ALBERT

Description: Ionospheric disturbances observed by bottom-side soundings of the ionosphere appear at many temporal and spatial scales. Australia has many simultaneous observations from vertically orientated ionospheric sounders with spatial separations on the scale of 1000 km. However, with this spatial sampling only large scale ionospheric disturbances can be mapped and subsequently modeled. DSTO has an experimental program in progress to investigate the smaller spatial scale disturbances. These are often seen on vertical incidence soundings and are uncorrelated with soundings from greater than 500 km away. They can also be uncorrelated with soundings from the same site only 15 min later. The DSTO program to investigate these ionospheric disturbances is called SpICE, for Spatial Ionospheric Correlation Experiment. SpICE uses a small set of transmitters and receivers with varying separations to achieve a geographically spread set of near-vertical incidence ionospheric “reflection” points separated by 50–150 km, allowing us to probe disturbances at this spatial scale. Using the latest digital receiver technology we can collect amplitude and phase information from the ionospheric returns of the continuous wave transmissions of a nearby transmitter that is rapidly sweeping through the HF band. The returned signal is processed at a very high resolution to achieve good signal-to-noise complex ionograms at better than one-minute time updates. To date there have been three SpICE campaigns. This paper will discuss the SpICE program goals and highlight some of the unusual features observed in the first campaign. Following papers will look more closely at this data set and the subsequent campaigns.

Description: [1]  DSTO has initiated an experimental programme, SpICE (Spatial Ionospheric Correlation Experiment), utilising state-of-the-art DSTO designed high frequency (HF) digital receivers. This programme seeks to understand ionospheric disturbances at scales 〈 150 km and temporal resolutions under 1 minute through the simultaneous observation and recording of multiple quasi-vertical ionograms (QVI) with closely spaced ionospheric control points. A detailed description of and results from the first campaign conducted in February 2008 were presented by [13]. In this paper we employ a 3D magneto-ionic Hamiltonian ray tracing engine (PHaRLAP), developed by DSTO, to (1) model the various disturbance features observed on both the O and X polarization modes in our QVI data and (2) understand how they are produced. The ionospheric disturbances which produce the observed features were modelled by perturbing the ionosphere with atmospheric gravity waves.

Description: [1]  Near-surface heterogeneities produce diffractions in common offset ground-penetrating radar (GPR) data from the Gnangara Groundwater Mound, north of Perth, Western Australia. These diffracted wavefields can be enhanced and show a dispersion pattern if they propagate along a waveguide caused by a low velocity surface layer, such as moist sand on top of dry sand. Until now, GPR waveguide dispersion has been analyzed and inverted using common midpoint data. Using numerical modeling, we demonstrate that the same dispersion information can also be recovered from a diffracted electromagnetic wavefield recorded with common offset geometry. Frequency-slowness analysis of shallow diffractions in common offset GPR field data reveals high resolution dispersion curves. Inverting picked dispersion maxima to modeled curves (i.e., modal wave propagation in waveguide layer) allows estimation of waveguide height and velocities of waveguide and the underlying material. Data analysis in the frequency-wavenumber domain provides an alternative technique for extracting dispersion curves. Preliminary results validate this approach, which could be favorable in large-scale applications due to minimal processing requirement and inherent yet adjustable spatial averaging. The differences between waveguide parameters recovered from two surveys appear to be consistent with seasonal changes in moisture content and lateral changes due to variations in depositional environment. Our approach presents a new method to quantify the shallow dielectric permittivity structure of the subsurface from common offset gathers—the most commonly acquired type of GPR data. Potential applications of this method include estimation of shallow moisture distribution, early target identification for unexploded ordnance (UXO) detection, concrete slab characterization, pedological investigations, or planetary exploration.

Description: On 10 January 2009, an unusual ionospheric scintillation event was observed by a Global Positioning System (GPS) receiver station in Fairbanks, Alaska. The receiver station is part of the National Geospatial-Intelligence Agency's (NGA) Monitoring Station Network (MSN). Each MSN station runs two identical geodetic-grade, dual-frequency, full-code tracking GPS receivers that share a common antenna. At the Fairbanks station, a third separate receiver with a separate antenna is located nearby. During the 10 January event, ionospheric conditions caused two of the receivers to loose lock on a single satellite. The third receiver tracked through the scintillation. The region of scintillation was collocated with an auroral arc and a slant total electron content (TEC) increase of 5.71 TECu (TECu = 1016/m2). The response of the full-code tracking receivers to the scintillation is intriguing. One of these receivers lost lock, but the other receiver did not. This fact argues that a receiver's internal state dictates its reaction to scintillation. Additionally, the scintillation only affected the L2 signal. While this causes the L1 signal to be lost on the semicodelessly receiver, the full-code tracking receiver only lost the L1 signal when the receiver attempted to reacquire the satellite link.

Description: 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.

Description: Observational data are presented from several low-level flights carried out during the afternoon over areas of the Sahel that had been previously wetted by rain. The measurements are used to quantify the response of boundary-layer circulations to surface heterogeneity over a range of ambient conditions. Satellite observations of surface temperature anomalies show that soil moisture is significantly correlated with the surface heterogeneity in a majority of flights. By analysing the flight data in frequency space, consistently high levels of coherence are found between surface and flight-level measurements at length-scales around 25 km, indicating the presence of mesoscale circulations induced by the surface variability. The circulations are detectable in all of the nine flights where the mean sensible heat flux is high enough and they persist in a range of background wind speeds up to 5 m s −1 . Further analysis confirms that the spatial phase-difference between surface and flight-level variables increases with the strength of the mean wind along the flight track. The boundary-layer thermal anomalies and circulations are advected downstream by the mean wind, and lead to convergent uplift on the order of 0.25 m s −1 at the 25 km scale. These results compare well with those from a cloud-resolving model and are broadly consistent with an analytical, linear model of a heated boundary layer. By demonstrating the significance of soil moisture in driving the circulations, the study shows that soil moisture is a likely cause of the negative precipitation feedback seen in recent remote sensing studies over the region. Copyright © 2012 Royal Meteorological Society

Description: Bulk rock lithium and oxygen isotope compositions from ODP Site 1256 were analyzed to investigate the seawater circulation in the upper oceanic crust formed at the East Pacific Rise (EPR). The upper extrusive basalts have δ18O values from +6.1‰ to +9.2‰, reflecting alteration of oceanic crust by seawater at low temperatures (