Sub-Bottom Profiling: An Alternative Approach to Shallow Water Automated Three-Dimensional Quantitative Sediment Classification
Authors: Lamprecht D.F. , Plekker E. 
Published: October 2008

Sub-bottom profiling in South Africa has, in the past, been used as a general mapping tool in deep water. Recently the shift has been towards shallow water engineering and mining applications within the costal zone where existing profiling systems fail to operate successfully. Commercially available systems were found to suffer from one or several limitations resulting in poor quality records. From tests, operational trials and experience a constant transmission frequency modulated profiler (CTFMP) was developed to address the need for a portable, versatile profiling system which can operate in very shallow water. A general description of the CTFMP system and the mode of operation is discussed with actual examples of data presentation and interpretation. It was found that this system has several advantages over available commercial systems. In an ongoing research and development drive, emphasis will be placed on a near real time, three dimensional, beamformed, swath-type sediment classification profiler.

INTRODUCTION

During the past two decades all sub-bottom profiling with transducers in South Africa was done in water generally deeper than 10 meters, and was used for sea-floor mapping for research purposes and later in the off-shore diamond mining industry. In recent years there has been more emphasis on engineering (for example the construction of bridges, pipeline routing and harbour dredging) and mining applications (heavy minerals and diamondiferous gravels) within the costal zone. A further need developed for profiling in harbours, rivers, reservoirs and in the inhospitable surf zone where existing systems fail to operate successfully.

STATEMENT

Sub-bottom profilers can be classified into on one of three general categories namely: linear, which operate between the frequency range of 3,0 - 7,0kHz; non-linear or parametric, with an operating frequency between 2 and 23kHz; and a chirp sonar which operates between 2 and 10kHz (References 1, 2, and 3).

In using some of these commercially available systems, it was found that there have been limitations when applied to shallow water environments. With all pulse type systems, a "water-window" is essential to distinguish between the transmitted and reflected signals. In general, where a transmitter array is also used as a receiver array, the minimum water depth without transmit/receive signal interferences, is approximately 10m (Ref. 4). A more useful alternative is to separate the transmitter and receiver arrays. Profiling systems, like the chirp profiler with separate arrays described by Schock and LeBlanc (Ref. 3) can operate in much shallower water.

When applying a high energy pulse to a transducer, a "ringing" effect has a tendency to "smudge" the received signal, resulting in poor vertical resolution. This makes the accurate positioning of sediment interfaces difficult and sometimes impossible. The restricted bandwidths of a pulse type system also determines the minimum thickness that can be resolved. Only fine grained sediments can be penetrated, due to energy scattering and absorption in the medium to coarser sediments. In most cases the deeper unconsolidated sediments are not fully penetrated.