Charles S. Cox

Measurement of the Roughness of the Sea Surface from Photographs of the Sun’s Glitter

A method is developed for interpreting the statistics of the sun’s glitter on the sea surface in terms of the statistics of the slope distribution. The method consists of two principal phases: (1) of identifying, from geometric considerations, any point on the surface with the particular slope required for the reflection of the” sun’s rays toward the observer; and (2) of interpreting the average brightness of the sea surface in the vicinity of this point in terms of the frequency with which this particular slope occurs. The computation of the probability of large (and infrequent) slopes is limited by the disappearance of the glitter into a background consisting of (1) the sunlight scattered from particles beneath the sea surface, and (2) the skylight reflected by the sea surface.The method has been applied to aerial photographs taken under carefully chosen conditions in the Hawaiian area. Winds were measured from a vessel at the time and place of the aerial photographs, and cover a range from 1 to 14 m sec−1. The effect of surface slicks, laid by the vessel, are included in the study. A two-dimensional Gram-Charlier series is fitted to the data. As a first approximation the distribution is Gaussian and isotropic with respect to direction. The mean square slope (regardless of direction) increases linearly with the wind speed, reaching a value of (tan16°)2 for a wind speed of 14 m sec−1. The ratio of the up/ downwind to the crosswind component of mean square slope varies from 1.0 to 1.9. There is some up/downwind skewness which increases with increasing wind speed. As a result the most probable slope at high winds is not zero but a few degrees, with the azimuth of ascent pointing downwind. The measured peakedness which is barely above the limit of observational error, is such as to make the probability of very large and very small slopes greater than Gaussian. The effect of oil slicks covering an area of one-quarter square mile is to reduce the mean square slopes by a factor of two or three, to eliminate skewness, but to leave peakedness unchanged.

Oceanic fine structure

Abstract Measurements by free fall instruments, in the San Diego Trough, the Florida Current, and the central Pacific, reveal the detailed structure of the vertical component of the oceanic temperature gradient. The temperature changes are concentrated into regions on the order of a meter thick wherein the measured gradients are often more than ten times the average gradient. The horizontal extent of the regions of high gradient is greater than 750 meters in the seasonal thermocline off San Diego, but is only a few hundred meters at depths greater than 400 meters. Fine scale measurements show that the layers of high gradient consist of even finer fluctuations in gradient which are only a few centimeters thick. Time scales of the thinnest of these regions of high gradient are of the order of five minutes. The data also yields an estimate of the entropy generation. According to the results of an idealized model relating entropy generation to the turbulent heat transport, only 240 to 700 ergs per cm.2 per se...

Marine controlled‐source electromagnetic sounding: 2. The PEGASUS experiment

The marine controlled-source electromagnetic sounding method developed over the past 15 years at Scripps Institution of Oceanography employs a towed seafloor electric dipole transmitter of moment 4 × 104 Am and multiple free-vehicle seafloor electric field recorders. A survey of 40 Ma normal oceanic lithosphere in the northeast Pacific using frequencies of 0.25 to 24 Hz and synchronous stacking of 0.25- to 12-hour-duration detected signals at transmitter-receiver ranges between 5 and 95 km. One-dimensional electrical conductivity structure is recovered from the data using the Occam process of nonlinear regularized inversion. Repeated inversion of a model terminated with an essentially infinite conductor or resistor demonstrates that the maximum depth of inference for this experiment is about 30 km, well into the upper mantle, with bounds placed on conductivity to depths of 60 km. Structure shallower than about 1 km is comparable to that obtained by a similar experiment on the East Pacific Rise and by borehole logging, with a sharp increase in resistivity at depths of 600–800 m, although strictly our experiment is sensitive only to integrated square root of conductivity, or total attenuation, in the surface layers. The lower crust and upper mantle has a resistivity between 2 and 7 × 104 Ω m and a transverse resistance of at least 109 Ω m2, suggesting at most 0.3% volume fraction of free water in the lower crust and some form of conductivity enhancement over mineral conductivity in the uppermost mantle. Although resolution is weak, below 30 km our data are compatible with a dry olivine model of mantle conductivity-temperature.

A Deep-Sea Differential Pressure Gauge

Abstract A pressure gauge configured to respond to the difference between the ocean pressure and the pressure within a confined volume of compressible oil is found to be especially useful for detecting pressure fluctuations in the frequency range from a few millihertz to a few hertz. In the middle of the range its noise level is lower than any other known gauge. The limitation of the gauge at the lower frequency limit is caused by unpredictable thermal expansion in the confined oil and at the upper limit by thermal agitation noise in the resistance of the strain gauge transducer. The gauge is insensitive to acceleration and tilting. Measurements with this gauge on the deep seafloor show two principal features in the spectrum of pressure fluctuations. At frequencies below 0.03 Hz there is evidence of pressures generated directly by long surface gravity waves. Above 0.11 Hz the pressures associated with microseisms are predominant. Between 0.03 and 0.11 Hz there is a spectral gap where the pressure level dr...

On the electrical conductivity of the oceanic lithosphere

Abstract Below the bottom of the sediments, the oceanic lithosphere is difficult to sound electromagnetically because the resistivity is high compared to over- and under-lying structures. There are three methods which can contribute information. 1. (1)|Magnetotelluric methods provide the impedance as a function of frequency for signals of sufficiently low frequency to penetrate through the ocean. In the deep sea the upper frequency limit is a few cph. 2. (2)|A quasi-static electric field can be measured at the ocean floor. The field is generated by charges built up by the motion of ocean water currents through the geomagnetic field. By providing a current path below the ocean, the lithospheric conductivity tends to reduce this field. In order to interpret the measurements, one needs to know the spatial character of the water motions. 3. (3)|An artificial EM source which transmits at frequencies near 1 Hz can be used at the ocean floor. Energy propagation through the lithosphere has been detected on the ocean floor to horizontal distances of 18 km. Detection is aided by the very low noise level at the high frequencies concerned. Estimates of the typical conductivity distribution throughout the ocean crust are given.

Measuring the two-dimensional structure of a wavy water surface optically: A surface gradient detector

A new method of measuring the slopes of a water surface covered with short waves is developed. A camera is placed far above the water surface looking downward so that it receives only approximately vertical rays of light emerging from the water surface from a source below. A large lens is positioned horizontally underwater. A plane light source in the form of a translucent colored screen is placed horizontally in the focal plane below this lens. Corresponding to each value of water surface slope, regardless of observer position, there is one and only one point of origin on the color screen from which light rays can enter the camera. When the color screen has a suitable two-dimensional color pattern, we are able to detect the gradient of the surface elevation throughout the field of view of the camera. This refraction slope detector has been used to find statistical properties of short wind waves in a wind-wave channel where a broad angular beam width of capillary ripples and short gravity waves contribute to the surface slopes. In these experiments waves were generated by winds ranging from 5 m/s to 10 m/s at a fetch of 24 m. The wavenumber spectra of short wave slopes have two distinguishing features: a dip at the capillary-gravity transition and steep slopes in the capillary range. Surface shapes resembling the shape of solitary capillary-gravity waves have been found from profiles of wave elevation deduced by integration of the elevation gradient.

Upper crustal resistivity structure of the East Pacific Rise near 13°N

An active source electromagnetic (EM) sounding has been conducted on the axis of the East Pacific Rise (EPR) at 13° 10′N. 1D inversion and modelling techniques, seeking resistivity as a function of depth, have been applied to 8 Hz amplitude data collected along the ridge crest. Resistivity is seen to increase monotonically between 50 m and 1 km below the seafloor, increasing from ∼1Ωm to around 90Ωm. We observe no intrinsic difference in upper crustal resistivity structure between the rise axis and 100,000 year old crust. Inferred surface porosities of 20% are larger than those recorded in 5.9 my old crust in DSDP hole 504B. Our data do not require, and lack sufficient information for, the reliable inclusion of a conductive termination to the model below 1.2 km.

Observations and Causes of Ocean and Seafloor Noise at Ultra-Low and Very-Low Frequencies

Although the term noise connotes troublesome phenomena that are to be eliminated in any useful experiment, seafloor seismo-acoustic noise is very highly colored and physically interesting. Modern seafloor seismic and acoustic instrumentation has largely eliminated instrumental effects in noise measurements and a clear picture of the behavior of Earth noise has emerged. In this paper, we present hypotheses for the generation and propagation of seafloor noise over a very broad frequency band; future and planned experiments will test these ideas. Furthermore, the quantification of seafloor noise, perhaps surprisingly, provides new insights into oceanic and geodynamic processes. The exploitation of these measurements has made new tools available for oceanographic exploration.

The Cartesian Diver: A Self-Profiling Lagrangian Velocity Recorder

Abstract The Cartesian diver is an autonomous velocity profiler capable of operating to 1 km depth in the ocean. It has a self-controlled buoyancy changer which is used to control the direction of profiling. The buoyancy changer has two states, full positive or full negative buoyancy. With correct ballasting the instrument has roughly equal up and down profiling speeds. A passive compressible volume appended to the pressure case is used to keep the buoyancy constant during an up or down run despite changes of density of the surrounding seawater, thus keeping the profiling speed and sampling intervals constant throughout a profile. A microcomputer system controls the buoyancy changer, data collection, and internal recording. Battery energy storage for the buoyancy changer is sufficient for 200 consecutive dives to 500 meters, thus 400 profiles. Horizontal components of velocity fluctuations with vertical wavelengths greater than 10 m are sensed using the method of geomagnetic induction. Since the instrumen...

Cox receives Ewing Medal

The Ewing Medal, given by AGU and the U.S. Navy for leadership in geophysics, was presented to Charles S. Cox at the 1992 AGU Fall Meeting Honors Ceremony on December 9, 1992. The meeting was held in San Francisco. The award citation, delivered by Michael Gregg, as well as Coxs response, are presented here.