We validate estimates of shallow sedimentary porosities made on the continental shelf by electromagnetic methods through a comparison with those measured directly in cores. Specifically, we show that an electromagnetic surveying system we have used to measure sediment resistivity in a number of seafloor locations returns porosity depth profiles that are related to those measured in cores, and that our data have good sensitivity to the structure within the uppermost few tens of centimeters of seafloor. We then consider inferences of sediment type made by observations of acoustic backscatter in the light of our porosity measurements. Our results suggest that electromagnetic methods provide complementary information that can be used to improve grain type estimates at and just below the seafloor, especially when combined with acoustic backscatter data. Because the system is pulled along the seafloor, data can be quickly collected from a large area of seafloor and can be used to interpolate facies conditions between core locations. ¿ 2001 American Geophysical Union