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Time to reflect

The powerful imaging capabilities of seismic reflection are now being employed to tackle civil engineering projects.

Seismic reflection technology is more often associated with large-scale offshore hydrocarbon and mineral exploration projects. But it is increasingly used by engineers looking for cost-effective, high quality subsurface information for the new raft of energy and infrastructure projects focused within the first 1km under our feet.

The technique measures reflected seismic waves, which give information as to the
depth of the feature that reflected them.

Significant depths

The technique can map geological strata and structure to significant depths - 10 km or more. It has been integral to offshore mineral, oil and gas exploration for almost a century. But its onshore application has been limited mainly to coal exploration.

Seismic reflection offers a view of the subsurface that would otherwise require many boreholes and a greater budget

Cambridge-based geophysical investigations specialist Fugro Aperio has overcome complex technical issues in scaling down seismic reflection equipment for wider onshore application. The new model is versatile and commercially viable for energy and infrastructure projects which need high integrity subsurface intelligence to help evaluate, design, cost, implement and risk-manage projects.

Key applications include foundation design for major civil structures, such as bridges, dams, wind farms, nuclear power stations, particularly in seismically active regions. Other areas set to benefit are geothermal and groundwater exploration, carbon sequestration, coalbed methane detection, underground coal gasification projects, carbon capture and storage as well as waste and natural gas storage.

Past drawbacks

In the past, seismic investigations have been expensive, unwieldy on site, and have involved lengthy processing times.

In contrast, Fugro Aperio’s fastrack method is mobile, scalable, can be deployed to short lead times. Turnaround of reporting is quick and efficient.

This means it may even be cost-effective for smaller, lower value developments where the engineering team may have expected a seismic-based survey to be too complex and expensive.

Reducing signals from ambient noise, particularly in urban areas, has been a recurrent challenge in making seismic reflection viable for smaller-scale onshore use.

Fugro Aperio has addressed the problem in a number of ways. Its methods include

  • building signal at the expense of noise, primarily through significant spatial oversampling;
  • applying quality control of raw records in the field;
  • by removing noise during data processing.

Offshore and early methods of seismic reflection involved the use of explosives, such as dynamite, to create seismic waves. Fugro Aperio utilises an Envirovibe swept-frequency source vehicle instead. This enables rapid data acquisition with minimal disruption to land owners, and none of the safety concerns associated with
explosives.

For major civil engineering projects, seismic reflection offers a comprehensive two-dimensional view of the subsurface that would otherwise require many boreholes and a much greater budget.

Major industrial site

In a recent project, it was used to evaluate structural geology below a major industrial site to determine the presence of capable faulting and to assess the risk of potentially damaging surface movement. At another site, the design team for a long span bridge in a seismically active area employed seismic reflection technology to assess the impact of deep geology on its foundation design.

Beyond a depth of about 30m, very few techniques can deliver the imaging detail and quality of seismic reflection. This increasingly popular branch of geotechnics will help engineers and developers better evaluate deep-lying conditions, assess the commercial viability of development sites, design robust underground structures and foundations, and reduce guesswork in managing risk and cost.

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