Seismic Interpretation of Sill-Complexes in Sedimentary Basins: The ‘Sub-Sill Imaging Problem’

Abstract

19Application of 3D-seismic reflection-data to igneous systems in sedimentary basins has led to 20 a revolution in the understanding of mafic sill-complexes. However, there is considerable 21 uncertainty on how geometries and architecture of sill complexes within the subsurface 22 relates those imaged in seismic reflection-data. To provide constraints on how sill complexes 23 in seismic data should be interpreted, we present synthetic seismograms generated from a 24 seismic-scale (22x0.25 km) outcrop in East Greenland constrained by abundant field-data. 25This study highlights how overlying igneous rocks adversely affect imaging of 26 underlying intrusions and rocks by decreasing seismic amplitude, frequency and making 27 steeply dipping features near-impossible to image. Furthermore, seismic modelling shows 28 that because of the high impedance contrast between siliciclastic host rock and dolerites, 29 thin (< 5m) intrusions should in principle be imaged at relatively high amplitudes. This is 30 contrary to many published 'rules of thumb' for seismic detectability of sill intrusions. 31However, actual seismic data combined with well-data shows significant amounts of un-32 imaged sill intrusions, and this is likely due to limited resolution, overburden complexity, 33 poor velocity-models, and interference between closely spaced sill-splays. Significant 34 improvements could be made by better predicting occurrence and geometry of sill intrusions 35 and including these in velocity models. 36 37

Seismic reflection data reveal the 3D structure of the newly discovered Exmouth Dyke Swarm, offshore NW Australia

“…2, 6, and 7) (e.g. Kirton and Donato, 1985;Wall et al, 2010;Ardakani et al, 2017;Holford et al, 2017;Eide et al, 2018;Minakov et al, 2018;Plazibat et al, 2019) and (ii) the geometry of individual VZDs, as well as that of the array they comprise, are akin to the morphology of dyke swarms exposed at Earth's surface (cf. Figs.…”

“…9d), a ∼ 48 m thick lava flow or sill should be seismically expressed as a high-amplitude, positive polarity, tuned reflection package (e.g. Eide et al, 2018;Rabbel et al, 2018). Yet the intra-Mungaroo seismic reflection coincident with the basalt in Chester-1 ST1 has a negative polarity and is of moderate amplitude ( Fig.…”

“…Dykes are rarely imaged in seismic reflection data because their sub-vertical orientation preferentially reflects seismic energy deeper into the subsurface rather than returning it to the surface to be recorded (e.g. Thomson, 2007;Eide et al, 2018). Dykes identified in the field and/or in aeromagnetic data have been indirectly recognised in co-located seismic reflection data where a localised reduction in returned seismic energy disrupts the continuity and strength (amplitude) of reflections associated with stratigraphic layering (e.g.…”

“…Here we carefully consider the errors associated with our quantitative analysis of VZD geometry. For example, synthetic seismic forward modelling indicates dyke-related VZD thickness is dependent on data quality and resolution, and thus it likely does not equal dyke thickness (Eide et al, 2018). Data quality and resolution, in turn, are influenced by a range of geophysical (e.g.…”

“…11b). However, synthetic seismic forward models suggest that the thickness of VZDs corresponding to sub-vertical dykes is greater than the true dyke thickness (Eide et al, 2018). Furthermore, because VZD thickness is partly controlled by the acquisition and processing properties of the seismic reflection data in which they are imaged in (e.g.…”