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The location of shipwrecks and other artifacts underwater, especially items buried underneath the bottom, can be aided with many scientific techniques, one of which is pulsed sound or sonar. The theory of sonar devices and several examples are discussed in this paper.

A sonar with a short acoustic pulse and high peak power (0.1 to 0.5 ms, 108 db) has been adapted for shallow water bottom penetration by speeding up the pulse rate to 30 cps. Known as the Pinger Probe, it has evolved from the EG and G sonar Pinger for positioning deep-sea cameras and is a device especially designed to show objects submerged in the soft bottom sediments of rivers, harbors, bays and coast lines. It emits a short, relatively powerful acoustic pulse capable of penetrating certain sediments and reflecting from objects or layers of different densities as a return signal. Coupled with a high resolution Alden recorder, it has a detection capability for locating a variety of sediment layering and shallow buried objects. The increased pulse repetition rate provides much improved resolution.

Three important parameters are among those governing the application of the many versatile sonar techniques. Penetration and resolution depend upon pulse length and frequency. The angle of the transducer is also important?a perpendicular beam will resolve geological strata, whereas a side-looking beam. will provide a "relief map" of the ocean terrain. Finally, several different types of transducers are available, both hull-mounted and towed, to provide the user with the best instrument for a specific application.

Preliminary experiments in harbor at Boston, Mass., with a sonar that has a high peak power output (about 105 db) and a very short ping length (about 0.1 msec) show that penetration at 12 kc/s can be as much as 50 feet below the bottom. Bedrock is reached in a few places. Sediment layers, probably of glacial origin, are clearly shown in others. Two vehicular tunnels that connect Boston with the airport are shown at 10 feet under the mud. The depth resolution in the sediment is less than ½ meter because of the short length of the signal. The transducer is mounted on a fish so that it can be lowered over the bow or side of a small boat and towed for making surveys. Thus there is a greater coupling of energy into the bottom and a smaller target area to reflect the signal.

The boomer, an electromagnetically driven sound source for seismic profiling, consists of a triggered capacitor-bank that discharges through a flat coil or coils. Eddy currents are induced in aluminum plates that are held against the coil by heavy springs or rubber bumpers. The plates are violently repelled, and a cavitation volume is produced in the water which acts as a source of low-frequency sound. The boomer has been used as the transmitter of a continuous seismic profiling system at sea. A 1000-joule system has been used in shallow, fresh and salt water. Reflections from sediment layers at least a kilometer below the bottom of the Puerto Rico trench have been recorded by Hersey and his group aboard the Woods Hole Oceanographic Institution's R.V. Chain with a 13,000-joule (maximum) boomer.

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.

Time-lapse photography indicates that biological activity and reworking acted as a catalyst for sediment erosion by bottom currents during a two-day period at a depth of 360 meters in the Hudson submarine canyon. Decapod Crustacea were responsible for the major tracks and burrows. Deep, sharp tracks made by the crab Cancer borealis were smoothed by currents within three hours, but a large burrow was maintained with intermittent excavation by a smaller burrowing decapod crustacean. The asteroid (seastar) Henricia did not track but smoothed the bottom. A polychaete worm appeared to forage in the Cancer tracks. Bottom water turbidity varied abruptly at intervals of less than one hour.

Amolitudes of high-resolution seismic reflections are reduced to within the dynamic range of facsimile profile recording by linear time-varied gain followed by 10:1 logarithmic compression so that relative amplitudes can be distinguished. The asymmetric pulse of the high-resolution Bogmer enables the polarity of isolated reflections to be determined. Seismic systems have been calibrated so that measurements of peak reflection amplitudes could be interpreted, yielding reflection coefficients of -50 to -80% in cases where no deeper penetration was observed. Presumed reduction of acoustic impedance by gas has been confirmed by samples containing gas and decomposing biologic material. These improvements to amplitude and polarity display and calibration help interpretation of biologic zones and limits to penetration on seismic profiles.

A 12-kilocycle pulsed transducer, with a 0.1 millisecond duration, is used for tracing a sub-bottom rock profile in Narragansett Bay. The short sonar pulse of high energy is produced by a capacitor discharge. Over-the-side installation of the transducer permits the use of any boat or ship for the survey work. Coherent presentation of the data on a wet paper recorder gives an instantaneous visual record. A cross a north-south rock formation, a recurring rise and fall of the rock is shown throughout the sedimentary deposit.

A method is described for photographing bats or other rapidly moving objects as they intercept in space a particular area which is covered by a camera system. Photographs taken at Carlsbad Caverns show that the tail membrane of the Mexican freetail bat is extended when the animal is in flight.

Lowering a camera to a position just above the sea floor is a critical operation. The "pinger" method of camera-to-bottom positioning has proved to be an excellent technique. A sonar transducer on the camera sends one sound signal directly to the surface, and another to the bottom. The bottom signal, reflected from the sea floor, rises to the surface where its delayed arrival time is measured against the camera signal beamed to the surface. An Alden recorder is used to record each direct or reflected signal as received.