Geotechnical Services

Marine Geo-Engineering using Seismic Data:

Acoustic Sediment Assessment (ASA)

The rapid assessment of seafloor sediment density/porosity by processing of shallow seismic data

  ASA is a highly effective geotechnical assessment, management tool that:

•   Reduces your risks in Front End Engineering Design and construction
•   Reduces your site investigation project durations and costs
•   Adds significant value to your engineering seismic surveys
•   Optimizes your in-situ geotechnical sampling and testing programs

Acoustic Sediment Assessment

ASA is computer-based processing of reflection seismic data to derive in-situ soil density, based on Biot Theory and empirical relationships. The technique determines propagation velocity, density and porosity values in a "single shot", continuously along all survey lines, optimising route development. Soil density predictions are possible 24-48 hours after data acquisition and the integration of CPT data improves the level of accuracy of geotechnical interpolations. Projects to date have achieved density predictions to 10% of true without ground-truthing and better than 5% with. Derived soil shear strengths can be correlated to plough types/operational parameters as is presently done with geophysically derived data. Delivery of soils engineering properties is well in advance of standard geotechnical techniques.

Applications include

• Route and Site Surveys - FEED, pre-construction support and burial assessment
• Dredging
• Aggregate and Mineral Exploration
• Offshore Construction and Reclamation
• Operations Planning and Management
• Subsea Construction
• Habitat Mapping
• GIS Applications

Enhanced Characterization

A cost effective and rapid means of mapping the distribution of marine sediments is required to facilitate project-critical decisions facing marine managers. The ability to effectively manage a given sediment environment is in part a function of the accuracy of the available site characterization data. In general, these data should provide accurate descriptions of the physical properties of the sediments and delineation of the lateral and vertical extents of all unique sediment units.

Marine environments often possess a high degree of sediment variability over relatively small geographic areas. This creates the potential for inappropriate project management and design based upon the assumption of widespread sediment homogeneity. ASA accurately maps a given site, from which an efficient physical sampling program can be developed.

Add Value to Engineering Seismic Surveys

Traditional geophysical methods yield stratigraphic information, showing how sediments are distributed spatially within a site. This product is now much improved due to integrated data acquisition technologies that geo-reference all data to a high-degree of accuracy. For some, this level of product may be all that's necessary.

With the added capability to analytically evaluate these same acoustic data sets using ASA in conjunction with sample data, enhanced sediment descriptions of engineering properties such as density, porosity, and mean grain size are obtained. This allows for a quantitative solution to the spatial distribution of surficial sediments and improved lithological assessments:

•   Engineering properties - porosity, density, grain size and elastic properties
•   Geospatial mapping of contiguous sediment units and quantities
•   Optimized sampling

Summary

•   ASA is a management tool that for geotechnical assessment, enabling reduction of overall project costs, schedules and risk.
•   The determination of soil mechanical properties is based on scientifically accepted principles and empirical relationships.
•   ASA optimizes route development as it determines propagation velocity, density and porosity values in a "single shot", continuously along all survey lines.
•   Soil density predictions are possible 24-48 hours after data acquisition.
•   ASA provides the earliest and optimised CPT planning and route engineering.
•   The integration of CPT data improves the level of accuracy of geotechnical interpolations.
•   Implementation of rigorous sub-bottom profiler data acquisition protocols greatly improves final interpretation.
•   Projects to date have achieved density predictions to 10% of true without calibration and better than 5% with.
•   Derived soil shear strengths can be correlated to plough types/operational parameters.
•   ASA overcomes the depth limitations and operational risks of seabed-towed refraction and resistivity methods.
•   The final ASA data products' quality is limited only by the actual quality of the acquired sub-bottom profiler data.

Technical Basis

(Images: Overview of the ASA methodology.  Analysis of digital sub-bottom profiler data as a historical exercise was used to delineate unconsolidated sands and calcarenite units for Dolphin Energy project.)

(Image: Plan view example of ASA-derived lithology and surficial porosity distribution in Port Gamble, Washington.)

(Images: Simple Ground Truthing. Collecting cores for analysis.)

(Images: Geoacoustic calibration reflection sequence through sediment substructure.)

(Images: Delivery of digital database of ASA-derived geoacoustic properties.)