Origins of seismic reflections in crystalline upper crust, Aberdeen, Scotland
Processing, interpretation, and modelling studies incorporating reflection seismic, VSP, and wireline data from onshore metamorphic basement in Aberdeen formed the basis for investigating the origins of unexpected sequences of coherent, laterally extensive seismic reflections, gently dipping to the NNE and occupying most of the crystalline upper crust. Wireline analysis from a vertical well drilled to 4800' in the metamorphic basement incorporated evaluation of formation lithology and fracturing. In absence of core, wireline characteristics were correlated with formation lithology using crossplots and histograms. Computed stratigraphy incorporating quartz-schist, mica-schist and igneous units agreed with cuttings analysis, outcrop geology, wireline characteristics, and known paleo-evironment of deposition. 1463 fractures counted on the FMS log showed significant NW-SE strikes and steep dips (40-50°) to the NEW. The significant fracture orientations may be an expression of the stress field affecting Scotland. Re-processing of the vibroseis seismic profiles using optimum parameters resulted in significant improvements despite limitations in the accuracy of RMS velocity estimations induced by survey characteristics. Two of the three profiles showed poor signal to noise ratio in consequence of traffic noise contamination of the data. Reflections were categorised into three kinds: (1) parallel, inclined, laterally extensive, sometimes weakly coherent, single packed reflections related to the syndepositional characteristics of the formation (metasedimentary and basic igneous units); (2) discordant, irregular, high amplitude, double or multiple peaked and usually short spaced reflections related to the post-depositional characteristics of the formation (fracture zone and acidic igneous units); and (3) two sequence boundaries, the upper of which was interpreted, on the basis of regional evidence, as both a stratigraphic boundary as well as a thrust plane. The NNE thickening and dip of the metasedimentary wedges defined by the sequence boundaries appeared to contradict inferred regional structure of the upper crust dictated by the recumbently folded Tay Nappe.