Wave-based imaging is an inverse problem for the wave equation, with a broad spectrum of applications in nondestructive testing, ultrasound for medical diagnosis, radar, underwater sonar, space surveillance, geophysical exploration, etc. Any imaging system uses sensors that measure sound, electromagnetic, or elastic waves generated by impulsive sources or by ambient noise sources. These waves carry information about the medium through which they travel and their initial state, and the goal of imaging is to extract this information from the sensor measurements, a.k.a. the data. The sensor configurations are application specific. The illustrations in Figure 1 show sensors that are closely spaced in a bounded set, so for imaging purposes they can be treated as a collective entity called an array. The