GBG GROUP NEWS

Contaminant Migration Mapping at QLD Power Plant

Concerns were held of a possible transport mechanism for saline contaminants detected in a freshwater stream north of a water storage dam on the power station site. GBG Australia was contracted to undertake a multi-discipline geophysical investigation.

Localised boreholes provided limited information on the stratigraphy of the area. A broader understanding of the greater area was necessary to understand how fluid may potentially be transported toward the fresh water stream. To gain this broader understanding, two geophysical techniques were employed, Frequency-Domain Electromagnetics (FDEM) and Electrical Resistivity Tomography (ERT).

Prior to the geophysical investigation these sand and gravel lenses were thought to have offered the permeability required for fluid transport and therefore were deemed to likely be the transport mechanism.

The requirements of the geophysical investigation were to identify the location and extent of sedimentary layers and to aid in the definition of more permeable layers. The data would then be used to generate a groundwater conceptual model. GBG Australia carried out FDEM to cover the areas broadly between the water storage dam and a freshwater stream to the north. ERT was conducted concurrently. Three ERT profiles were positioned perpendicular to the expected direction of flow towards the freshwater stream, and at equal distances moving away from the power stations water storage dam. The locations the survey are shown in Figure 1. The rationale to these positions was to attempt to intersect any sediment lenses or migration paths, as well as to understand how they may change between the power station and the freshwater stream.

The FDEM results revealed a high conductivity response that appeared to be originating and migrating from the water storage dam in question. The results showed the response decreasing in intensity towards the freshwater stream. This was interpreted as either a deepening migration path to the north and being beyond the penetration of the instrument or some sort of barrier between the power station site and property to the north separated by an unsealed road.

The ERT results show small lenses of low resistivity, or conversely, higher conductivity, that deepen towards the freshwater stream. This corroborated the theory that a possible migration path is deepening towards the North.

Processing Plant Seismic Investigation

GBG was asked to investigate a mineral processing plant overseas to test the ground beneath the processing plant.

Concern about the integrity and life of the structures meant that remediation or replacement was required for the plant infrastructure. The material beneath the plant has a risk of being eroded by the acidic contents of the tank. Access below the tanks was limited to tunnels lined with corrugated iron or concrete lined rooms. Concrete cores were taken using a drill to test the concrete and soil conditions.

Determining the size and location of areas that had been eroded chemically or physically required a geophysical approach. Areas of low density or voiding would impede the velocity of seismic waves. The geophones were placed at regular intervals along the length of the access tunnels – one row in the ceiling and another in the floor. Hammer blows were struck around the circumference of the tank at the surface. The travel times from the surface to each geophone were analysed and measured in order to determine variations.

By mapping the raypaths and comparing their known distances and the time for the wave to travel, the velocity was determined. Each raypath was then inverted to generate a model of the subsurface in GeoTomCG software. Because the elevation varied between the geophones, as well as the shot points, a three dimensional model could be generated. Areas of overlapping low bulk velocity raypaths were able to be modelled as discrete anomalies.

The geometry of this survey was unusual and required specific processing that can be applied to other jobs where traditional seismic surveys would be unable to generate an appropriate model.

Multi Disciplinary Marine Survey of a Bay in Sydney Harbour

A Sydney wharf required an upgrade and GBG was hired to provide geotechnical information and to detect any foreign objects on the seabed.

Due to the high expense of undertaking overwater drilling and the requirement for continued access to the wharf structure for vessel refuelling it was decided that geotechnical information on the seabed depth and the sub bottom geology could best be provided using shallow marine geophysics.

The project requirements were to map geological units with emphasis on the mapping of bedrock to assist in the design and placement of piles for an upgrade to the current wharf facility. GBG Australia carried out marine seismic reflection, marine seismic refraction as well as single beam bathymetry for an area of 150m either side of the wharf and 100m in front of the wharf based on the proposed wharf expansions. Marine gradiometer and sidescan sonar were also collected to locate/image any objects on the seabed.

Marine seismic reflection data were processed and presented as 3D contours and marine seismic refraction data was processed to obtain indicative material strengths required for pile design.

GBG Australia were employed as Geophysical Consultants for the project. The project was undertaken by Simon Williams acting as project manager and lead geophysicist and David Mitchell as deckhand and onsite electrical and acoustic engineer.

NDT Investigation of Timber Structural Elements at Campbell’s Stores

Campbell’s Stores was established in 1839 and remains one of the most important heritage-listed building in Sydney. Included in its remediation, GBG Australia (GBG) was commissioned by Taylor Thompson and Whitting to conduct Non-Destructive Testing (NDT) of the timber structural elements. Ground Penetrating Radar (GPR) and Ultrasonic Pulse Velocity (UPV) testing were utilised as the technique to provide information on the building.

The purpose of the investigation was to provide a structural assessment of the current condition of the support and bearers of the heritage-listed building. The results provided were to allow the client to assess any repair and maintenance costs as well as the structural integrity of the building. Visual and tap testing were already completed and provided target areas for NDT testing.

GPR data was collected with a 1.5 Hz surface-couple antenna connected to a GSSI SIR3000. The technique was able to locate evidence of internal rot, soft timber, termite damage, pipes and large splits or shakes. Bearers were images from south to north along the beam where profiles were scanned on either side of the beam as well as the underside. Columns were then scanned from their highest point to the ground on all accessible sides to ensure detection of anomalies in the corners of the beams.

The UPV measurements were recorded to find the material strength of the timber, that any timber loss, moisture content and defects (such as air-filled cracks and voids) will be detected. However, as the natural growth of the timber changes the density of the material and its directional elements, a fixed elastic modulus could not be calculated accurately.

Geophysical Investigation to Map Areas of Archeological Significance, Victoria

GBG Australia undertook a geophysical investigation to map areas of archeological significance at the site of an old Inn near Melbourne, Victoria. For this investigation Frequency Domain EM (FDEM), Ground Penetrating Radar (GPR) and Magnetic Gradiometry were utilised to gain information on any possible archeological remains within the area of investigation.

MASW data was collected using the GSSI EMP-400 Profiler. A frequency range of between 15 kHz and 10 kHz was used for this investigation which equated to a depth of investigation of approximately 0.5 m to 1.5 m.

The GPR investigation was undertaken using a SIR-3000 acquisition system and 400 MHz ground coupled antenna. The system was mounted in a cart for ease of investigation.

The Magnetic Gradiometry survey was undertaken with the GEM GSPM-35G Potassium gradiometer. The investigation was performed at 1 m centres with readings being undertaken every 15 ms.

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