John H. Northrop

Underwater 20‐Hz Signals Recorded near Midway Island

From time to time, strong underwater acoustic signals with frequencies near 20 Hz have been recorded from hydrophones in nearly all oceans. Further details, such as source level, waveform, and movement, are not so well known. These signals may be divided into two main classes: short (∼1 sec) and long (up to 35 sec) emissions of sound at or near 20 Hz. We describe some low‐frequency long signals that were recorded on hydrophones near Midway Island, thus providing a further example of these interesting phenomena. The slow (∼1 knot) meandering of the source, its relatively high signal level (∼70 dB re 1 dyn/cm2 at 1 yd) and low frequency (23–25 Hz) suggest a large biological source.

20‐Hz Signals Observed in the Central Pacific

Twenty hertz signals, similar to those reported by other investigators from different locations, were recorded on hydrophones of the Pacific Missile Range at Midway, Wake, and Oahu Islands. Year‐round recordings revealed the signals during the winter months with a peak in October at Midway, December at Oahu, and January at Wake. There was no daily pattern of occurrence. More signals were recorded at Midway than at the other locations. Signals appeared in pairs with members separated by 16 sec between Type I and Type II, and 20 sec between Type II and Type I. Both signals had maximum energy near 20 Hz and lasted about 1 sec. Type I ranged from 18–27 Hz; whereas Type II ranged from 18–44 Hz. Signal trains lasted 10–12 min, and they were separated by 1–3 min. Estimated source levels ranged from 85–120 dB re 1 μ (at 1 yd). Source movement was evident at stations where a series of signals was recorded at two or more hydrophones.

SOFAR propagation paths from Australia to Bermuda: Comparison of signal speed algorithms and experiments

Signal speeds (average horizontal ray velocities) are calculated on the basis of bathymetry and nine sound speed profiles (SSPs) for three great circle paths from shot detonation sites off Perth, Australia to receivers near Fernando de Noronha (14 552 km) and Bermuda (19 766 and 19 810 km). We use two signal speed algorithms, one a long‐range ray‐tracing code and the other an adiabatic invariant approximation (AIA) method, which assumes J = ∮ c−1 sin ϑ dz is constant for a given ray (where ∮ dz is an integral over one‐ray cycle, c the local depth‐dependent sound speed, ϑ the ray angle, and z the depth coordinate). Comparison with field measurements made 21 years ago shows that the ray‐tracing and AIA methods, when applied to the same eigenrays, yield values averaging, respectively, 5.9±2.3 and 4.5±2.4 m/s below the observed values. The AIA method, however, yields values only 2.0±1.6 m/s below measurements when applied to rays vertexing just above the chief bathymetric obstructions near the Crozet and Kerg...

SOFAR accuracy in the North Pacific

SOFAR fixes were obtained by a computer program on a number of underwater explosions and implosions of various sizes and depths off the coast of California. Results are: (1) Accuracy of SOFAR fixes was best for events detonated in the sound channel; (2) SOFAR fixes on the implosions were within one standard deviation of those for explosions at the same depth; (3) SOFAR accuracy, compared to delivery vehicle navigation, was better for events dropped by ships than for those dropped from aircraft; and (4) SOFAR accuracy improved when a larger number of detection hydrophones was used in obtaining the fix.

Location and Enumeration of Underwater Explosions in the North Pacific

Hydroacoustic signals from underwater explosions were documented from analog recordings of 20 widely spaced SOFAR depth hydrophones in the North Pacific for a year. The number of shots per month ranged from 300 in winter to 4000 in summer. Source solutions, of accuracy between 10 and 30 miles, were grouped off the west coast of North America, north of Hawaii, and seaward of the Japanese and Kuril Islands.

Detection of low‐frequency underwater sounds from a submarine volcano in the Western Pacific

Underwater sound signals in the frequency band below 100 Hz were recorded at several widely spaced SOFAR depth hydrophones in the North Pacific. The signals, which typically lasted from 5 to 30 sec and were spaced about 1 min apart, continued at a high level of activity during the fall and winter of 1973–1974. SOFAR fixes obtained on discrete events in the series plot in the volcano islands near 22°N, 144°E. Some of the signals exhibited spectral banding with fundamental frequencies in the 1–2‐Hz region. If one assumes the spectral banding is due to constructional interference of surface‐ and bottom‐reflected sound in the vicinity of the source, the calculated depth of the explosions was between 375 and 750 m. The volcanic noises are therefore believed to have originated from explosive volcanic activity associated with the shallow seamount charted at 21.9°N, 143.4°E with a minimum sounding of 150 m (Navy Hydrographic Office Chart No. 970000).

Underwater Sound Propagation Across the Hawaiian Arch

Sound signals from 300 small ( <10 lb) underwater explosions at various depths in the Northeastern Pacific Ocean were monitored on SOFAR depth hydrophones off Midway and Wake Islands. Travel paths to Wake crossed the Hawaiian Arch while those to Midway were unobstructed. The signals received were analyzed, in the 5‐ to 180‐Hz band, for both peak sound pressure level and spectral density. Results indicate that (1) the Hawaiian Arch, as far west as Midway Island, caused complete terrain shadowing of hydrophones at Wake except for small “bathymetric windows” formed by 4‐km deep passages through the Arch; (2) peak pressure levels recorded at Wake, for paths through the “windows,” were as much as 35 dB below those at Midway; (3) acoustic shadowing through the “windows” was frequently independent.

Long‐Range Multiple Arrivals Received at Deep Hydrophones from Shots in the Gulf of Maine

Shots of 1000, 2000, and 3000 lb of TNT, detonated on the bottom of the 150‐f (fathom) Northeast Trough of the Gulf of Maine, were monitored at deep hydrophone locations on the continental slope off the United States of America, Bermuda, and the Antilles. Four types of arrivals were received from these shots: (1) ground arrivals, which were received at distances up to 250 miles from the shot; (2) direct arrivals that traveled along great circle, all‐water paths to receivers up to 1800 miles away; (3) indirect arrivals that traveled down the Northeast Trough and thence to the receivers; and (4) reflections from seamounts that lie off the mouth of the Northeast Trough.

Changes in Protein-Content of Bacterial Suspensions during Lysis and Autolysis.

The lysis of bacteria by phage has been considered as a simple plasmolysis, quite different from autolysis, and in fact many attempts to detect protein split products in the solution after lysis have failed. Bronfenbrenner and Hetler, 1 however, did find evidence for the appearance of protein split-products in solution after lysis and hence concluded that lysis was due to hydrolysis of the protein of the bacterial cell. Meyer, Palmer, Thompson, and Khorazo 2 have reported the appearance of non-protein nitrogen during lysis of Sarcina by lysozyme. The detection of minute amounts of protein split-products in culture-media is a difficult matter but the loss of protein may be readily and accurately followed provided the culture-medium itself contains little or no protein. In the present experiments bacteria were grown on the protein-free yeast-extract media previously described. 3 The quantity of cells in suspension was determined by comparing the turbidity of the suspension in water with that of a standard casein-suspension by means of a Klett photoelectric colorimeter. This figure was called “water-insoluble protein” and was found to agree quite closely with the protein-content of growing bacterial suspensions as determined by Kjeldahl determinations of the centrifuged and washed sediment. A second turbidity-measurement was made on a sample of the suspension diluted in 2% trichloracetic acid. This figure is called “total protein.” The relative changes in those 2 quantities is best shown by plotting the water-insoluble protein of each sample against the total protein for the same sample. The changes in water-insoluble protein and total protein during growth and lysis have been determined for Staphylococcus aureus, Staphylococcus musca, B. coli, and B. megatherium. The changes in the staphylococcal cultures during autolysis under anaerobic conditions 4 have also been determined.

Long‐range Pacific acoustic multipath identification

Acoustic signals from three long‐range (500–700 km) transmission paths in the Northeast Pacific were examined for multipath structures. Sound propagation along each path encountered both different sound‐speed provinces and unique bathymetry which, together with the range differences, caused characteristic pulse arrival patterns at each hydrophone site. The receivers were all on a sloping bottom at depths between 1200 and 1400 m and the source depth was at 450 m in deep water. Ray‐path arrivals were modeled using impulse, a new ray‐theoretical impulse response code written by one of us (RCS). This code uses piecewise continuous cubic polynomials to fit the sound‐speed profile and bathymetric profile, and Runge–Kutta methods to solve the ray equation of motion. It allows arbitrary ray density in launch angle, and identifies eigenrays by searching for rays whose depths bracket the receiver and which were adjacent at the source. Using this procedure, we were able to identify most of the major pulse arrivals o...