CASE STUDIES
Seismic Survey on Quarry Site
We’re pickin' up good vibrations!… and the good vibes were the results of a SUMO seismic refraction survey.
The Brief:
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Colas outlined an area for quarry development. However, before any excavation could start, it was crucial that the geology and depth to bedrock was determined to ensure the viability of the extraction. Information on the extent of voiding, sedimentary structures and faults was also crucial to avoid unexpected quarrying difficulties that can result when these features go undetected.
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SUMO’s expertise in non-intrusive geophysical surveying was called on to devise a methodology to maximise survey outcomes for the intended purpose while providing the best value solution.
The Process
Selecting The Survey Methodology -
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Test pits and boring offer definite results however, they are costly and time-consuming. What’s more, the subsurface geology is only detected at the borehole location and the geology between them needs to be estimated.
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Geophysical methods are quicker, lower cost and they provide thorough coverage of the entire area. There are a range of separate and combined technologies for the identification of subsurface features and properties for different purposes. In each case, the application of the correct technology is important to yield accurate and reliable geological information for a specific objective.
SUMO’s choice of geophysical survey technique was seismic refraction, which would yield the best results and prevent misleading or inconclusive analysis. It provides fast data collection in the field at a lower cost compared to other technologies, plus the equipment used is easily portable and quick to activate.
How Seismic Refraction Works -
The seismic refraction technique detects the geological interface between layers of rock and soil. Seismic waves are generated in the subsurface via a source such as a heavyweight drop or sledgehammer blow.
These waves travel through the subsurface and are refracted from geological layers in the ground with contrasting seismic velocities. A line of ground motion sensors called Geophones detect the arrival time of the waves from the source and their speed, this information is collected and give the geological layer thickness and composition.
Applications for Seismic Refraction typically include:
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Estimation of depth to bedrock
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Rippability estimation (the ease with which ground can be ripped by an excavator)
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Hydrological Study
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Fault/fracture detection
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Landfill detection.
Costs And Benefits of Seismic Refraction -
The proposed site for development measured 24 hectares. Using test pit and borehole methodology would have been time-consuming and costly compared with a seismic survey and would have provided an incomplete representation of the entire site geology.
The results from the seismic survey were delivered in just 8 days and successfully indicated the depth to bedrock and furthermore, it pinpointed the problem extends across the entire site.
The cost for the seismic survey represented a huge saving in time and money over an intrusive borehole survey while providing more meaningful and consistent information.
The Results:
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The seismic refraction survey completed by SUMO was highly effective in showing three geological layers, one of soil, one of conglomerate and then a layer of limestone bedrock which was the primary interest to the client.
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The conglomerate overlying the layer of limestone was detected in the north of the survey area, which resulted in the limestone being at a greater depth than the client expected. In other locations, a layer of soil at a depth of between one and two metres was detected.
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The seismic velocities of the limestone varied across the survey area revealing a potential area of increased fracturing towards the south. This revealed the rock quality which was of particular concern for the client.
Above: Section View of data and interpretation showing the depth of conglomerate and limestone.
Above: Plan View of the site showing depth to Bedrock.
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