Seismic phases diffracted around Earth's core contain information about lowermost mantle wave speeds. By measuring the slowness of incident diffracted energy from array recordings, seismic velocity along the diffracted path can be estimated. Here we apply this principle to diffraction of the major arc seismic phase PKKPab recorded at the Canadian Yellowknife array to estimate P wave velocity variations along the core-mantle boundary. We observe PKKPabdiff about 7.5¿ past the ray theoretical cutoff distance for PKKPab. We utilize 330 western Pacific rim earthquakes that allow us to probe the core-mantle boundary beneath the North Atlantic and the South Pacific oceans using PKKPabdiff. Slowness and back azimuth are measured by frequency--wave number analysis. Mapping PKKPabdiff slowness variations suggest 4--19% P wave velocity reductions relative to PREM, in good agreement with the magnitude of velocity reductions previously mapped in ultralow velocity zones. The PKKPabdiff slowness and back-azimuth variations combined with results from previous ULVZ studies using SPdiffKS imply that the lowered velocities occur at the base of the mantle beneath the North Atlantic Ocean, along the receiver side of raypaths. PKKPabdiff array measurements thus hold important potential for mapping ultralow velocity zone structure in so far unprobed regions of the lower mantle as well as for providing additional and independent information about lower mantle structure.