Browsing by Subject "crystalline bedrock"

Seismic modelling was conducted to investigate the extent to which seismic methods, and vertical seismic profiling (VSP) in particular, can be used to image steeply dipping faults and fractures in a crystalline bedrock environment typical of southern Finland. Modelling is based on the geology and subsurface geometry found in Kopparnäs, Inkoo, where a steeply dipping fault zone is intersected by a borehole. The goal of modelling was to design the optimal survey for seismic data acquisition to image the fault zone. Borehole geophysical data analysis and computing of 1D synthetic seismograms gave a first insight into the expected response of the subsurface structures. Simple travel-time modelling was used to define the time-window for direct and reflected waves as well as gaining some understanding of useful source positions, based on the separation of direct and reflected waves. To assess the compatibility of distributed acoustic sensing (DAS) in this setting, a modelling software for comparing the response of geophones with that of a fiber optic cable / DAS was used. For more accurate modelling of the propagating wavefield, a finite-difference based full waveform modelling scheme was used to create shot gathers for both acoustic and elastic wave propagation through a 2D model. The raw shot gathers were first briefly analysed before further processing. Using a common VSP processing sequence resulted in migrated and stacked shot gathers to determine the optimal source positions. High frequencies are needed for imaging the subsurface structures in Kopparnäs, largely due to the high velocities of the crystalline bedrock and the shallow geometry of the fault. It was found that a high-resolution image of the upper part of the fault can be obtained using only four shot points located on the south side of the borehole collar, away from the fault. Shear wave reflections provided the best image of the fault, even with noise added to the shot record. The feasibility of using DAS for data acquisition was evaluated, and due to the imaging ability comparable to geophones, this method can be suggested for data acquisition in Kopparnäs. Further modelling can be conducted if desired, but good imaging results should be obtained if the suggested survey geometry is used. The practical and financial benefits of using DAS technology for data acquisition could enable some testing in the field, reducing the need for additional modelling.