ORIGINS OF POROSITY AND VELOCITY VARIATIONS AT CASCADIA ACCRETIONARY PRISM

At Cascadia accretionary prism, log-based porosities exhibit complex l ateral changes in response to variations in cementation, horizontal co mpression, and fracturing. Porosities at the prism toe are lower than in reference sediments, whereas porosities landward of the toe are sli ghtly higher than those of reference sediments. Ideally, a porosity/ve locity relationship can be used to determine two-dimensional porosity structure from seismic velocities. Porosity is the major control on ve locity at Cascadia, but variations in grain framework strength (frame bulk modulus) cause deformed and undeformed rocks to have quite differ ent velocity/porosity relationships. Within the prism, cementation can raise velocities substantially above those for unconsolidated referen ce sediments of the same porosity; also, compression-induced compactio n and fracturing may affect this velocity/porosity transform. Because these factors vary both within and between prisms, velocity/porosity t ransforms are local, not universal.