RIASSUNTO
ABSTRACT
Acoustic velocity measurements were made aboard ship on sediment samples encased in core liners using a newly developed velocometer. The measuring equipment takes advantage of a water interface to avoid coupling problems between the transducer and core tube.
Velocity measurements transverse to the sample axis were performed on both 2"" and 3"" gravity cores. Examples of the velocity distribution and wave shape with depth in the sample are presented. The paper describes how such information is useful in correlating the sediment stratigraphy with geophysical reflection profile records.
INTRODUCTION
Preliminary site information is usually required for the planning and design of mining, dredging, construction and all other types of underwater projects. In many cases, preliminary studies require information and a representative picture of the subbottom sediments over large areas of the ocean bottom to locate the most promising areas where more detailed study is warranted. In such cases, a small coring program combined with geophysical profiling can provide a useful picture of the subbottom sediment profile and an evaluation of sediment engineering properties.
The purpose of this paper is to describe an onboard method for measuring the acoustic properties of sediments while they are retained in their core tubes. It will also show how these velocity values may be used to provide a depth scale to link the results of subbottom acoustic profiling and stratigraphic records from core tube samples.
CORE SAMPLE EVALUATION
Sediment core samples provide basic information necessary to develop a detailed picture of the geology and geotechnical properties of subbottom soils for underwater projects of all types. Visual inspection of these cores provides detailed information on thickness, distribution and stratigraphic relationships between sediment layers. Tests on representative samples from various depths in the core provide knowledge of the grain size distribution, natural water content, specific gravity, organic content and Atterberg Limits of the soils in the sediment column. Geotechnical studies of undisturbed samples obtained from these cores provide measures of the stress strain, shear strength, and time dependent consolidation behavior of the different sediment layers (Buchan, et al; 1967). Finally, the interpretation and evaluation of all this data provides the basic information for engineering planning and design.
In many situations, the use of sediment coring programs to define subbottom soil profiles may prove to be both expensive and time consuming and may not provide the quantity of information required to adequately picture large subbottom areas. As a result, geophysical profiling is often used to gather preliminary subbottom information (Feldhausen and Silver, 1971).
HIGH RESOLUTION SEISMIC REFLECTION PROFILING
Proven reliability, speed and economy have made seismic reflection profiling popular for determining the orientation and relative thickness of bottom sediment layers. Reflection profiling is similar to fathometer techniques that measure and continuously record the travel time of a sonic pulse as it travels downward from the water surface to the bottom where it is reflected back up to the receiver.