TECHNICAL ARTICLE
Improved coastal geotechnics with integrated marine seismic
reflection and refraction geophysics: Case studies
R. J. Whiteley & S. B. Stewart, Coffey Geotechnics Pty. Ltd.
ABSTRACT
Strong world demand for energy, mineral and agricultural products and the advent of larger transport vessels is underpinning new construction and upgrades at many Asia-Pacific and Australian ports. Overwater geotechnical investigations are required at the feasibility and design stages of these projects, directed mainly at entrance channels, pipeline routes and supporting land-based facilities. These are costly and difficult when overwater drilling is involved due to the costs of jack-up rigs and barges and restricted drilling sites within busy waterways. Consequently, there is increasing reliance on marine geophysics to provide the necessary subsurface information, typically in water depths of less than 20 metres.
Since water is acoustically transparent, continuous seismic reflection profiling (CSP) using boomer, sparker or airgun sources has been applied to these projects for many years, despite its limitations in certain conditions. From a geotechnical perspective a more important problem is that it is very difficult to determine engineering properties from single channel marine CSP data as Australia’s near shore marine environment is essentially a drowned continental land mass that has experienced a wide range of both terrestrial and coastal weathering and depositional processes over an extended geological time scale. These have created wide range of materials with very different geotechnical properties and behaviours that are not easily quantified with marine seismic reflection alone. Recently, single-ended, continuous underwater seismic refraction (CUSR) with near-bottom towed equipment and air-gun sources and static USR (SUSR) systems have been developed. These provide sub-bottom seismic P-wave velocities that can be correlated with engineering properties.
We present a series of case studies to demonstrate the application and integration of CSP, CUSR and static SUSR methods using advanced geophysical analysis processes to port infrastructure and near shore construction. In Victoria, combining conventional boomer CSP and CUSR improved the definition of a submerged, buried basalt flow and assisted dredging design along the Geelong Ports navigation channels. In East Malaysia, the same technologies assisted assessment of the viability of HDD (Horizontal Directional Drilling) as a pipeline installation option in variably weathered granites. In Western Australia, SUSR imaged the granitic regolith beneath sediments and indurated layers at a proposed new berthing where deep piling was required. This allowed preferred piling sites to be identified that minimised pile lengths.
USR technologies supported by advanced processing and analysis methods have demonstrated an ability to improve marine geotechnics in a diverse range of applications and will be increasingly applied in Australia’s coastal waters.
1 INTRODUCTION
In recent years population growth and increasing commodities demand have driven major port, harbour and infrastructure developments. Typical projects involve deepening and widening of navigation channels, berth and nearshore construction and require geotechnical investigations at the feasibility and design stages. These are expensive and difficult when overwater drilling is involved due to the costs of jack-up rigs and barges and restricted drilling sites within busy ports and waterways. Consequently, there is increasing reliance on marine geophysics, integrated with geotechnics, to provide this subsurface information (Whiteley, 2002).
Since water is acoustically transparent at seismic frequencies, shallow reflection using boomer, sparker or airgun sources has enjoyed considerable application to marine exploration for many years (e.g. Mosher and Simpkins, 1999). Despite their widespread use these, so called continuous seismic profiling (CSP) techniques have limitations in certain conditions, for example, in shallow water where strong multiple reflections obscure deeper primary reflections and in areas of restricted water circulation and/or rapid sedimentation when shallow gas layers form (Bertin and Chaumillon, 2005).
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