using a 10 kg sledge hammer as a source and three-component 10 Hz geophones to record the data. We used the reduction in seismic wave amplitudes and the delay in phase arrival times in the geometrical shadow of the vertical shaft to independently detect and locate the object in space. Additionally, we use narrow band-pass filtered data combining two orthogonal transmission surveys to detect and locate the object. Our analysis showed that ambient noise recordings may generate data with sufficient signal-to-noise ratio to successfully detect and locate subsurface voids. This is a result of the low intrinsic attenuation of seismic waves by the limestone and dolomite rocks that are ubiquitous throughout Qatar. 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.