Analysis of amplitude, reflection strength, and acoustic impedance of AUV sub-bottom profiles with application to deepwater near-surface sediments
Authors: Zhang Zijian , Digby Adrian 
Published: May 2013

Understanding chirp sub-bottom profiler (SBP) data and selecting attributes are essential to improve SBP interpretation. Convolution-based numerical simulations of SBP responses reveal that amplitude of waveform of the SBP is a function of impedances. Reflection strength and its variations in different layers are controlled by the absolute value of the impedance contrast that causes the reflection. The reflection strength is more diagnostic than the amplitude alone for structural interpretations and identification of horizons. However, ambiguities occur when negative impedance contrasts generated by shallow gas and positive impedance contrasts generated by hard soils/sediments give similar responses. The ability to derive acoustic impedance from the SBP data allows geologists, geophysicists, and engineers to assess near-surface geologic conditions and characters of the soils/sediments based on its physical properties. The acoustic impedance data derived from SBP amplitude data provides a separation between the shallow gas and hard soils/sediments. A low impedance anomaly is the salient indicator of shallow gas while a high impedance anomaly is inferred to be hard soils/sediments. We conclude that integrating the reflection strength and acoustic impedance is a better interpretation strategy. With log data or geotechnical data control, amplitude data can be used to reasonably predict physical properties of soil away from the wells.

Introduction

High-resolution chirp sub-bottom profiler data has been collected in marine geophysical and geotechnical surveys for years. It is a favorable tool for analysis of sediment types, identification of faults and gas seeps, and detection of pipelines or other subsea objects. The autonomous underwater vehicle (AUV) chirp SBP commonly provides a wide spectrum pulse in a range of 2–20 KHz with a central frequency of 3.5 KHz, typically penetrating approximately 40 m in marine clay layers of the deepwater environment. Identifying variations in geotechnical properties using chirp SBP would be a huge benefit to the evaluation of the near-surface conditions in deepwater oil and gas exploration and development.

The limited number of locations that can be drilled and sampled offshore because of time and cost constraints suggests that deriving geotechnical properties from acoustic data could be of great benefit. This paper shows the that SBP data has the potential to be used with pre-conditioning from velocity and density logs in shallow boreholes to allow construction of a 2D or 3D representation of geotechnical variability of soil layers that could be tied directly to samples and seabed cone penetration tests (CPTs).

Numerical chirp SBP data processing and analysis have been presented by a number of authors. Previous studies on chirp SBP have used forward modeling to characterize marine sediments (Bull et al., 1998; Forno and Gasperini, 2008; Rakotonarivo et al., 2011), or used frequency and amplitude analysis to estimate physical and acoustic properties of sediments, such as P- and S- wave velocities, attenuation, bulk density, and mean grain size (Schock et al., 1989, LeBlanc et al., 1992; Stevenson et al., 2002; Schock, 2004a, 2004b, Pinson et al., 2008). However, trace-based inversion for acoustic impedance is rarely used in the SBP data processing, probably because high signal-to-noise-ratio true amplitude and phase information required for the inversion have only recently become available. The paper presented below shows that the trace-based inversion is able to produce inverted acoustic impedance from SBP data and is potentially a useful tool in the interpretation of SBP data.