Petrology and porosity of an oceanic crustal site: Results from wave form modeling of seismic refraction data

P and converted S waves observed in refraction stations FF1, FF2, and FF4 of the 1959 Fanfare cruise of the Scripps Institution of Oceanography are analyzed by synthetic seismogram modeling of the data using the reflectivity algorithm and by inversion of the P and S wave travel time data to obtain extremal bounds on the P and S velocity (vp and vs) profiles. While the FF1 data are inadequate for detailed analysis, the FF2 and FF4 data yield vp profiles displaying rapidly increasing velocity with depth in layer 2, a small velocity discontinuity between layers 2 and 3, gently increasing velocity with depth in layer 3, and a 1-km-thick Moho transition. The vs profiles for FF2 and FF4 show rapidly increasing velocity with depth in layer 2, fairly uniform velocities in the top of layer 3, a slight low-velocity zone extending through most of layer 3, and a 1-km-thick Moho transition. Using theories of seismic wave velocities in cracked media and a laboratory velocity measurement made on a basalt sample from this site, a porosity of 18% is inferred for the top of the igneous crust at this site. A further reduction of porosity to 2% can explain the observed velocity gradients only to a depth of 0.6 km into the igneous crust. In the 0.8- to 1.5-km depth interval, Poisson's ratio appears to drop below 0.27 to a minimum of 0.24, which may indicate a zone of trondhjemites or other quartz-rich rocks at this depth or which may be related to state of fluid saturation of the rocks. Within layer 3, observed vp and vs agree well with laboratory velocity measurements of ophiolite samples from the western U.S. and from the Bay of Islands, Newfoundland. The observed velocities suggest the disappearance of hornblende and the appearance of augite and olivine with increasing depth in layer 3. There is no evidence for more than 30% serpentine anywhere within the crust or upper mantle at this site, except possibly within unresolvably thin zones or pods. Evidence is also given which suggests that velocities and velocity gradients in the shallow crust may be partly controlled by differential pressure (externally applied pressure minus pore fluid pressure) and its spatial gradients and that laboratory velocity measurements made on water-saturated basalt samples at zero differential pressure are more representative of in situ velocities in the shallow crust than lab measurements made at which are usually employed as in situ conditions, namely, elevated externally applied pressure and zero pore fluid pressure. The factors affecting the efficiency of shear wave conversion at the sea floor are investigated, and the important role of basement vp and especially vs are shown. Since basement vs is very sensitive to fracture geometry, the high lateral variability of shear wave conversion may be related to variability in the extent and character of basement porosity. A useful explosive source function for marine synthetic modeling is presented, and a nomenclature for marine seismic phases is suggested.

[1]  E. Wielandt.  Generation of Seismic Waves by Underwater Explosions , 1975 .

[2]  H. W. Menard.  Fragmentation of the Farallon Plate by Pivoting Subduction , 1978, The Journal of Geology.

[3]  Bernard Budiansky,et al.  Seismic velocities in dry and saturated cracked solids , 1974 .

[4]  B. Kennett,et al.  Reflections, rays, and reverberations , 1974, Bulletin of the Seismological Society of America.

[5]  T. Francis,et al.  Seismic refraction measurements in the southern Indian Ocean , 1967 .

[6]  N. Christensen,et al.  The seismic velocity structure of a traverse through the Bay of Islands Ophiolite Complex, Newfoundland, An exposure of oceanic crust and upper mantle , 1978 .

[7]  N. Christensen,et al.  Structure and constitution of the lower oceanic crust , 1975 .

[8]  D. Helmberger,et al.  A travel time and amplitude interpretation of a marine refraction profile: Primary waves , 1969 .

[9]  C. G. Engel,et al.  Composition of Basalt Cored in Mohole Project , 1961 .

[10]  C. G. Knott.  III. Reflexion and refraction of elastic waves, with seismological applications , 1899 .

[11]  R. Whitmarsh.  Seismic refraction studies of the upper igneous crust in the North Atlantic and porosity estimates for Layer 2 , 1978 .

[12]  D. G. Moore,et al.  Deep‐sea tests of a new nonexplosive reflection profiler , 1963 .

[13]  D. C. Krause.  Geology of the sea floor east of Guadalupe Island , 1961 .

[14]  M. Ewing,et al.  SEISMIC-REFRACTION MEASUREMENTS IN THE ATLANTIC OCEAN BASINS, IN THE MEDITERRANEAN SEA, ON THE MID-ATLANTIC RIDGE, AND IN THE NORWEGIAN SEA , 1959 .

[15]  J. Watt,et al.  The Elastic Properties of Composite Materials , 1976 .

[16]  R. Kirkpatrick.  The physical state of the oceanic crust: Results of downhole geophysical logging in the Mid‐Atlantic Ridge at 23°N , 1979 .

[17]  E. E. Horton.  Preliminary Drilling Phase of Mohole Project I. Summary of Drilling Operations , 1961 .

[18]  N. Christensen,et al.  Velocities, elastic moduli and weathering-age relations for pacific layer 2 basalts , 1973 .

[19]  D. Helmberger,et al.  Glorified optics and wave propagation in nonplanar structure , 1978, Bulletin of the Seismological Society of America.

[20]  E. Hamilton,et al.  SHEAR‐WAVE VELOCITY VERSUS DEPTH IN MARINE SEDIMENTS: A REVIEW , 1976 .

[21]  R. Parker,et al.  Travel time inversion: A geometrical approach , 1979 .

[22]  K. Fuchs,et al.  Computation of Synthetic Seismograms with the Reflectivity Method and Comparison with Observations , 1971 .

[23]  C. Fowler.  Crustal structure of the Mid-Atlantic ridge crest at 37"N , 1976 .

[24]  D. Helmberger,et al.  A travel time and amplitude interpretation of a marine refraction profile: Transformed shear waves , 1970 .

[25]  R. Hyndman.  The Physical Properties of Oceanic Basement Rocks From Deep Drilling on the , 1976 .

[26]  H. S. Ladd,et al.  Preliminary Drilling Phase of Mohole Project II. Summary of Coring Operations , 1961 .

[27]  P. Spudich,et al.  Synthetic seismograms from model ocean bottoms , 1979 .

[28]  N. Christensen.  Ophiolites, seismic velocities and oceanic crustal structure , 1978 .

[29]  Kazim Ergin.  Energy ratio of the seismic waves reflected and refracted at a rock-water boundary , 1952 .

[30]  P. Spudich,et al.  Ophiolites found in oceanic crust , 1978 .

[31]  D. C. Krause.  Tectonics, Bathymetry, and Geomagnetism of the Southern Continental Borderland West of Baja California, Mexico , 1965 .

[32]  S. Shtrikman,et al.  A variational approach to the theory of the elastic behaviour of multiphase materials , 1963 .

[33]  D. Helmberger.  The crust-mantle transition in the Bering Sea , 1968 .

[34]  J. Ewing,et al.  Upper crustal structure as a function of plate age , 1976 .

[35]  M. N. Toksoz,et al.  Velocities of seismic waves in porous rocks , 1976 .

[36]  D. Helmberger,et al.  A note on velocity, density, and attenuation models for marine sediments determined from multibounce phases , 1979 .

[37]  T. Jordan,et al.  Estimation of the attenuation operator for multiple ScS waves , 1977 .

[38]  B. Kennett,et al.  A comparison of travel time inversions for marine refraction profiles , 1976 .

[39]  A. R. Gregory,et al.  An experimental investigation of factors affecting elastic wave velocities in porous media , 1958 .

[40]  Gene Simmons,et al.  The effect of saturation on velocity in low porosity rocks , 1969 .

[41]  Bernard Budiansky,et al.  Viscoelastic properties of fluid-saturated cracked solids , 1977 .

[42]  E. Hamilton.  SOUND SPEED AND RELATED PHYSICAL PROPERTIES OF SEDIMENTS FROM EXPERIMENTAL MOHOLE (GUADALUPE SITE) , 1965 .

[43]  B. Lewis,et al.  Converted shear waves as seen by ocean bottom seismometers and surface buoys , 1977, Bulletin of the Seismological Society of America.

[44]  B. Kennett,et al.  Structure of the East Pacific Rise from an Ocean Bottom Seismometer Survey , 1976 .

[45]  W. H. Somerton,et al.  Physical properties of Mohole test site basalt , 1963 .

[46]  G. Simmons,et al.  Effect of pore pressure on the velocity of compressional waves in low‐porosity rocks , 1972 .

[47]  A. Arons,et al.  Underwater Explosion Shock Wave Parameters at Large Distances from the Charge , 1954 .

[48]  D. E. Weston,et al.  Underwater Explosions as Acoustic Sources , 1960 .