Case Study - Seismic Survey on Quarry Site
We’re pickin' up good vibrations!
…and the good vibes were the results of a SUMO seismic refraction survey on a quarry which indicated better than expected limestone deposits
Colas outlined an area for quarry development. However, before any excavation could start, it was crucial 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.
The survey technique selected
Although test pits and boring offer definite results they are costly and time consuming. The sub-surface geology is only detected at the bore hole location and the geology between them needs to be estimated.
Geophysical methods however are quicker and 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 expertise in non-intrusive geophysical surveying for site investigation was called on to devise a methodology to maximise survey outcomes for the intended purpose while providing best value solution.
SUMO elected for seismic refraction technology to yield the best results and to prevent misleading or inconclusive analysis. The seismic refraction technique detects the geological interface between layers of rock and soil and the equipment used is easily portable and quick to activate. It provides fast data collection in the field at comparatively low cost compared to other technologies.
How it works
Seismic waves are generated in the sub surface via a source such as a heavy weight drop or sledge hammer blow. These waves travel through the subsurface and are refracted from geological layers in the ground with contrasting seismic velocities. A line of geophones (ground motion sensors) detects the arrival time of the waves from source and their speed, this then informs geological layer thickness and composition.
Applications for Seismic Refraction include: Estimation of depth to bedrock • Rippability estimation (the ease with which ground can be ripped by an excavator) • Hydrological Study • Fault/fracture detection • Landfill detection.
Costs and benefits...
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 extents 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 survey was highly effective in showing three geological layers, a soil, a conglomerate and the limestone bedrock which was the primary interest to the client. The conglomerate overlying the limestone was detected in the north of the survey area which resulted in the limestone being at greater depth than the client expected. In other locations, a layer of soil at a depth of between one and two metres was detected.
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 is particular concern for the client.