Comparative analysis of step and pulse time domain transmission techniques using laboratory sands and saline pore water

Abstract

Step and pulse time-domain transmission methods are used to measure the velocity of an electromagnetic signal transmitted through sand-filled columns with variable water content. Pore water electrical conductivities ranging from 0.3 dS/m to 10 dS/m are tested, and four different laboratory sands are used: #30, #60, graded, and a mixture of graded and #60 sand. While the composition of the electromagnetic signals used in each of the two methods is different, all other conditions at the time of measurement are identical; consequently, differences in the behavior of the methods are attributable to the variations in the composition of the step and pulse signals as well as differences between the time-picking methods of the two techniques. The relationship between the travel time of the signal and the volumetric water content of the sand column is shown to change with changing pore water salinity, and it changes at different rates for sands of different porosities. The slope of travel time versus volumetric water content increases with increasing pore water electrical conductivity when measured with a step signal. The slope of travel time versus volumetric water content decreases with increasing pore water electrical conductivity when a pulse signal is used. While the root mean square errors for the two techniques are similar, the magnitude of the change in slope is smaller for the step signal, suggesting that this method may give more precise measurements under field conditions where pore water electrical conductivity changes over time. The combination of the two measurement techniques is shown to provide more stable results over the range of pore water electrical conductivities tested, which remain closer to theoretically predicted values than either of the two methods used alone.