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

A comparative evaluation of the efficacy of manual, magnetostrictive and piezoelectric ultrasonic instruments: an in vitro profilometric and SEM study

OBJECTIVES: The debridement of diseased root surface is usually performed by mechanical scaling and root planing using manual and power driven instruments. Many new designs in ultrasonic powered scaling tips have been developed. However, their effectiveness as compared to manual curettes has always been debatable. Thus, the objective of this in vitro study was to comparatively evaluate the efficacy of manual, magnetostrictive and piezoelectric ultrasonic instrumentation on periodontally involved extracted teeth using profilometer and scanning electron microscope (SEM). MATERIAL AND METHODS: 30 periodontally involved extracted human teeth were divided into 3 groups. The teeth were instrumented with hand and ultrasonic instruments resembling clinical application. In Group A all teeth were scaled with a new universal hand curette (Hu Friedy Gracey After Five Vision curette; Hu Friedy, Chicago, USA). In Group B Cavitron(TM) FSI - SLI(TM) ultrasonic device with focused spray slimline inserts (Dentsply International Inc., York, PA, USA) were used. In Group C teeth were scaled with an EMS piezoelectric ultrasonic device with prototype modified PS inserts. The surfaces were analyzed by a Precision profilometer to measure the surface roughness (Ra value in µm) consecutively before and after the instrumentation. The samples were examined under SEM at magnifications ranging from 17x to 300x and 600x. RESULTS: The mean Ra values (µm) before and after instrumentation in all the three groups A, B and C were tabulated. After statistically analyzing the data, no significant difference was observed in the three experimental groups. Though there was a decrease in the percentage reduction of Ra values consecutively from group A to C. CONCLUSION: Within the limits of the present study, given that the manual, magnetostrictive and piezoelectric ultrasonic instruments produce the same surface roughness, it can be concluded that their efficacy for creating a biologically compatible surface of periodontally diseased teeth is similar

Seismoelectric imaging of the vadose zone of a sand aquifer

We have acquired a 300-m seismoelectric section over an unconfined aquifer to demonstrate the effectiveness of interfacial signals at imaging interfaces in shallow sedimentary environments. The seismoelectric data were acquired by using a 40-kg accelerated weight-drop source and a 24-channel seismoelectric recording system composed of grounded dipoles, preamplifiers, and seismographs. In the shot records, interfacial signals were remarkably clear; they arrived simultaneously at offsets as far as 40 m from the seismic source. The most prominent signal was generated at the water table at a depth of approximately 14 m and had peak amplitudes on the order of 1 V/m. A weaker response was generated at a shallower interface that is interpreted to be a water-retentive layer. The validity of these two laterally continuous events, and of other discontinuous events indicative of vadose-zone heterogeneity, is corroborated by the presence of reflections exhibiting similar characteristics in a ground-penetrating radar profile acquired along the same line

Feasibility of an Off-Road Landstreamer

A series of tests compared the performance of a geophone sensor with plate coupling to the ground versus the conventional spike coupling in sandy and hard-packed ground with uneven surfaces. The purpose of these tests were to see if weight and footprint of a self-levelling sensor, where the geophone self-rights under gravity, were critial for performance. The sensor is intended to operate off-road being twoed on a cable behind a vehcle as part of a landstreamer. Our tests indicate that these factors don't matter significantly. The plate sensor data match geophone data to better than 10% RMS difference in the 10-150 Hz band. Thus, using landstreamers with self-levellling geophones appears to be a robust approach to automating 2D surveys

GPR for large-scale estimation of groundwater recharge distribution: Investigation of the shallow hydrogeology at the central Gnangara Mound, Perth

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 odeling of the ground water in this region of the world. ©2009 IEEE

Time-lapse seismic monitoring of CO2 injection into a depleted gas resevoir - Naylor Field, Australia

The CO2CRC Otway Project conducted under the Australian Cooperative Research Centre for Greenhouse Gas Technologies is the first of its kind, where CO2 is injected into a depleted gas reservoir. The use of depleted gas fields for CO2 storage and or enhanced gas recovery is likely to become globally adopted. Therefore, the CO2CRC project provides important experience for monitoring under these conditions. However, injection of CO2 into a depleted gas reservoir (residual gas saturation zone in this case) does not present a favorable condition for the application of geophysical monitoring techniques, and particularly seismic methods