**1. Introduction**

The geometry, composition, and mechanics of crustal intrusions, including sills, have been subject of numerous field and lab investigations; the associated literature is extensive [1–6]. Although sills are often observed in surface geological outcrops and thus available for direct sampling [7,8], their subhorizontal geometry makes them less likely to outcrop than near-vertical dykes, especially if buried beneath later sedimentary basins. The detection and delineation of magma, especially if still molten, at depth has been an explicit goal of many geophysical surveys using a variety of techniques, each with its own strengths and limitations. Gravity, for example, can be used to identify mass excess or mass deficiency that can be attributed respectively to mafic or granitic materials (molten or frozen) at depth [9–12]. Magnetotelluric methods have been widely used to detect magma at depth due their sensitivity to the high conductivities associated with magma and magmatic fluids [13,14]. However, gravity is notoriously non-unique [15], as are the various electrical methodologies [16,17]. Both offer relatively limited resolution at depth, especially if the target is a thin planar structure. Seismic methods using both artificial and earthquake sources have also been widely used to define crustal structure in general and magmatic additions both hot and cold. Thybo and Artemieva [18]) review many of the controlled source refraction/wide angle results that have been used to infer massive magmatic underplating in the crust. Seismic tomography is also now a commonly used tool to search for magma in all its forms at depth [19–21]. However, even the most recent tomography is limited in spatial resolution to tens of kilometers [22,23]. A somewhat greater resolution can be achieved using receiver functions computed from teleseismic recordings. Receiver functions have been interpreted to indicate an extensive sill beneath the Altiplano-Puna volcanic zone of the central Andes [24,25].

In many applications, the seismic reflection technique [26], also known as seismic reflection profiling or multichannel seismic profiling, offers the highest resolution of any geophysical technique. Seismic reflection surveying, the primary geophysical method used for oil and gas exploration, has become a highly sophisticated tool for imaging the subsurface in both 2D and 3D at depths ranging from the near surface to the upper mantle. Oil industry reflection surveys have already had a major impact on our understanding of the sill distributions in sedimentary basins [27–29].

Here, we focus on published examples of identified or inferred sills in the continental basement hidden beneath the sedimentary cover, with special attention to frozen sills that may be fingerprints of ancient thermal processes and the large-distance lateral transport of magma in the crystalline crust.
