Marine Geophysics: a handy tool in an engineer’s toolbox

Marine geophysics can be used to clarify engineering design parameters and reduce the risk associated with such work. For example, marine geophysics can be used as an integral tool for:

• Oil & gas lease-block clearing: mapping pre-existing hazards prior to drilling
• Port development
  • Bedrock mapping for construction purposes such as piledriving and foundation design
  • Existing utility mapping such as pipelines
  • Bathymetry and full-coverage bottom mapping to map submarine hazards and meet international hydrographical standards for ports, anchorages and approaches
  • Route-mapping surveys for cable-laying projects
• Sewage outfalls
• Aggregate mapping
• Offshore / near-shore oil-platforms
  • Pipeline shore-landings
  • Post-drilling remediation verification
• Diamond exploration
• Mine tailings - environmental monitoring of tailings ponds
• Transportation - bridges, tunnels, ferry terminals
  • Construction
  • River scours
  • River-crossings for pipelines
• Military applications: mapping sediment caps above ordnance test ranges.

Associated Mining Consultants Ltd. (AMCL) has recently expanded its marine geophysical capabilities and can now equip a geophysics team to obtain marine surveys at a wide range of scales:

• Fine-scale (cm-resolution) images of the water bottom and subsurface structures such as bridge piers and outfall pipes
• Medium scale (dm-resolution) images of boulder layers that can cause havoc with horizontal directional drilling operations
• Large scale images of layering to depths of approximately 15 metres at 1 metre resolution, and images of bedrock at depths up to 200 metres beneath the water bottom at four metres resolution.

AMCL offers a range of marine geophysical surveys, including:

Reflection seismic is the most common marine seismic survey. Several marine seismic sources are available, the choice of which to use depends on the depth range of interest. Lower frequency sources provide greater penetration at the expense of resolution. The seismic source directs acoustic waves to the water bottom, and echoes are reflected back to the surface from the water bottom and from acoustic boundaries below. These echoes are recorded, then processed and interpreted to obtain a two-dimensional cross-section of the sub-surface. Reflections will occur from changes in acoustic impedance, as in this example (see figure left) from a set of surveys that AMCL conducted for BC Ferries. The objective of this survey was to

identify the boundary between saturated sediments and bedrock in order to plan upgrades in the ferry dock and terminal.

Bathymetry is similar to reflection seismic, but uses a much higher frequency to obtain depth to water bottom at very high resolution – in the order of centimetres. Bathymetry can be collected in grid fashion to create a contour map of the water bottom.

Side scan sonar uses a fan-shaped acoustic signature to sweep the water bottom producing high-resolution images of the ground surface. Side scan surveys can be used to identify scours at the base of bridge piers and to identify underwater hazards. In the example of side scan data shown below, the base of the bridge pier and accompanying scour can be seen on the right and a steel girder protruding several meters from the river bottom on the left. The steel girder is identified by the horizontal, high amplitude line. The shadow angled below it is an indication that the girder is protruding from the water bottom, rather than lying on it. Identification of such marine hazards can be critical for all marine work.

In addition to these services, AMCL also offers marine video surveys for inspection of underwater structures, fresh-water ground penetrating radar (GPR) surveys for bedrock mapping, and marine magnetometer surveys for mapping pipelines.

Associated Mining Consultants Ltd. is an affiliate of the Associated Engineering group of companies. AMCL has offices in Burnaby and Calgary. For more information on AMCL’s Marine Geophysics capabilities contact David Butler at butlerd@amcl.ca tel. 604-293-2800.