A seismic source rich in low frequencies is essential for deep crustal imaging, using wide-angle and long-offset (>6 km) recording, to construct a good, whole-crustal velocity model for use in structural interpretation and as a starting model for waveform inversion. When ocean-bottom seismometers (OBS) are used to record data at large distances, the offset and hence the depth of penetration of the seismic energy is limited only by the low-frequency output of the seismic source. Data from a 48-element, 9324 in3 air-gun source have been recorded to offsets of 400 km and compare favorably with 200-kg explosive sources (Staples et al., 1999). Where thick basalt flows are present, as on volcanic rifted margins such as those of the North and South Atlantic, they strongly scatter the high frequencies, produce strong reverberations, and make imaging of deeper structures difficult. Deep crustal arrivals, such as diving waves in the upper mantle, are often observed only at long offsets (typically >50 km in continental crust). In this paper, we investigate the effectiveness of low-frequency sources in providing good quality, long-offset, sub-basalt and deep crustal penetration.