ABSTRACT

The ease or difficulty of dredging I and the suitability of foundation materials for marine facilities, can frequently be predicted on the basis of seismic velocity values. Data for bottom and near-bottom materials (0-100 feet) are the primary concern. The velocity values are determined by means of continuous seismic refraction procedures requiring a towed I submerged, multi detector cable I a high energy source near one end of the cable, and closely spaced detectors (2040 feet) and shot point (l00± feet) locations.

Velocity values exceeding 7 I 000 ft. / sec. are usually indicative of hard dredging and good foundation conditions. Velocity values of approximately 10, 000 ft. / sec. or greater correlate with materials requiring blasting for removal and materials which will refuse driven piles.

Velocity inversions consisting of softer layers underlying harder materials are observed if the hard layer is of limited thickness (approximately 5± feet).

The reversal of profiles is required to distinguish localized anomalous zones from sloping interfaces and to determine true velocities. Accurate positioning is a prime requirement when comparing a run of refraction data recorded in one direction with that from the reversed direction. Crosser intersecting profiles are another requisite for the fullest interpretation and the highest confidence level.

INTRODUCTION

The nature and condition of sub bottom materials are a primary concern for dredging and marine facility foundation considerations. The harder or more cemented the materials are I the more difficult and more costly is the dredging and the more suitable is the foundation When the areas of interest are large in extent, miles in length or hundreds of feet in cross section, it is virtually impossible to economically drill enough holes to obtain adequate sub-bottom information.

In recent years the high-resolution seismic reflection methods have been proven as an effective exploration tool for determining stratification and continuity of the looser, sub-bottom materials. In an area where the correlation borings are reliable enough to confirm the absence of compacted or cemented materials and rock, no further exploration may be necessary. However, when compacted or cemented materials are known or suspected to be present in the given area, their detection and delineation can be beyond the economic and/or technical capability of a seismic reflection and correlation boring program. Accordingly, another exploration "tool" that can be used for exploration of such compact or cemented material is the seismic refraction method, adapted for continuous underwater profiling by use of a large number of shot points and a towed multi detector cable.

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