Acoustic Interrogation of Complex Near Surface Attributes

J.Y. Guigné¹, I. McDermott², and M.N. Cahill²

Abstract

Geophysical imagings are made problematic by discontinuous fractured bedrock or sediments, by Karst features, and by near surface formation structures characterized by buried boulders, sand lenses and cobbles and gravels. Seismic data related to reservoirs often become of limited value because of these masking influences. The questions often posed relate to what exactly constitutes the "real" state of near surface conditions. Details are required on the presence of buried geo-hazards, sediment property discontinuities, boulder clusters, gas in the sediments and on such sensitive concerns as trapped pollutants that could be released during excavation operations. On land, the difficulties in imaging the near surface has hindered the development of more exact statics and velocity models that account for the dispersive effects of the near surface on 3D seismic. A move to multiple echo energy and signal-shaped sonar and seismic systems delivered with broadened bandwidths, shorter pulse lengths, customized pulse shapes and beam-widths have provided datasets that capture more complete acoustic responses. However these data are still subject to a range of degradation effects when the near surface soils are inhomogeneous. There remains a technology gap for dealing with complex site investigations. Geophysical approaches do not hold the fine scales, density, or multiplicity of data to capture the true distribution and spatial variability of complex soils. In addition, the sampling scales of geotechnical probes and corers hold little spatial distribution knowledge on a scale equivalent to the features to be mapped. This paper is an illustrative description and introduction to a stationary acoustic interrogating approach, which delivers a wide volumetric 'acoustic core' product. This pioneering method yields vertical and lateral scales of meters and tens of meters in depth, thus producing a detailed, volumetric, layer-by-layer footprint, unprecedented in physical coring and in acoustic profiling. Densely collected data results in a multi-aspect and multi-folded, volumetric answer product which morphs into a coherent summation of the backscattered wave-field through focusing into the data, hence the term 'interrogation'. Creating a volumetric acoustic core through acoustically interrogating the sub-seabed and/or land based soils in a stationary manner holds value in bridging the confidence valley between the seismic collected, borehole and in situ test datasets.

AAPG Search and Discovery Article #90188 ©GEO-2014, 11th Middle East Geosciences Conference and Exhibition, 10-12 March 2014, Manama, Bahrain