Exploration Geophysics - Volume 23, Issue 4, 1992

Community problems such as soil and groundwater contamination by toxic waste and other pollutants, landslides, disruption of buildings and roadways due to ground subsidence, the need to detect buried objects such as pipes and cables and for crime scene investigations, radon gas accumulation in buildings, and stray radioactive sources are all problems which can be usefully investigated by traditional geophysical methods. Their remote-sensing ability offers advantages over more invasive and disruptive alternatives. The most appropriate methods include magnetics, ground penetrating radar, electrical, electromagnetic, radiometrics and to a lesser degree, seismic and micro-gravity techniques. The urban situations in which these methods are applied are often challenging due to such factors as a large amount of background noise, confined spaces to work in and shallow targets, often in highly disturbed ground. Special approaches and methods of interpreting the data are therefore sometimes required. As the applications of these techniques become more widely accepted it can be expected that new methods and ways of interpreting the results will be developed.

The use of electromagnetic (EM) methods for the mapping and monitoring of groundwater contamination in urban environments presents difficulties because of the presence of cultural noise and impediments to surveys from manmade structures. This paper shows the successful application of EM and dc methods for the detection and monitoring of groundwater conductivity changes caused by leachate contamination of groundwater, and for monitoring saline intrusions in these difficult environments. This paper discusses results obtained at four sites in the Canning Vale, Morley, Mindarie and Quinns Rocks suburbs of Perth, where any contamination of the groundwater poses a major threat because the city and suburbs rely on groundwater resources for up to 60% of their total water supply. Geophysical measurements were made by the transient electromagnetic (TEM) method using SIROTEM, dc sounding using a Schlumberger array and the down-hole frequency-domain electromagnetic (FEM) method using a Geonics EM-39 instrument. At Morley, where waste disposal began about 12 years ago, and TEM and dc measurements commenced 5 years later, inversion of the data assuming a layered earth showed that a region of low resistivity was developing in 1987 in the saturated zone to the southwest of the waste pit. Data obtained since then have shown a continuing development of this resistivity low further southwest. At the Tamala Park site in Mindarie, waste disposal commenced at the beginning of 1991 and, by November 1991, borehole sampling near the waste pit showed no significant change in groundwater conductivity. From background data collected at Tamala Park, it is possible to quantify seasonal variations in the apparent resistivity derived from the TEM and dc response and to predict that changes caused by leachate contamination should readily be detected above the seasonal variations.

The data obtained from one TEM and one dc survey at the Quinns Rocks-Mindarie site appear to indicate that sea water could be affecting the conductivity of the aquifer at a distance up to 180 m inland. This interpretation of the surface TEM data has been difficult to verify with down-hole FEM measurements in the area because the boreholes do not penetrate to the base of the aquifer and measurements have not been made at sufficient depth to detect a significant increase in conductivity below the watertable.

A review of physical property contrasts at landfills indicates that they are highly variable due to different disposal methods, materials and active chemical/biochemical processes. Substantial density, seismic velocity, electrical conductivity and dielectric permittivity contrasts can be expected. Old landfills normally have a low density and seismic velocity. High electrical conductivities may be present where contaminants and operative biological processes increase the free ion concentration of leachate. Organic contaminants may dramatically reduce dielectric constant. Seismic refraction field data from the Lucas Heights Landfill in south-western Sydney has been re-interpreted using the latest computer method. This re-interpretation defined an irregular sedimentary bedrock beneath the fill but did not locate the actual base of the fill due to the lack of seismic velocity contrast between saturated fill and weathered sandstone. Re-interpretation required the incorporation of a hidden layer representing the waste zone and underlying sand/weathered sandstone aquifer. At the same site interpretation of resistivity sounding data in terms of a 3-layered model defined the base of the cover material and the bedrock but could not distinguish fill from the underlying sand and weathered shale. The intermediate layer was highly conductive apparently due to leachate which flowed from the waste into the permeable sand within bedrock depressions defined in the seismic interpretation. The Lucas Heights field example raised problems of the minimum level of contamination which could be detected and the effect of thin contaminated layers. Analysis of these problems using synthetic examples demonstrated that conventional resistivity sounding alone may not provide sufficient vertical resolution if contamination is at a low level or, alternatively, if thin, highly conductive contaminated layers are present. Studies on synthetic electromagnetic (EM) examples with thin conductive layers also indicated that transient electromagnetics (TEM) may have an ability to markedly improve the resolution of highly conductive layers through the process of joint inversion with resistivity sounding data. However, TEM accuracy at landfill sites may be limited as their minimum depth of investigation could be too large for shallow targets. Ground penetrating radar has the potential to resolve fine detail in the subsurface. However, the rapid attenuation of radar signals in conductive materials poses a major limitation to its application at landfill sites. At Greenacre, in western Sydney, ground penetrating radar failed to achieve significant subsurface penetration in fills and saturated clays. Also, erroneous interpretations of contrasts between these materials and buried drums demonstrate the limitations of this technique. It is concluded that although no single geophysical technique can deliver at all contaminated landfill sites, a combination of techniques, underpinned by resistivity should provide the best possible resolution of contaminated zones.

Gravity surveys, as applied to geotechnical engineering problems, have been extensively used as a method for the detection of cavities. Through careful data analysis and modern presentation techniques, a much better overall geological model can be derived. A case history is presented from the Thompsons Creek Dam Site, NSW which illustrates that the gravitational method, through the careful use of standard survey procedures and equipment, has potential as a geological mapping tool in urban or industrial settings deemed inappropriate for other techniques due to environmental issues or geophysical noise considerations. Rock density determinations based on regression analysis of gravity data detected significant anomalies over the survey area. Image processing was used to enhance Bouguer gravity intensity maps, which enabled the location of additional cavities in the vicinity of the proposed dam site.

Earth anchors are steel cables used in the construction of earth-retaining walls to transmit load to earth or rock at greater depth away from the structure. In the construction of large multi-storey buildings it is common for percussion drilling to be conducted in close proximity to buried anchors. If an anchor is intercepted, there is a significant risk that the retaining wall and supported site offices would collapse. The detection of earth anchors beneath pavement in an urban environment is a difficult geophysical problem because the target has a dimension of about 0.01 m at a depth of about 10 m. Since one end of the anchor is exposed, the detectability of the target can be enhanced by injecting electrical current into it. Modelling of a buried inclined source has shown that the electric potential at ground surface is diagnostic of the anchor, and that the half-width of the anomaly is proportional to depth. A field trial has been conducted at one of the major building sites in the Sydney Central Business District. Various combinations of point sink and line sink current electrodes were tested in the field, but the most effective combination proved to be a pair of line electrodes set up by electrifying two earth anchors about 2 m apart in a vertical plane. Measurements of total potential and potential difference were made at electrodes driven into the contact between the concrete gutter and asphalt road. Because the anchor plates are in contact with the network of steel girders which form the skeleton of the retaining wall, most of the injected current was short-circuited through the shoring and the terminals of the transmitter. The magnitude of the observed response is consistent with about one per cent of current entering the ground through the anchors. The mean depth of the anchors is estimated at 8.5 m, compared to an expected depth of 7.8 m if the anchors are inclined at 45 degrees.

arthquakes pose a major threat to life and property, particularly in major conurbations where there is the potential for considerable damage to buildings. In the Sydney Basin region, which is home to approximately four million people and contains the largest urban development in Australia, earthquakes have been experienced since European settlement in 1788. However, it is only in the last thirty years that significant damage has been caused. In this qontext, the 1989 Newcastle earthquake dominates. It killed 13 people and caused damage estimated at $1.5 billion. Before that event, the 1961 Robertson and the 1973 Picton earthquakes, which were of similar size to the Newcastle earthquake (M˜5.5), also caused significant, but much less damage — $3 million and $2 million respectively in 1990 dollars. Current estimates of earthquake risk in the Sydney Basin indicate that on average, there is a 10 per cent chance of ground accelerations exceeding 0.11g in 100 years, or in terms of velocity, a 1000 year return period of approximately 90 mm/s. The earthquakes tend to occur within basement rocks beneath the basin and are caused by NE-SW compressive stress in the earth’s crust. This information is derived from in-situ stress measurements in basin rocks and focal mechanism determinations from earthquakes. It is evident that while the earthquake risk is much lower than in cities such as Tokyo and Los Angeles, which are on active plate margins, the risk in the Sydney region must be addressed, particularly for special buildings like schools, hospitals, dams and high rise office blocks.

Field and laboratory experiments and tests were carried out to investigate the response of buried steel and plastic drums to the magnetic, transient electromagnetic and resistivity profiling techniques in a magnetically quiet weathered Hawkesbury Sandstone environment in the central Sydney Basin. Steel drums, 2001, 501, 201, and 51 in size, and plastic containers, 701, and 201 in size, were used as buried targets in controlled profiling surveys. The proton precession magnetic method located individual drums buried at shallow depths but with increasing difficulty as drum size decreased. The survey profiles provided data that could be modelled by sphere or dipole sources magnetized by induction as viscous and permanent magnetization contributions were minor. Clusters of buried drums were readily detected by magnetometry and the data successfully modelled with an array of dipole sources. The TEM method clearly located individual steel drums, but the drum anomaly was less evident in the Wenner resistivity data. Plastic drums were not detected by any of the methods applied. Magnetic surveying is the primary method of choice for the location of shallowly buried steel drums in environmental studies.