Seafloor Models Research Articles

Acoustic–elastic wave scattering from realistic seafloor models.

Acoustic–elastic wave propagation was simulated with a 2-D finite-difference method for models of an ocean layer, sediment layer, and rough basalt layer. The roughness profiles along the upper surface of the basalt, specified at 1- or 10-m intervals, are from a self-affine bathymetry model. A low-velocity sediment layer was included in some models as a pond or a blanket on the basalt. A bandlimited point source (maximum frequency 150–300 Hz) was initiated in the water within 150 m of the seafloor. Although the wave field in the water column scattered from the 1-m roughness is more complex than that from the 10-m roughness, the dominant features are similar. The 1-m roughness reduces the coda amplitude but increases its complexity and duration. The scattered field is dominated by diffractions from the larger irregularities along the irregular basalt interface. Sediment layers produce a long coda resulting from P–SV conversion and SV reverberation. SV energy trapped in sediment ponds persists for long travel times, becoming chaotic and contributing to the field observed in the water column.

Comparison of a stochastic seafloor model with SeaMARC II Bathymetry and Sea Beam data near the East Pacific Rise 13°–15°N

A recent SeaMARC II survey along the Hanks and crest of the East Pacific Rise between 13° and 15° N (Edwards et al., 1988) provides us with a unique opportunity to compare stochastic models of the seafloor against bathymetric data. Stochastic models can be used to quantitatively characterize intermediate to small‐scale (<100 km) seafloor features such as abyssal hills. In a recent paper, Goff and Jordan (1988) introduced a model for the two‐point covariance function having five free parameters which describe the amplitude, orientation, characteristic wavelengths, and Hausdorff (fractal) dimension of seafloor topography. These parameters can be estimated from multibeam data such as Sea Beam by inverting auto‐ and cross‐covariance functions for the beams. The stochastic model can be used to generate synthetic topography to arbitrary resolution and scale. By assuming appropriate Sea Beam noise and response characteristics we can use the synthetic topography to produce synthetic data sets. Comparison of real and synthetic data is an important means of assessing the performance of the stochastic model. Unfortunately, the limited spatial extent of Sea Beam in the across‐track direction and lack of 100% coverage over large areas severely limit our ability to compare the full two‐dimensional stochastic model with real seafloor morphology, especially with respect to abyssal‐hill length characteristics. The recent SeaMARC II survey included sufficient off‐axis topography (over 100 km, 70,000 km2) to permit a comparison of a complete two‐dimensional synthetic topographic field with a region of abyssal hill terrain that has close to 100% data coverage. We have estimated stochastic models for various regions of the SeaMARC II survey area from several Sea Beam tracks which cross it. Synthetic fields are generated from these models using a Fourier method. These comparisons indicate the extent to which the model succeeds in characterizing seafloor topography, and the directions we will need to take to improve our modeling. A change in ridge complexity at 14° N and the location of a propagator pseudofault appear to be correlated with changes in stochastic character modeled in this region. Such correlations may provide important new constraints on the understanding of ridge processes.

Kirchhoff-Helmholtz modeling of seafloor scattering in a Mid-Atlantic Ridge seismic reflection line: Origin of scatterers and possible masking of a magma chamber reflector

In January 1989, multichannel seismic reflection data were collected over the MARK area (23° N) of the slow-spreading Mid-Atlantic Ridge (MAR) in hopes of imaging an axial crustal magma chamber. Unfortunately, the seismic reflection data are dominated by what appears to be scattering from a very rough unsedimented seafloor. No magma chamber reflection could be seen through the scatter. Some stronger coherent events, however, could be interpreted as dipping intracrustal reflectors. Kirchhoff-Helmholtz scattering synthetics from digital SeaBeam bathymetry maps indicate that almost all of the events seen in the reflection data originate from the seafloor and not the subsurface. A back projection algorithm was used to locate the points of origin of these seafloor scattered events. The sources of most of these scattered events are from smaller holes in the bathymetry rather than from large features, implying focusing within a Fresnel zone as the mechanism for the scattering. Combining Kirchhoff-Helmholtz transmission synthetics, using reflectivity-derived starting amplitudes, with the scattering synthetics, show that a magma chamber with an amplitude reflection coefficient as high as 0.10 would be made unobservable because of high-amplitude MAR seafloor scattering.

Experimental studies of sound propagation using a scaled marine geoacoustic model

This paper will describe a sound propagation experiment with a scaled shallow-water geoacoustic model. This two-layer seafloor model has dimensions 1.2×2.5 m and is composed of an epoxy layer 5.8-cm thick overlying a concrete basement. With a water depth of 15 cm, this model represents a shallow-water marine environment where the surficiai sediment possesses enough rigidity to transmit shear waves. This experiment concentrates upon the pressure magnitude as a function of depth in the water column using a fixed cw source with frequencies between 8 and 30 kHz. These pressure-depth shapes are compared with the effective depth theory of Chapman et al. [J. Acoust. Soc. Am. 85, 648–653 (1989)]. [Work supported by ONR.]

Geoacoustic propagation through random seafloor models

Finite difference forward models of elastic wave propagation through laterally heterogeneous upper oceanic crust are presented. The finite difference formulation is a two‐dimensional solution to the elastic wave equation for heterogeneous media and implicitly calculates P and SV propagation, compressional to shear conversion, interference effects, and interface phenomena. Random velocity perturbations with Gaussian and self‐similar autocorrelation functions and different correlation lengths are presented that show different characteristics of secondary scattering. The presence of a water‐solid interface in the models allows for the existence of secondary Stoneley waves, which account for much of the seafloor noise seen in the synthetic seismograms for the laterally heterogeneous models. “Random” incoherent secondary scattering increases as the product of wavenumber and correlation distance (ka) approaches 1. Deterministic secondary scattering from larger heterogeneities is the dominant effect in the models as ka increases above 1. Secondary scattering also shows up as incoherence in the primary traces of the seismograms when compared to the laterally homogeneous case. Cross‐correlation analysis of thc initial P‐diving wave arrival shows that, in general, the correlation between traces decreases as ka approaches 1.

An Advanced Sea-Floor Spreading Model

Although sea-floor spreading models that illustrate transform fault mechanics are fairly easy to construct, all the models so far devised illustrate constant-rate sea-floor spreading; that is: 1) they do not illustrate spreading that varies in rate from place to place; 2) they do not clearly show transform faults as arcs of small circles; and 3) they do not illustrate what happens near a pole of rotation. Models that illustrate the correct geometry of these concepts are mechanically possible and in fact nearly as easy to construct as the simpler variety.

Statistical averaging of marine magnetic anomalies and the aging of oceanic crust

Visual comparison of Mesozoic and Cenozoic magnetic anomalies in the North Pacific suggests that older anomalies contain less short‐wavelength information than younger anomalies in this area. To test this observation, magnetic profiles from the North Pacific are examined from crust of three ages: 0–2.1, 29.3–33.1, and 64.9–70.3 m.y, B.P. For each time period, at least nine profiles were analyzed by (1) calculating the power density spectrum of each profile, (2) averaging the spectra together, and (3) computing a ‘recording filter’ for each time period by assuming a hypothetical seafloor model. The model assumes that the top of the source is acoustic basement, the source thickness is 0.5 km, and the time scale of geomagnetic reversals is according to Ness et al. (1980). The calculated power density spectra of the three recording filters are complex in shape but show an increase of attenuation of short‐wavelength information as the crust ages. These results are interpreted using a multilayer model for marine magnetic anomalies in which the upper layer, corresponding to pillow basalt of seismic layer 2A, acts as a source of noise to the magnetic anomalies. As the ocean crust ages, this noisy contribution by the pillow basalts becomes less significant to the anomalies. Consequently, magnetic sources below layer 2A must be faithful recorders of geomagnetic reversals.

A comparison of finite difference and reflectivity seismograms for marine models

Summary Finite difference and reflectivity synthetic seismograms are compared for laterally homogeneous seafloor models with step and ramp discontinuities in elastic properties. Given suitable numerical parameters in each case excellent agreement can be obtained. For step discontinuities between a liquid and a solid the explicit finite difference formulation of the elastic wave equation for heterogeneous media is unstable. It is necessary to introduce the boundary conditions specifically at the interface and suitable formulations correct to first and second order in the space increment are given. For ramp discontinuities between a liquid and a solid the finite difference formulation for heterogeneous media is stable and agrees with the reflectivity method when, in the latter case, the gradient is approximated by homogeneous layers thinner than one-fifth of the minimum compressional wavelength at the upper half-power frequency. For a step discontinuity between two solids the explicit finite difference formulation of the elastic wave equation for heterogeneous media, although stable, is inaccurate and boundary conditions must again be specifically introduced.

Travel-time curves for a simple sea floor model

This paper reviews a simple technique for interpreting the velocity structure of upper oceanic crust from travel-time data of sonobuoy and ocean bottom receiver refraction experiments. The technique does not involve sophisticated digital processing or synthetic seismogram analysis. Interpretations can be carried out with a pencil, paper and slide rule.

Implications of Heat Flow and Bottom Water Temperature in the Eastern Equatorial Pacific

Summary The distribution of terrestrial heat flow through the sea floor of the eastern equatorial Pacific Ocean is compared to recent tectonic interpretations of this complex region. Some 500 values between 25° N to 20° S latitude, including 146 newly reported here, show that the theoretical distribution of heat flow for sea-floor spreading models matches well with smoothed data over portions of the Pacific, Cocos and Nasca lithospheric plates created within about the past 18 My from the East Pacific Rise. For the Galapagos Rift Zone, the fit is reasonable for lithosphere formed within the last 5 My only if a significant proportion of low heat-flow values is ignored. The heat flow on the Mathematicians’ Ridge and Galapagos Rise indicates that these were sea-floor spreading axes along which activity terminated about 5 and 10 My ago, respectively; this interpretation and its time scale are generally consistent with tectonic histories deduced from magnetic and topographic evidence. Detailed surveys and repeated measurements at stations suggest that localized heat-flow variability is frequently associated with topographic variability of the sea floor. Bottom water temperature profiles show that the East Pacific Rise is a barrier to eastward flow of waters at depths greater than 3 km, the approximate sill depth over the Nasca plate.