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

21The geometry and distribution of igneous dykes, sills, and inclined sheets has long been used