The Earth's mantle above the core-mantle boundary (CMB) is characterized by a variety of seismically visible structures at many different length-scales, ranging from small-scale features such as scatterers (e.g., Mancinelli et al., 2016) to large-scale structures such as large low seismic velocity provinces (LLSVP), visible in tomographic inversions (e.g., French & Romanowicz, 2015;Ritsema et al., 2011). The D″ layer (Bullen, 1949), defined as the lowest 200-300 km of the mantle, is considered to be a thermochemical boundary layer (e.g., Lay et al., 1998;Lay, 2015) and studies focusing on this layer have detected signals indicating a variety of different structures, such as ultralow velocity zones (ULVZ;