depths if the sound velocities of the media equal the calibration velocity. This 

 value is a reasonable average velocity for seawater, and deviation from this figure 

 will result in only slight depth errors in shallow water. In unconsolidated and 

 porous sediments the sound velocity is different than the instrument calibration 

 velocity, but here again the velocity differences are usually not great, and the 

 thickness of sediment penetrated not large, so that errors in sediment thickness are 

 not excessive for most applications. 



Recordings may be made at two scale settings: either normal or one-half scale. 

 Changes in the scale settings do not change the vertical scale of 1/2 inch per 10 

 feet, only the relative position of the zero line is affected. Using the normal set- 

 ting, zero depth is at the top of the paper. At the one-half scale setting, the zero 

 position Is located at the 100-foot line of the normal scale, or the center of the 

 recording paper. When the pulse is transmitted a black line is recorded directly 

 beneath the zero position. 



Scale changes are made during a run in an attempt to keep the bottom and 

 subbottom reflections in proper sequence on the recording. (In this report the repro- 

 ductions of the Sonoprobe records have been adjusted so that the bottom trace is 

 continuous regardless of scale changes.) In general, the water bottom, subbottom 

 reflections, and bedrock can be recorded in the 200-foot span of the recorder. 

 Multiples may, however, overlap the trace of actual bottom and subbottom features. 

 Multiples occur when the original reflected signal is again partly reflected at the 

 water-air interface or other reflecting surfaces above the horizon in question. 

 This energy is then directed downward to start another cycle and to be eventually 

 recorded. In most cases the multiples are easily distinguished (section V, C below), 

 but they may become a problem In shallow water where the bottom and subbottom 

 features are horizontal . 



In operation, various frequencies and filter settings are used to obtain the best 

 penetration and subbottom definition . Many of the notations on the records refer 

 to these changes. The effects of these changes are monitored on the display scope 

 mounted on the Control-Display Unit. When the best penetration is displayed the 

 printing controls are adjusted to obtain the desired subbottom record. 



III. THE SURVEY AREA: PENOBSCOT BAY, MAINE 



A. Geographic Setting 



Penobscot Bay is an estuary located in the central portion of the Maine coast 

 that opens into the Gulf of Maine at 69°\N , The bay, extending northward from 

 the coastline at 43''58' N . to the mouth of the Penobscot River, is about 28 miles 

 long. Of the many islands in the bay, Long Island, North Haven Island, and 



