A Technique for the In-Situ Measurement of Shear Wave Velocities (V s ) for Deep Marine Foundations
Authors: Schwarz Sigmund D. , Conwell Fred R. 
Published: October 2008

A seismic recording system has been developed for making direct, in-situ shear wave velocity (Vs) measurements on the sea floor by the refraction method. The system can be deployed from a ship in the open sea in water as much as 300 m deep. Reliable shear wave velocity (Vs) measurements of horizontally stratified, multilayered bottom sediments can be made to a penetration depth of approximately 100 beneath the sea floor. The data can be used, together with _other parameters, to determine the seismic response of sea-bottom sediments for use in establishing seismic design criteria for structures founded on or in the sea floor. Uncertainties associated with the derivation of dynamic elastic moduli, based upon assumed values of Vs or the testing of laboratory samples, are eliminated.

INTRODUCTION

The environmental aspects of developing offshore petroleum resources is contingent upon many factors, including the engineer r S ability to design and construct facilities that can function safely under the severest conditions of seismic abuse.

One key element of information necessary to complete an adequate design is accurate, reliable Vs and thickness distribution of sea-bottom sediments. These, together with other data, can be used to predict the seismic response of the foundation ai1d structure to a given design earthquake.

The objective associated with a specific project in Santa Barbara Charnel was to design, fabricate, and test a geophysical system capable of making in-situ Vs measurements on the sea bottom and to use the system for developing the dynamic elastic module of bottom sediments.

Basically, the procedure involves deploying a multiple-element array of gimbals-mounted horizontal geophones in a line on the sea bottom, together with an electromechanical, bidirectional energy source. The sensitive axes of the geophones, and that of the energy source, are oriented in a horizontal plane transverse to the line and the direction of seismic wave level. This offers the optimum condition for the detection of shear waves, and their differentiation from compress ional (Vp) waves, by reversal of polarity when the energy source is actuated in opposite directions.

Deep marine foundation conditions were explored at two offshore sites in water as much as 300 m deep. The results provided the thickness and Vs of a layered system of bottom sediments to a depth of 100 m below the mud line. This information was used to provide the basis for developing dynamic elastic module to establish seismic design criteria for the structure and foundation.

GEOPHYSICAL EQUIPMENT

The seismic energy source is basically a bidirectional electromagnetic hammer that can deliver a force of approximately 68 ft-tons of impulse energy along a single axis. The impulse energy is derived by the impact of a 170-lb iron slug against either endplate of the source tube. The force to move the slug is-generated by the electrical discharge of a large capacitor bank into either of two large wire coils located on opposite sides of the source tube. The basic configuration of the energy source system is shown in Fig. 1.