Sub-bottom penetration in Boston Harbor: 2.

Abstract

A sonar record, taken with a 7700-cps sound pulse of 0.3-msec duration, is presented to show superior penetration of the sediments in Boston Harbor when compared with records previously made with a 12-kc/s sound pulse. Area studied. The continuous profile was made from near red buoy R6 to the Deer Island light at slow speed (see Figure 1 ). A distance of about 2 miles was covered in about 40 minutes. The approximate path of the vessel is shown on the chart. Records of the penetration have been published previously [Edgerton, 1963] showing weak, if any, penetration in the same vicinity. It will be noticed that the 7.7-kc/s penetration is superior in this particular area to the previous records made with a 12-kc/s sound pulse. This information checks the penetration experiments of Smith [1958]. Notice that there is no penetration at the beginning of the record but that there are three strong multiple bottom echoes where the bottom apparently is covered with a surface of high sound reflectance. The actual course at the start of the record was out of the channel, to the north. A side echo from buoy R6 can be seen. It is believed that gas bubbles in the surface layers of the bottom are responsible for these multiple bottom echoes. The water currents from the tide and the ships probably keep the main channel swept free of the high reflecting layer.