Use of the Echo Sounder as an Offshore Exploration Tool

Abstract

ABSTRACT Precise delineation of sea floor morphology can reveal geologic features of significance to offshore exploration. A wide variety of echo sounding devices can be used to record geologically meaningful parameters of sea bottom relief such as abrupt changes in slope, degree of roughness, etc. Basic simplicity of the equipment required and the high speed of data acquisition make the echo sounder an attractive exploration tool where the full potential is yet to be realized. INTRODUCTION A great deal of information about the structure of the rocks immediately below the earth's surface can be derived from the study of the shape and distribution of the land forms which constitute the topography of the land and the floor of the sea. Most of the experience in the interpretation of surface morphology has been gained on land where it is possible to study directly the relations between land forms and underlying geologic structures under a variety of conditions. This experience can be applied to the interpretation of land forms offshore, particularly in shelf areas where the surface morphology and the underlying structures are comparable to those found on shore. Several of the more common and important aspects of the sea bottom morphology are listed in table 1. Escarpments, ridges, changes in slope and, diagnostic shapes such as cones are the features most readily recognized whether one is viewing a bathymetric contour map constructed from discrete soundings or, an analog echo sounder profile. The definition of valleys requires a high density of soundings to allow diagnostic contouring or profiling. A properly placed single sectional or longitudinal traverse will, however, yield a diagnostic profile where the echo sounder is used. A series of diagnostic topographic features that might be found on the sea floor are shown in Figure 1. Possible relations of these features to underlying geologic structures are shown diagrammatically in Figure 2 The features shown in Figures 1 and 2 represent the grosser aspects of the sea floor topography. In some areas, much additional information can be derived from a study of the details of the bottom topography. Modern echo sounders yield a remarkable amount of topographic detail at shallow and moderate depths. Figure 3 illustrates the concepts of orders of topographic relief used in this paper. The definition locally of high or low relief depends, of course, on scale. Within this context one can define high relief where there is a significant difference in the local elevations. Where maximum depths range to 100 fathoms, for example, changes in elevation of 50 fathoms or more would certainly constitute high relief, whereas changes of 5 fathoms would constitute low relief. The concept of roughness relates to whether the small changes in elevation take place abruptly or smoothly.