Preliminary report on the Passamaquoddy bedrock survey, July-August, 1951

Abstract

In order to understand the results of the Passamaquoddy bedrock survey and the details of the method of obtaining them, it is desirable to have, in outline, a general idea of the methods and their application. In this part of the report a general account of the operation will be developed. The details will be found in subsequent parts of the report. At this point a statement of the general objective and some mention of the plans for reaching it are in order. The purpose of the Passamaquoddy survey was to determine the submarine geology in certain areas of Passamaquoddy and Cobscook Bays--the sea-bottom topography, the bedrock topography, the thickness of sediment overlying the bedrock beneath the sea, and the type of sediment. The information is needed for the planning of major engineering structures that may be built in the area. It developed in planning the study that detailed surveys could be limited to several areas where dams may be located. To determine bottom or bedrock topography, elevations of each were determined by sonic methods at a number of closely spaced locations (about 400 feet apart), evenly distributed on range lines 1,000 or 800 feet apart, in each of eight areas. Contour lines were then drawn to produce the desired topographic maps. Sonic soundings are necessarily referred to the existing water surface. To ascertain specific elevations in an area of large tidal variations is a complex problem. The details are discussed in the section on Vertical Control by Charles E. Knox (part 8). The basic datum is mean low water as determined by the U. S. Coast and Geodetic Survey. Equally important is the accurate positioning horizontally of each sounding location or fix. All such positioning was by the use of the standard three point fix procedure, using sextants. A net of shore points tied to the first-order control net of the Coast and Geodetic Survey was set up for this purpose, and all positions were thus accurately located. These problems are discussed in a section on Horizontal Control by Norman Duckworth (part 7). The general planning was done in conferences held in May 1951 at the Boston office of the Corps of Engineers, and in Eastport, Maine, after a brief reconnaissance of the area. A detailed account of the conferences is given in a memorandum by Herman J. Kropper of the New England Division, Corps of Engineers, reproduced in this report as part 4. Detailed survey procedures were developed on May 18, 1951, at a conference attended by P. J. A. Scott and Norman Duckworth of the Corps of Engineers and W. O. Smith of the Geological Survey. In order to make the necessary geophysical interpretation of the sound records, adequate geologic control had to be established. A preliminary field study of the geology was made by G. G. Parker of the Geological Survey during the period May 15-17, 1951. Subsequently Dr. T. E. Upson of the Geological Survey made the detailed geologic study required for the project. Two sonar units were used for the survey. One was a low-frequency high-power sonar unit developed by the Edo Corp. to meet specifications established by the Geological Survey. It was used to collect the bulk of the bedrock and bottom data. The other, of higher frequency and lower power, was manufactured by Bludworth Marine, Inc., end was required for adequate determination of water depths in shoal areas. The Edo equipment is described by Dr. Charles E. Mongan, Jr., in part 6 of this report.