A Three-Dimensional Acoustic Site Surveying Probe
Authors: Inkpen S.T. , English J.M. , Pike C.J. , Guigne J.Y. , Guzzwell J.A. , Hunt P.H. , Tuff W.K. 
Published: October 2008

A new acoustic site surveying probe is being developed. The device rests on the seabed during the investigation and is controlled remotely from a supporting vessel. Multi-channel digital data is processed on a personal computer. The final data is displayed as a three dimensional image of a five meter diameter cylinder of marine soils. The accuracy of the temporal and spatial resolution of the data may provide the direction and confidence needed to translate acoustic responses into physical and behavioral properties of marine soils.

I. INTRODUCTION

Development of offshore facilities in a safe and cost-effective manner requires a knowledge of the nature and behavioral properties of the seabed. Offshore site investigations are conducted to such acquire information as: seabed topography; geotechnical engineering soil properties; the presence of sub-surface hazards such as shallow gas and loose glacial till; and, the depth and distribution of ice scours in ice prone areas. Costly losses can result if the engineering properties of the seabed are not accurately determined. Thus there is a growing demand for more accurate three dimensional, qualitative and quantitative representations of subsurface properties [1].

Over the past several years a phased approach to site characterization has emerged to better evaluate the inherent variability of marine sediments [2]. Such a program may include: geotechnical drilling; surficial sediment sampling; in situ geotechnical testing such as cone penetration and self-boring pressuremeter testing; and, seismic sub-bottom profiling.

A new acoustic site surveying technique is being researched at the Centre for Cold Ocean Resources Engineering at Memorial University of Newfoundland. This technique is based on the Acoustic Sub-Seabed Interrogator (ASI?) concept [2] [3] [4] [5], which involves very precise on site imaging of near surface sediment and the statistical estimation of a soil's homogeneity.

A shallow water prototype probe has been developed to further the ideas of an ASI? and to establish the limitations inherent in this technique. The prototype rests on the seabed during the investigation and is controlled remotely from a supporting vessel. Tunable, broadband acoustic sources, the stationarity of source and receiver during the investigation, and very fast sample rates, all contribute to the acquisition of precise acoustic images. Conventional multi-channel digital seismic data processing techniques, running on a personal computer, are used to process the data as it is being acquired. The final data is displayed as a three dimensional image of a five meter diameter cylinder of marine soils to an anticipated depth of fifty meters or more. The accuracy of the temporal and spatial resolution of the data should be unprecedented. Therefore inferences on the physical and behavioral properties of the seabed may be possible. A deep water tool will be the focus of future research and development.

The prototype and its control systems will be described in this paper. The operational and investigative procedures will be outlined and data from a recent field test will be displayed.

II. SYSTEM DESIGN

The prototype probe has been field tested several times and has undergone extensive development and refinement.