Resistivity

Brand new data from the ARP! The three panels above depict porosity data collected over a 19-hr period in a 20 by 100-cm laboratory tank (time flows L to R on the top, then R to L in the middle, and again L to R on the bottom).

The test went something like this... a partially consolidated, clumpy mixture of bentonite (a clay mineral) and seawater was stirred vigorously, and then allowed to settle. Almost immediately a thin layer of particle-free fluid appeared and over the next several days that interface migrated slowly downward (note the ever increasing thickness of the red layer (the water column) above). Also in the early stages of the test, 300 mm glass beads were added to the tank where they created a 1-3 cm thick layer over a roughly 40-cm patch (blue, low porosity in panels). A second interface between the rapidly consolidating upper layer and the clumpy "bed" (green) is also readily apparent. More tests are ongoing...

IRP in lab mode on the Seward Johnson II (February 2003).	IRP doing its thing in the Bahamas (leg spacing 0.6 m).
The porosity of marine sediments is a fundamental parameter with diverse implications. For example, seabed porosity affects the biovolume of sediments (i.e., the amount of room that may be occupied by microbes and meiofauna), as well as the propagation of solutes, solids and sound through the matrix. Porosity may also prove to be the best surrogate for predicting the erodibility of fine-grained sediment. To explore this latter possibility we have improved upon a long-standing geophysical technique: resistivity profiling. Briefly, our systems are 4-electrode Wenner probes in which the 2 outer electrodes drive a current and the 2 inner electrodes sense the resistivity. As the probes are pushed slowly into the seabed, more and more non-conducting sediment falls within the sensing volume, thus the resistivity increases. The so-called "formation factor" (ratio of bottom-water resistivity to pore-water resistivity) is then empirically related to porosity. We currently use two systems, both built for us by Rex Johnson, a talented engineer at the University of Washington. The first, the in situ resistivity profiler (IRP) shown above, is a small system that can be positioned by divers on features of interest on the shallow sea floor (see Wheatcroft, 2002). We also use IRP in ship board laboratories when sampling beyond SCUBA depths. The down side of IRP is that it can provide data only as frequently as you can collect cores. Hence, we may only get porosity data every few weeks. But since consolidation and sediment transport events can occur over short times scales (hours to days), it would be nice to get more frequent information... A substantial improvement on IRP that addresses the above shortcoming is a new instrument called ARP (the Autonomous Resistivity Profiler). We're clever with the acronyms, aren't we? Anyway, ARP (shown below) is designed to move in 3-dimensions and can cover a 20 by 100 cm piece of seafloor. Profiles can be up to 58-cm long, and a total of 2000 separate profiles can be taken during a 2.5 month deployment (that's 1 an hour!). We plan to deploy ARP off the Tet River in the western Gulf of Lions as part of the 2004-2005 EuroSTRATAFORM Program.
ARP tripod during recent field tests in Yaquina Bay, OR (leg spacing 4 m).
Brand new data from the ARP! The three panels above depict porosity data collected over a 19-hr period in a 20 by 100-cm laboratory tank (time flows L to R on the top, then R to L in the middle, and again L to R on the bottom). The test went something like this... a partially consolidated, clumpy mixture of bentonite (a clay mineral) and seawater was stirred vigorously, and then allowed to settle. Almost immediately a thin layer of particle-free fluid appeared and over the next several days that interface migrated slowly downward (note the ever increasing thickness of the red layer (the water column) above). Also in the early stages of the test, 300 mm glass beads were added to the tank where they created a 1-3 cm thick layer over a roughly 40-cm patch (blue, low porosity in panels). A second interface between the rapidly consolidating upper layer and the clumpy "bed" (green) is also readily apparent. More tests are ongoing...