Effect of soil grain size on the geophysical response of graves: Clay vs silt vs sand

Invited contributionNon-invasive geophysical imaging of Maori ancestral burial sites (urupa) have allowed us to test when and where geophysical surveys are most likely to succeed. Results from five sites, with both marked and unmarked graves, in three coastal settings along the east coast of the South Island of New Zealand show that burials in clay and silt (loess) can be readily identified using geophysical techniques, but burials in sand frequently have no anomalous responses. The differences in responses are likely due to the depositional setting. Clay and loess are usually deposited as layers or massive beds so any disturbance due to burial is relatively clear. In contrast, nearshore, fluvial and dune sands contain sedimentary structures that can be difficult to distinguish from burials, and can mask the geophysical responses of the grave

Non-invasive imaging of hydrocarbon contamination in permafrost soils at Marble Point, McMurdo Sound region, Antarctica

Summary: Hydrocarbon spills can cause extensive environmental damage and can last a long time in dry permafrost environments like that in Antarctica. Intrusive sampling and attempts at remediation can cause more harm to a site already damaged, particularly in the harsh yet fragile Antarctic environment. Additionally, even carefully planned intrusive sampling programmes can miss the target, in this case the location and extent of contamination. With negligible physical disturbance, non-invasive, non-destructive geophysical methods determined the extent of hydrocarbon contamination in the permafrost soil at Marble Point, in the McMurdo Sound region. Limited sampling was done for calibration, with minimal and highly restricted disturbance. The contamination is 35 to 40 years old; however the electromagnetic response was electrically resistive, the same as relatively young contamination in temperate soils, whereas older temperate spills tend to be electrically conductive. Radar profiles were acquired across the eastern half of the site, crossing two contaminated locations. The radar reflections were enhanced, again as observed for young contamination in temperate soils. Correlation between the radar and electromagnetic responses was excellent. The cold polar climate slows chemical and physical changes to contaminants, so that they respond as if young and relatively fresh. Geophysical imaging provided a viable non-invasive means to map the extent of hydrocarbon contamination in Antarctic permafrost soils with little or no site disturbance

Migrating and merging polarised GPR profiles: does it matter if migration is before or after merger?

A group of radar profiles was gathered across what may have been the site of a canal that was part of the late 16th to early 18th century landscaping at Castle Ward, an estate in Northern Ireland. The canal was filled early in the 19th century, and its exact location was unknown since then. Two sets of profiles were acquired for each of the three survey lines: one set had the antennas perpendicular to the line direction, the common cross-line survey configuration; the other set had the antennas parallel to the survey direction, which we call here the inline orientation. In all cases, transmitting and receiving antennas were parallel to each other. The raw data were compared and there are the obvious differences expected when acquiring data using different polarisations. In addition, we also tested migrating then merging the profiles, versus merging then migrating, to look for any systematic difference in the results. In principle, the final profiles should be the same. However, it appears that migrating first, then merging yields a clearer image of the shallower subsurface (the upper part of the profile image), whereas merging then migrating yields a clearer image of the deeper parts of the profiles. The same features are readily apparent in both profiles; there is no net loss of information nor any difference in interpretation in either case. Nonetheless, the interpretation is aided by processing using both orders – migrating then merging and merging then migrating – so that all relevant features are clearly identified

Geophysical imaging of subsurface earthquake-induced liquefaction features at Christchurch Boys High School, Christchurch, New Zealand

On February 22, 2011, a magnitude Mw 6.2 earthquake affected the Canterbury region, New Zealand, resulting in many fatalities. Liquefaction occurred across many areas, visible on the surface as ‘‘sand volcanoes’’, blisters and subsidence, causing significant damage to buildings, land and infrastructure. Liquefaction occurred at a number of sites across the Christchurch Boys High School sports grounds; one area in particular contained a piston ground failure and an adjacent silt volcano. Here, as part of a class project, we apply near-surface geophysics to image these two liquefaction features and determine whether they share a subsurface connection. Hand auger results enable correlation of the geophysical responses with the subsurface stratigraphy. The survey results suggest that there is a subsurface link, likely via a paleo-stream channel. The anomalous responses of the horizontal loop electromagnetic survey and electrical resistivity imaging highlight the disruption of the subsurface electrical properties beneath and between the two liquefaction features. The vertical magnetic gradient may also show a subtle anomalous response in this area, however the results are inconclusive. The ground penetrating radar survey shows disruption of the subsurface stratigraphy beneath the liquefaction features, in particular sediment mounding beneath the silt ejection (‘‘silt volcano’’) and stratigraphic disruption beneath the piston failure. The results indicate how near-surface geophysics allow the characteristics of liquefaction in the subsurface to be better understood, which could aid remediation work following liquefaction-induced land damage and guide interpretation of geophysical surveys of paleoliquefaction features