An electrical resistivity method for evaluating the in‐situ porosity of clean marine sands

A well‐known parameter in electrical bore‐hole logging is the formation factor (formation resistivity/pore‐water resistivity) and its relationship to formation porosity (or void ratio). This in turn can be used to give an assessment of the foundation characteristics of a saturated sand. To use this relationship in marine sediment investigation's, a device has been constructed which measures the resistivity of a seafloor sand by passing a focused current into the sediment from a superior position, rather than by physically penetrating the sediment with an electrode array which would inevitably produce a mechanical disturbance. At present, the investigated depth (which is a function of the size of the electrode array) relates to a zone within 0.5 m of the water‐sediment interface. Knowing the sediment resistivity, the formation factor can be determined from a separate assessment of the pore‐water resistivity. The formation factor is empirically related to the porosity of the sand by means of a graph prepared from measurements made in a formation factor‐porosity cell, which allows both parameters to be determined simultaneously throughout a range of packing conditions. From the determination of in‐situ porosity the relative density of the sand can be subsequently calculated.