Brett Harris

Colloid release and clogging in porous media: Effects of solution ionic strength and flow velocity

The release and retention of in-situ colloids in aquifers play an important role in the sustainable operation of managed aquifer recharge (MAR) schemes. The processes of colloid release, retention, and associated permeability changes in consolidated aquifer sediments were studied by displacing native groundwater with reverse osmosis-treated (RO) water at various flow velocities. Significant amounts of colloid release occurred when: (i) the native groundwater was displaced by RO-water with a low ionic strength (IS), and (ii) the flow velocity was increased in a stepwise manner. The amount of colloid release and associated permeability reduction upon RO-water injection depended on the initial clay content of the core. The concentration of released colloids was relatively low and the permeability reduction was negligible for the core sample with a low clay content of about 1.3%. In contrast, core samples with about 6 and 7.5% clay content exhibited: (i) close to two orders of magnitude increase in effluent colloid concentration and (ii) more than 65% permeability reduction. Incremental improvement in the core permeability was achieved when the flow velocity increased, whereas a short flow interruption provided a considerable increase in the core permeability. This dependence of colloid release and permeability changes on flow velocity and colloid concentration was consistent with colloid retention and release at pore constrictions due to the mechanism of hydrodynamic bridging. A mathematical model was formulated to describe the processes of colloid release, transport, retention at pore constrictions, and subsequent permeability changes. Our experimental and modeling results indicated that only a small fraction of the in-situ colloids was released for any given change in the IS or flow velocity. Comparison of the fitted and experimentally measured effluent colloid concentrations and associated changes in the core permeability showed good agreement, indicating that the essential physics were accurately captured by the model.

Heat and mass transport during a groundwater replenishment trial in a highly heterogeneous aquifer

Changes in subsurface temperature distribution resulting from the injection of fluids into aquifers may impact physiochemical and microbial processes as well as basin resource management strategies. We have completed a 2 year field trial in a hydrogeologically and geochemically heterogeneous aquifer below Perth, Western Australia in which highly treated wastewater was injected for large-scale groundwater replenishment. During the trial, chloride and temperature data were collected from conventional monitoring wells and by time-lapse temperature logging. We used a joint inversion of these solute tracer and temperature data to parameterize a numerical flow and multispecies transport model and to analyze the solute and heat propagation characteristics that prevailed during the trial. The simulation results illustrate that while solute transport is largely confined to the most permeable lithological units, heat transport was also affected by heat exchange with lithological units that have a much lower hydraulic conductivity. Heat transfer by heat conduction was found to significantly influence the complex temporal and spatial temperature distribution, especially with growing radial distance and in aquifer sequences with a heterogeneous hydraulic conductivity distribution. We attempted to estimate spatially varying thermal transport parameters during the data inversion to illustrate the anticipated correlations of these parameters with lithological heterogeneities, but estimates could not be uniquely determined on the basis of the collected data.

Time-lapse borehole radar for monitoring rainfall infiltration through podosol horizons in a sandy vadose zone

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.

Semiautomatic and Automatic Cooperative Inversion of Seismic and Magnetotelluric Data

Natural source electromagnetic methods have the potential to recover rock property distributions from the surface to great depths. Unfortunately, results in complex 3D geo-electrical settings can be disappointing, especially where significant near-surface conductivity variations exist. In such settings, unconstrained inversion of magnetotelluric data is inexorably non-unique. We believe that: (1) correctly introduced information from seismic reflection can substantially improve MT inversion, (2) a cooperative inversion approach can be automated, and (3) massively parallel computing can make such a process viable. Nine inversion strategies including baseline unconstrained inversion and new automated/semiautomated cooperative inversion approaches are applied to industry-scale co-located 3D seismic and magnetotelluric data sets. These data sets were acquired in one of the Carlin gold deposit districts in north-central Nevada, USA. In our approach, seismic information feeds directly into the creation of sets of prior conductivity model and covariance coefficient distributions. We demonstrate how statistical analysis of the distribution of selected seismic attributes can be used to automatically extract subvolumes that form the framework for prior model 3D conductivity distribution. Our cooperative inversion strategies result in detailed subsurface conductivity distributions that are consistent with seismic, electrical logs and geochemical analysis of cores. Such 3D conductivity distributions would be expected to provide clues to 3D velocity structures that could feed back into full seismic inversion for an iterative practical and truly cooperative inversion process. We anticipate that, with the aid of parallel computing, cooperative inversion of seismic and magnetotelluric data can be fully automated, and we hold confidence that significant and practical advances in this direction have been accomplished.

Open Source Interactive Electromagnetic Modelling

Summary The marine controlled source electromagnetic method has developed during the last decade for direct hydrocarbon indication. Marine controlled source electromagnetic software is still in its infancy with only a small number of open source algorithms and even fewer integrated software environments. We have developed an open source software package to encourage the development and use of the marine controlled source electromagnetic method in both industry and educational institutions. The software was written in Java and was made to perform interactive real-time synthetic modeling for varying earth models or survey parameters.

GPR for large-scale estimation of groundwater recharge distribution

The Gnangara Mound, north of Perth, Western Australia, has been investigated using Ground-Penetrating Radar (GPR). Several hundred line-kilometers of GPR of common offset data have been acquired over an area of approximately 800 km2. The acquisition of these datatasets was performed at two different center frequencies (50 and 250 MHz) in order to better resolve the complexity of the hydrogeological targets of interest which are water retentive layers found above the water table. These layers impede the recharge of the surficial aquifer and may have important impact on local ecosystems but also on the management of the ground water resource. The data presented here-in demonstrate the successful imaging of the regional water table and of these water retentive layers. For the first time, these data provide insight into the spatial distribution and the continuity of these water retentive layers and provide important information to be included in the flow modeling of the ground water in this region of the world.

Cross well radar and vertical radar profiling methods for time lapse monitoring of rainfall infiltration

The relationship between electromagnetic velocities derived from in-hole radar surveying and soil saturation can be exploited to map changes in recharge from rainfall infiltration in the vadose zone against time. We have completed time-lapse cross-well radar and vertical radar Profiling (VRP) experiments with the objective of monitoring rainfall infiltration during the winter season at two sites on the Gnangara Mound in the Perth Basin, Western Australia. Depth-velocity profiles have been derived from the direct transmission measurements. Results obtained from Vertical Radar Profiling and Zero vertical offset cross well profiling are evaluated and the influence of different geometries and test-site conditions are discussed. We find that zero vertical offset cross well radar experiments were highly repeatable. Further changes in ground conditions such as an increase in moisture content can be observed with great confidence. The interpretation of vertical radar profiles was more challenging. However both techniques successfully reveal the time-lapse response of water migrating through the unsaturated soil profile for the two trial sites.

Stoneley wave dispersion in a high permeability sandstone: Perth Basin, Western Australia

Summary There is increasing support for the existence of a relationship between Stoneley wave characteristics and permeability in sandstone formations. We evaluated monopole full waveform sonic data sets acquired in a mudded drill hole at the Mirrabooka Aquifer Storage and Recharge trial site in Perth, Western Australia. To increase the spectral range of the full wave form sonic data the hole was logged three times with transmitter centre frequencies at 1, 3 and 15 KHz. Data were recorded in four receivers spaced at 1ft intervals with the first receiver at 3ft from the transmitter. Stoneley waves were clearly identified in the low frequency range of 1–5 KHz, which is characteristic of Stoneley wave propagation in a slow formation. A semblance slowness technique was used to determine the slowness of Stoneley wave. Slowness values ranged from 950 μs/m (Vst = 1050 m/s) for sandstone to 1650 μs/m (Vst = 600 m/s) for shaley sediments. Observations of the dependence of phase velocity on frequency were made by using multi filter and phase shift transform techniques. The relationship between Stoneley wave dispersion and fast flow, high permeability pathways, as identified in flow and time lapse induction logging data, was clearly observed in an interval from 330 to 333m below ground level. This high permeability sandstone layer can be identified in dispersion curves by assessing frequency and phase velocity shifts. Our outcomes are significant, as they present the possibility of identifying narrow high permeability layers in wells where full waveform sonic logs have been completed.

Time-lapse borehole radar measurements in a sandy groundwater system during a winter recharge cycle

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.

Estimation of Water Content in Partially Saturated Soil Horizons with Ground-penetrating Radar

Ground-penetrating radar (GPR) measurements have been used to map the absolute water content of water retentive layers found within the vadose zone of the superficial aquifer at the Gnangara Mound near Perth, Western Australia. The lateral water content distribution was derived from a pseudo 3D GPR dataset acquired over a podzolic soil profile. The water content of the bulk vadose zone was estimated from travel times. We discuss the parameters that influence the quantification of water content within a water retentive layer. By constraining key variables as layer thickness, porosity, Lichteneker-Rother model parameters, background permittivity and point of reflection, we can make a reasonable quantification of water content in the water retentive soil horizon. Calculated water content is shown to lie within reported values. The horizons were mapped laterally and reveal large variations throughout the test site. Our GPR derived hydrogeologic model is confirmed by time-lapse neutron logs and a high-resolution electrical resistivity image. Finally we compare results with total reflected energy from the interval above the regional water table and obtain a good correlation with the water content map.