Sonar Bathymetry

Mapping the ocean bottom requires continuous measurement of depth using sonic means and accurate recording of the data, referenced to time and geographic position. From when Michelson was put into service through c. 1964 a single beam sonar was used to acquire depth information. This "trackline" method involves steaming along preset North/South and East/West lines in a checkerboard pattern to cover the area to be surveyed. Accumulated data is later plotted to create maps (charts) of the underwater topography.

The sonar transmitter/receiver, was located in the survey control center. Michelson was equipped with the SQN-6 (XN-1) sonar, essentially a modified UQN-1B depth finder set. Nearly every naval vessel of that time had a UQN-1B on the bridge. This was the successor to the "lead line" used to measure depth in the days of sailing ships.

UQN-1 Sonar Depth Finder with cover off (left) and closed.

According to the UQN-1B tech manual the sonar transmitter delivered 800 watt pulses at an audio frequency of 12 khz. Somehow I remember that our sonar, for whatever reason, pulsed ("pinged') a bit higher at approximately 14 khz, controlled by a pair of quartz crystals. The indicator scope and recording mechanism in the SQN-6 cabinet was not used. The unit's transmitter ping was keyed by a separate precision depth recorder (PDR) which also took the SQN's receiver output to be recorded. Two large glass tetrode tubes furnished the powerful audio frequency output. These 4-65A tubes were commonly used in radio transmitters of that time. 

Few people could hear the 14 khz sonar pings, at about the human ear's high frequency hearing limit. Most of the time I could hear it below decks in the living areas, barely audible if listening carefully, a ping every 1 1/2 seconds. American TV sets had horizontal sweep oscillators running at 15.750 khz. I could hear that sound from TVs when I was younger!

4-65A Tetrode

The second sonar element was the transducer. This was the thing that transmitted the pings down into the water and received the resultant echoes. An off-the-shelf UQN-1B came with a transducer made for simple depth finding use. Ours was stabilized such that the transducer always pointed straight down, in line with the local vertical. This electro mechanical apparatus was located down a vertical trunk below the fourth deck, two levels beneath our living area. It probably had some sort of gyro reference to help keep it stable.

Electrically, the transducer was magnetostrictive. Electrical pulses (pings) applied to it changed its shape, converting the transmitter's pulses to acoustic mechanical energy in a narrow beam.

Looking at the stabilized transducer could make you seasick. It stood straight up while the ship rolled and pitched around it. The whole affair worked quite well, keeping the sonar beam more or less straight down unless we experienced really heavy seas. 

Our two precision depth recorders (PDR) were in survey control. Mechanical monsters, each about the size of a washing machine, initiated the transmitter's pings and recorded the results. These worked in a similar manner to plotters but with an electrified stylus instead of ink and printhead. The sonar guy spent a lot of time fiddling with these intensive care machines. A company called Timesfax made them, which identifies the origin of the technology. Fax (facsimile) was invented to transmit low resolution newspaper quality photos over phone and radio circuits.

A wide (26-28") roll of electrosensitive paper was loaded into the PDR's left side. The paper ran horizontally across the top to the take up roller on the right side. A track for the styluses ran from front to back on the machine's left. When turned on, a stylus would start its trip across the width of the paper.

The first point to be marked was the start line corresponding to sea level. After moving a very short distance the stylus encountered the keying block. Here contacts closed very briefly, initiating the transmitter's output (ping). The stylus would then record a short black mark where the sonar heard its own outbound ping. The stylus continued to travel across the paper. If the sonar receiver heard a return echo it would cause the stylus to mark the paper accordingly. One trip across the paper's 24 inch wide track took 1 1/2 seconds and represented a scale of 0 to 600 fathoms. One fathom equals six feet.

Timesfax Precision Depth Recorder (PDR) Operation. Click for larger image.

As the first stylus reached the 600 fathom mark (exactly 24 inches from the start line) another stylus started its trip across the paper, marking the start, keying the transmitter and recording return echoes. Another stylus appeared every 1 1/2 seconds, the equivalent of 600 fathoms of depth. The PDR also recorded some sort of time codes as well as manually inserted event marks. Trace intensity was adjustable.

Another type of PDR, this one from EDO Corporation.

All this actually worked well if everything was adjusted correctly. Before each trip our sonar guy had to check with the first mate or the bosun to get the ship's draft in order to set the keying block to correspond with how deep we were in the water. Precise stuff, indeed.

So what happens if the depth is greater than 600 fathoms? I knew somebody would ask that. Well, if the depth below the keel were say, 1000 fathoms, then the echo return would appear on the next pass across the paper, making that the 600-1200 fathom range. Periodically the oceanographer was required to make a phase check. He could disable the normal pinging, initiate a single ping manually from the next stylus pass, then listen with headphones for the return echo, noting on the PDR paper in which pass (or phase) the return was heard. Simple, huh?

Again, all of this electro mechanical stuff sounds hopelessly complex but it was the best we had and best there was at the time!

An example of a PDR bottom profile trace. Paper moved through this PDR from left to right, giving a continuous profile of the ocean bottom.