In situ measurements of near-surface porosity in shallow-water marine sands

An in situ resistivity profiler was developed to measure with minimal disruption, the near-surface porosity of shallow water marine sands. Results from a siliciclastic site off NW Florida and two Bahamian carbonate sites (an ooid shoal and coral reef sand flat) suggest the following general features. First, there is a 5- to 15-mm thick zone of elevated porosity adjacent to the sediment-water interface. Porosity in this layer was from 0.05 to 0.25 (decimal porosity) greater than the subjacent values, and would be difficult to resolve using traditional measurement techniques. Second, average porosity at > 10-mm depth was 0.38 +/- 0.01 at the siliciclastic site, 0.39 +/- 0.01 at the ooid shoal site, and 0.49 +/- 0.02 at the coral reef sand flat site; consistent with literature values. Third, individual profiles exhibited 0.05-0.15 fluctuations about the mean, with vertical length scales of 5-15 mm. These fluctuations may be the result of grain packing heterogeneities caused by hydrodynamic sorting during deposition and subsequent physical and biological mixing or could be artifacts caused by disruption of the grain framework. Fourth, ripple troughs at the siliciclastic sand site had a significantly higher near-surface porosity compared to ripple crests, due most likely to the presence of detrital material in the troughs. In addition, sediments within a microalgal film present at the coral reef sand site had a significantly higher near-surface porosity than the adjacent microalgae-free sand. All,of these findings indicate that there is pervasive small-scale variability in the near-surface porosity field of shallow-water marine sands, which potentially has important implications for sediment transport and high-frequency acoustics.