[1] We measured the variations of the self-potential (SP) during periodic pumping tests performed at a test site located near a freshwater reservoir (Kemnader See, Bochum, Germany). Successions of injection and production intervals were applied in a borehole penetrating a jointed sandstone aquifer. We report the SP observations for tests with periods ranging between 10 and 60 min and flow rates between 10 and 25 L min À1 . The SP responses at the surface exhibit the imposed period but are not truly harmonic contrary to the hydraulic pressure and SP measured in monitoring wells. In the grassy zone around the injection well, the amplitude of the SP signals decreases with distance from the injection well (around one order of magnitude at 10 m) in rough agreement with predictions for radial flow in a homogeneous medium around an infinite source. The shape of the SP responses also evolves with distance. Fourier spectral analysis reveals that the surface signals generally contain two main components at the main period and at half the period with the relative weight of the subperiodic components increasing with distance. Furthermore, the characteristics of the SP responses depend on whether the boreholes are left open or closed by packers. The comparison between surface and borehole measurements suggests that nonlinear phenomena are acting, probably related to the saturation and desaturation processes occurring in the vadose zone.

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The shallow aquifer on the Gnangara Mound, north of Perth, Western Australia, is recharged by winter rainfall. Water infiltrates through a sandy Podosol where cemented accumulation (B-) horizons are common. They are water retentive and may impede recharge. To observe wetting fronts and the influence of soil horizons on unsaturated flow, we deployed time-lapse borehole radar techniques sensitive to soil moisture variations during an annual recharge cycle. Zero-offset crosswell profiling (ZOP) and vertical radar profiling (VRP) measurements were performed at six sites on a monthly basis before, during, and after annual rainfall in 2011. Water content profiles are derived from ZOP logs acquired in closely spaced wells. Sites with small separation between wells present potential repeatability and accuracy difficulties. Such problems could be lessened by (i) ZOP saturated zone velocity matching of time-lapse curves, and (ii) matching of ZOP and VRP results. The moisture contents for the baseline condition and subsequent observations are computed using the Topp relationship. Time-lapse moisture curves reveal characteristic vadose zone infiltration regimes. Examples are (I) full recharge potential after 200 mm rainfall, (II) delayed wetting and impeded recharge, and (III) no recharge below 7 m depth. Seasonal infiltration trends derived from long-term time-lapse neutron logging at several sites are shown to be comparable with infiltration trends recovered from time-lapse crosswell radar measurements. However, radar measurements sample a larger volume of earth while being safer to deploy than the neutron method which employs a radioactive source. For the regime (III) site, where time-lapse radar indicates no net recharge or zero flux to the water table, a simple water balance provides an evapotranspiration value of 620 mm for the study period. This value compares favorably to previous studies at similar test sites in the region. Our six field examples demonstrate application of time-lapse borehole radar for characterizing rainfall infiltration.

Borehole Radar has demonstrated to be an effective method to determine water content profiles within the vadose zone. Time-lapse measurements shown in this study were acquired on the Gnangara Mound north of Perth, Western Australia. Borehole radar experiments using vertical radar profiling and zero-offset crosswell profiling acquisition geometries have been performed under different soil moisture conditions. Time-lapse repeatability proved robust for zero-offset profiles, while vertical radar profiles provided mixed results. The variations observed in the zero-offset profiling are expected to be related to the seasonal variations of soil moisture content since results from the saturated zone remained constant during the tests. We quantify the wetting front developing through the dry soil profile on a monthly basis. Infiltration behaviours are characterized under different scenarios ranging from homogeneous soil moisture distribution with shallow water tables to heterogeneous soil profiles including water retentive layers and the watertable at 12 m depth. The measured soil moisture profiles and infiltration observations have implications for groundwater recharge estimation and unsaturated flow parameter estimation.