Refraction Velocity Research Articles

An Introduction to the generalized reciprocal method of seismic refraction interpretation

The generalized reciprocal method (GRM) is a technique for delineating undulating refractors at any depth from in‐line seismic refraction data consisting of forward and reverse traveltimes. The traveltimes at two geophones, separated by a variable distance XY, are used in refractor velocity analysis and time‐depth calculations. At the optimum XY spacing, the upward traveling segments of the rays to each geophone emerge from near the same point on the refractor. This results in the refractor velocity analysis being the simplest and the time‐depths showing the most detail. In contrast, the conventional reciprocal method which has XY equal to zero is especially prone to produce numerous fictitious refractor velocity changes, as well as producing gross smoothing of irregular refractor topography. The depth conversion factor is relatively insensitive to dip angles up to about 20 degrees, because both forward and reverse data are used. As a result, depth calculations to an undulating refractor are particularly convenient even when the overlying strata have velocity gradients. The GRM provides a means of recognizing and accommodating undetected layers, provided an optimum XY value can be recovered from the traveltime data, the refractor velocity analysis, and/or the time‐depths. The presence of undetected layers can be inferred when the observed optimum XY value differs from the XY value calculated from the computed depth section. The undetected layers can be accommodated by using an average velocity based on the optimum XY value. This average velocity permits accurate depth calculations with commonly encountered velocity contrasts.

The effects of refractor topography and overburden anisotropy on time-term solutions of refractor anisotropy

Summary It is generally held that a non-iterative time-term solution gives a satisfactory model if refractor dips are everywhere less than 10|Mo. Although this is a reasonable assumption for a refractor with a constant dip, for structures with strong directional characteristics such as anticlines and syn-clines the solution can be seriously degraded and will exhibit a refractor velocity that could be erroneously interpreted as anisotropic. By ray tracing through a continental model of an isotropic crust and upper mantle separated by an anticlinal Moho nowhere dipping more than 8|Mo, it is demonstrated that the Pntravel time can be well fitted by a non-iterative time-term solution with a 4.4 per cent upper mantle anisotropy. The effect is smaller for oceanic cases. These studies illustrate the importance of carrying out an iterative sequence of solutions and indicate that convergence at the correct solution will occur when a good azimuthal distribution of observations is available. It is further demonstrated that an anisotropic crust can perturb the travel times in a similar way to an anisotropic upper mantle. The correct separation of crustal and upper mantle effects requires the independent utilization of Pn and crustal phases.

Sedimentary and crustal velocities in the Norwegian‐Greenland Sea

Refraction and wide‐angle reflection velocities from the oceanic crust and the overlying sediments in the Norwegian‐Greenland Sea have been analyzed. There is a relatively rapid increase in velocity with depth in the uppermost sediments, whereas a low constant gradient is representative of the main sediment sequence. Two regions, the northeastern Greenland Sea and the oldest parts of the basins, have particularly high velocity gradients, 0.71 and 0.85 s−1 respectively. The high gradient in the northeastern Greenland Sea is associated with the shear‐rifted margin off Svalbard. Relatively high sea floor sediment sound velocities are typical of the Norwegian‐Greenland Sea. The crustal velocities fit the Houtz‐Ewing model which divides layer 2 into layers 2A, 2B and 2C. Average velocities in the Norwegian‐Greenland Sea are: 3.88(2A), 5.18(2B), 6.05(2C), 6.66(3), and 7.80(4) km/s. Layer 2A is age dependent and has not been observed on crust older than 36 m.y. The model is not representative of the uppermost crust near the active spreading axis where one has to introduce additional layers or assume continuous velocity changes with depth.

Comparison of velocity–depth characteristics in western North Atlantic and Norwegian Sea sediments

The western North Atlantic was divided into 17 sediment provinces, and sonobuoy sound velocities were compiled from each region. The regions were analyzed statistically to develop least-squares regressions of the form V=V0+Kt, where t is one-way vertical travel time. Eight regions provided velocity functions, four were inadequately sampled, and five provided mean values from single-layer solutions in sediment bodies less than 1 km thick. The eight values of K are grouped very closely about a mean of 1.14±0.17 km/s2. By contrast, 12 regions from the Norwegian Sea yield 10 values of K that range from 0.6 to 2.5 km/s2. The three highest values (≳2.1 km/s2) are clearly separable from the rest and were measured on data from the Voring plateau, Jan Mayen ridge, and eastern Greenland margin. These three regions are uniquely floored by thick Mesozoic epicontinental sediments that predate the breakup of the Norwegian Sea. These older sediment bodies account for most of the regional heterogeneity in the velocity functions of the Norwegian Sea. Approximately one fifth the surface area of the North Atlantic is floored by sediments thinner than 200 m. Within these vast areas, principally near the midocean ridges, the sound velocity of acoustic basement is a crucial factor in developing low-frequency propagation models. A contour map of basement refraction velocities in the North Atlantic shows a rough increase from 3.5 km/s near the mid-Atlantic ridge to 5.0 km/s at variable distance from the ridge. The region in which geoacoustic models are most sensitive to basement velocity is between 15 ° and 30 °N, where sediment cover thinner than 200 m account for about half the surface area. A significant portion of this area is topographically rough and does not yield reliable basement refraction data. Interval velocities from the thinner deposits of pelagic sediment seaward of the abyssal plains, are greater by about 200 m/s than those from the continental rise and abyssal plains at comparable depths of overburden. The difference suggests that sound velocities in the predominantly pelagic sediments increase more rapidly with depth than they do in the silty hemipelagic clays and turbidites nearer the continents.

Seismic stratigraphy of the western Venezuela Basin

Multi-channel seismic reflection records from the central Venezuela Basin reveal planar reflections within the upper half of the oceanic crust as well as within the overlying sediment layer. The high resolution of the reflection data allows a division of the sediment section into three seismic intervals bounded by strong laterally persistent amplitude peaks which can be traced throughout much of the basin. The underlying crustal reflection zone is divided into two seismic intervals based on an angular relationship between the reflections which constitute each zone. Deep-sea drilling permits local determination of age and lithology of the seismic intervals of the sediment section and the top of the uppermost crustal interval. The lateral continuity of high-amplitude peaks that separate the sedimentary seismic intervals suggests that the boundaries are for the most part time-synchronous throughout the Venezuela Basin. Thinning and local pinch-outs of the deeper sedimentary seismic intervals in several parts of the basin contrast with the continuity in thickness of the shallower sedimentary seismic intervals. This suggests that the deeper intervals were affected more than shallower intervals by bottom currents which scoured and redistributed sediment from Cretaceous to Eocene time, but which became much weaker in post-Eocene or Early Oligocene time. Perhaps the weakening of bottom currents in Late Eocene or Early Oligocene was related to the development of the Lesser Antilles island arc and the isthmus of Panama in Eocene time. Planar seismic reflections within the high-velocity crust of the Venezuela Basin extend 1.5 sec below the base of the sediment section. These planar reflections extend tens of kilometers laterally. The upper interval of this crustal section consists of fairly level internal reflections compared to the lower interval with more steeply dipping reflections. The boundary between these two seismic intervals is not a distinct reflection, but rather a zone of apparent merging of the two intervals. The reflections of the lowest seismic interval become lost in the noise at a depth approximately equivalent to the top of the region in the lower Venezuela Basin crust with refraction velocities of 7.2 km/sec.

Changes in the crust and upper mantle near the Japan‐Bonin Trench

Depths and reflection times to mantle have been computed in the west Pacific from 60 sonobuoy refraction solutions, many of which could be compared with observed mantle reflection depths from multi‐channel data obtained at the same time. After repicking some of these sonobuoy records, all were eventually adjusted to agree within 0.05 s with the observed mantle reflection times. This added constraint produces solutions that are clearly more reliable. Crustal velocities (exclusive of water and sediment) from the study area are rather tightly distributed about a mean value of 6.53 km/s with a standard deviation of only 0.23 km/s (n = 47). Results show that the crust thickens in a westerly direction from the west Pacific basin, where mantle depths are 11–11.5 km to a belt 200 km east of the Japan trench, coinciding with the outer gravity high, where mantle is at an average depth of 14 km. Several sonobuoys in the zone of maximum crustal thickness just east of the outer slope of the Japan trench record two deep reflectors about 0.6 s apart in the vicinity of the upper mantle. Two values of interval velocity obtained from a reduced T2/X2 analysis of the layer bounded by these reflectors are 7.5 and 7.2 km/s. These sonobuoys and a few others with weaker double reflections are all located within the outer gravity high. To the south a well‐observed mantle reflection and its strong 8.2‐km/s refraction disappear from our records just as the crust begins its descent into the Bonin trench. Within the outer trench slope a 7.3‐km/s refractor, which is a weak arrival elsewhere, becomes the dominant refractor. The peculiar double reflector near mantle and the marked change in velocity structure and upper mantle reflectivity at the edge of the outer slopes seem to confirm the changes in upper mantle refraction velocity reported by Talwani et al. (1977) in the Curacao trench, Caribbean Sea, but our evidence is not conclusive. In spite of the ambiguity it is clear that velocity structures near active trenches cannot be confidently defined by correlating sections with similar velocities.

Crustal structure of the North Atlantic on the basis of large-airgun—sonobuoy data

Owing to improvement of the signal/noise ratio, large airgun arrays consisting of 2 to 4 × 466 in. 3 (7.64 l) guns fired at 12- to 18-s intervals provide more refracted arrivals from the upper crust than can be obtained with a small 25 in. 3 (.41 l) airgun. In particular, sonobuoy data obtained previously with small airguns indicated a 1.5-km-thick layer with a refraction velocity of 3.6 km/s overlying a major refractor with a velocity of about 5.1 km/s near the Mid-Atlantic Ridge. However, large-airgun-sonobuoy results reveal intermediate velocities between the 3.6 and 5.1 km/s refractors and commonly achieve penetration down to the mantle. Large-airgun data have been used to contour acoustic basement refraction velocities in the North Atlantic; these show a clear increase of velocity with age. A contour map of mantle depths is shown for the North Atlantic; it is based on 100 measurements, more than half of which are from high-quality, two-ship refraction data (solutions that include layers 2 and 3). A comparison between mantle depths determined from sonobuoy and two-ship solutions shows that the two-ship solutions have three times the variation of sonobuoys from the same geographical region. Solutions are plotted as velocity-depth points to show variations in the entire crust as a function of age. As the crust ages, the uppermost basement velocity increases and total crustal rock thickness increases rather abruptly on Upper Jurassic crust. These effects combine to decrease the gross crustal velocity gradient (exclusive of water and sediment) from about 1.0 sec −1 near the Mid-Atlantic Ridge to about 0.5 sec −1 near the continental margins.

Crustal parameters estimated fromP-waves of earthquakes recorded at a small array

TheP-arrival times of local and regional earthquakes that are outside of a small network of seismometers can be used to interpret crustal parameters beneath the network by employing the time-term technique. Even when the estimate of the refractor velocity is poorly determined, useful estimates of the station time-terms can be made. The method is applied to a 20 km diameter network of eight seismic stations which was operated near Castaic, California, during the winter of 1972–73. The stations were located in sedimentary basins. Beneath the network, the sedimentary rocks of the basins are known to range from 1 to more than 4 km in thickness. Relative time-terms are estimated fromP-waves assumed to be propagated by a refractor in the mid-crust, and again fromP-waves propagated by a refractor in the upper basement. For the range of velocities reported by others, the two sets of time-terms are very similar. They suggest that both refractors dip to the southwest, and the geology also indicates that the basement dips in this direction. In addition, theP-wave velocity estimated for the refractor of mid-crustal depths, roughly 6.7 km/sec, agrees with values reported by others. Thus, even in this region of complicated geologic structure, the method appears to give realistic results.

REFRACTOR VELOCITY DETERMINATION—CAUSE AND NATURE OF SOME ERRORS*

AbstractFor detailed determination of refractor velocities of in‐line reversed profiles, two principally different systems have been employed, the ABC method and Hales's method. The two systems differ in the travel‐path arrangements used. The more conventional approach to the problem, the ABC method, makes use of critically refracted rays converging on a common surface position, while Hales's method deals with the common position on the refractor surface from which critically refracted rays diverge towards the ground surface.Because of the travel‐path system used, Hales's method has proved to be particularly applicable to high‐relief structures and to cases where the refractor lies at considerable depth. Some of the ambiguities in more commonly used interpretation techniques can be solved by Hales's method.Some attention has also been paid to errors caused by non‐critical refractions and to the diffraction problem.

Compressional velocities from multichannel refraction arrivals on Georges Bank—northwest Atlantic Ocean

Velocities were obtained from unreversed, refracted arrivals on analog records from a 48‐channel, 3.6-km hydrophone cable (3.89 km from the airgun array to the last hydrophone array). Approximately 200 records were analyzed along 1500 km of ship track on Georges Bank, northwest Atlantic Ocean, to obtain regional sediment velocity distribution to a depth of 1.4 km below sea level. This technique provides nearly continuous coverage of refraction velocities and vertical velocity gradients. Because of the length of the hydrophone cable and the vertical velocity gradients, the technique is applicable only to the Continental Shelf and the shallower parts of the Continental Slope in water depths less than 300 m. Sediment diagenesis, the influence of overburden pressure on compaction, lithology, density, and porosity are inferred from these data. Velocities of the sediment near the water‐sediment interface range from less than 1500 m/sec on the north edge of Georges Bank to 1830 m/sec for glacial deposits in the northcentral part of the bank. Velocity gradients in the upper 400 m range from [Formula: see text] on the south edge of the bank to [Formula: see text] on the north. Minimum gradients of [Formula: see text] were observed south of Nantucket Island. Velocities and velocity gradients are explained in relation to physical properties of the Cretaceous, Tertiary, and Pleistocene sediments. Isovelocity contours at 100-m/sec intervals are nearly horizontal in the upper 400 m. Isovelocity contours at greater depths show a greater difference from a mean depth because of the greater structural and lithological variation. Bottom densities inferred from the velocities range from 1.7 to [Formula: see text] and porosities range from 48 to 62 percent. The most significant factor controlling velocity distribution on Georges Bank is overburden pressure and resulting compaction. From the velocity data we conclude that Georges Bank has been partially overridden by a continental ice sheet.

Kerguelen Plateau bathymetry, sediment distribution and crustal structure

The Kerguelen Plateau is a large topographic high situated in the south-central Indian Ocean, adjacent to Antarctica. The plateau rises some 2000 fathoms (∼ 3700 m) above the adjacent deep seafloor and, along its NW-SE-trending axis, exceeds 2000 km in length. The rugged northern part of the Kerguelen Plateau is dominated by NW-SE-trending normal faults and associated sediment-filled troughs and is believed to be young relative to the rest of the plateau. The central and southern parts lie in deeper water and are topographically subdued; fault trends in the central region are N-S, in the southern they are NW-SE, identical to the northern region. Sediments of Cenomanian age were sampled by a piston core taken on the eastern flank of the central region; we judge that the core penetrated the acoustic basement. Seismic profiles and piston cores from the central and southern parts of the plateau reveal a prominent sub-bottom reflector of Eocene age. It appears that significant areas of the plateau were peneplaned during the Eocene; this event was followed by general subsidence and normal faulting. Beneath the acoustic basement in the central plateau, sonobuoy measurements reveal a layer up to 1 km thick with a refraction velocity of 3.7 km/sec, underlain by material with an average velocity of 4.9 km/sec, presumably a crystalline basement. The 4.9 km/sec material must be considerably older than Cenomanian. No significant structural connection between Antarctica and the Kerguelen Plateau was observed. A sediment isopach map of the plateau and the deep ocean area to the east has been compiled. A sediment ridge paralleling the eastern margin of the plateau in abyssal depths appears to be the result of deposition by a westward-flowing bottom current that has been deflected northward by the plateau. Gravity data indicate a crust about 20–23 km thick beneath the plateau. We interpret this intermediate crustal thickness, the short-wavelength large-amplitude magnetic anomalies observed over the plateau, and other evidence discussed in the text, as evidence that the plateau is probably oceanic in origin. Anomalous depths to the top of the oceanic crust adjacent to the northeastern edge of the plateau and the plateau elevation itself are interpreted to result from a mantle hot spot, now centered beneath the active volcanoes in the northern part of the plateau. We suggest that the composite Kerguelen Plateau is an uplifted remnant of the Cretaceous ocean basin that existed to the west of Australia following the separation of India from Western Australia-Antarctica. Furthermore, it is speculated that the time transgressive eastern margin of the plateau has been created by a modest northerly drift of the locus of volcanic activity relative to the Antarctic plate.

The physical properties of oceanic basement rocks from deep drilling on the Mid-Atlantic Ridge

The physical properties of compressional and shear velocity, electrical resistivity, bulk and grain density, porosity and water content, and thermal conductivity have been measured on a large collection of basalt samples and a few gabbros and serpentinized peridotites recovered by drilling on the mid-Atlantic ridge near 37°N. Samples were recovered to 582 m into basaltic basement in one hole, and rocks that may be representative of the lower crust were recovered from another hole. The mean compressional velocity of the basalts of 5.94 km s−1 agrees well with previous fresh unweathered sea floor basalts. It, however, can be reconciled with the much lower average upper crustal layer 2 seismic refraction velocities generally observed only by the presence of extensive large-scale fracturing and voids in the crust. The laboratory velocities of the gabbros are consistent with upper layer 3, or oceanic layer, refraction velocities. Lower layer 3 refraction velocities are consistent with a composition of a mix of gabbro and unserpentinized peridotite. Poisson's ratio for little-weathered laboratory samples of basalt is found to decrease systematically with decreasing velocity. The mean electrical resistivity of the basalts is 220 ohm m at 25°C. A close dependence of resistivity on porosity and a moderate increase in resistivity with pressure suggest conduction primarily through pore fluid. The basalt resistivity decreases rapidly with increasing temperature, the mean becoming about 20 ohm m at 100°C and about 5 ohm m at 150°C. The mean basalt bulk density is 2.795 g cm−3, the mean porosity is 7.8%, and the mean thermal conductivity at 21°C is 3.97 mcal cm−1 s−1 °C−1.

Upper crustal structure as a function of plate age

Each Lamont-Doherty sonobuoy located on well-dated crust has been carefully analyzed to determine crustal structure down to oceanic layer 3. Results from the Atlantic and the Pacific are compiled separately in order to study crustal structure as a function of plate age in both oceans, since they have very different spreading rates. Layer 2A (refraction velocity about 3.6 km/s) in the North Atlantic is 1.5 km thick at the ridge crest and thins consistently to about 100 m as the crust ages to about 60 m.y. Layer 2A in the east Pacific is 0.7 km thick at the ridge and thins to about 100 m at about 30 m.y. This difference in thickness is probably attributable to the much faster spreading rate in the Pacific. A poorly refractive acoustic basement layer about 200 m thick with similarities to layer 2A but not necessarily composed of the same materials is measured sporadically in the Pacific M-Series plates and even less consistently in the Atlantic. This layer is not recorded in the Cretaceous or the Jurassic quiet zones. Regressions of refraction velocities in layer 2A as a function of age show that its velocity increases from about 3.3 km/s at the ridge crests to that of layer 2B on crust about 40 m.y. old. There is no corresponding increase of velocity with age in any of the deeper layers. The high resolution of the air gun/sonobuoy records shows that the layer 2B refraction line breaks directly to layer 3 velocities (46 times in the present work) or to a line with a velocity of 6.1 km/s (114 times in the present work), which we call layer 2C. The variance of the velocities in 2C is one fourth that of 2A and 2B, which indicates relative lithological uniformity in 2C. It does not seem likely that layer 2A really thins; what appears to be a thinning of the layer may actually be the result of an increase of its refraction velocity with age. However, the 2A/2B interface is well-defined by large amplitude refractions from 2B on crust that is younger than about 30 m.y., which seems to rule out a transitional zone at the base of layer 2A where it ‘converts’ to 2B. The seismic observations seem to require a diagenetic process or repeated basaltic intrusions; both processes raise serious objections.

Discussion

The President thanked Dr Brooks for this further contribution to data bearing on the structure of the Bristol Channel basin. He expressed surprise at the similarity of energy propagation velocities quoted for the basal Lias (limestones with at least equal parts of shale) and for the Carboniferous Limestone, presumed to be developed as a massive carbonate, and asked if there was some local explanation. Dr Brooks replied that although the lower part of the lower Lias of the Vale of Glamorgan contains about equal parts of limestone and shale, there are sequences (notably in the bucklandi Zone) in which the proportion of limestone approaches 80% of the total thickness and within which individual massive limestone bands may be up to a few metres thick. Recent land refraction investigations west of Aberthaw produced a lower Lias velocity of 4.05 ± 0.15 km/s (provisional value). This value lies close to assumed Carboniferous Limestone velocities encountered in the present surveys and draws attention to the problem of assessing the geological significance of refractor velocities in the range from 4.00 km/s to 4.50 km/s. D. R. Tappin commented on I.G.S. Borehole 72/60 (Brooks & James fig. 5) from which a Tremadocian age has been obtained. The core is 2.5 m in length and is vertically cleaved. It was recovered in a shattered condition beneath 5 m of sand and gravel and 3 m of stiff, brown, stoneless clay. The rock is dark grey, thinly laminated and highly cleaved. Microscopic examination (K. S. Siddiqui personal

Structure and constitution of the lower oceanic crust

Any petrologic model of the lower oceanic crust must be consistent with three sets of data: (1) the seismic structure of the oceanic crust, (2) the petrology of oceanic dredge samples, and (3) laboratory measurements of seismic velocity through such samples. A review of these data indicates that within the framework of earlier three‐layer models of oceanic seismic structure the crust is internally complex and varies markedly with age, azimuth, and tectonic province. Mantle compressional wave velocities Vp are anomalously low under the ridge (7.2–7.7 km/s) but increase to 8.0–8.3 km/s beyond 15 m.y.; layer 3 thickens by 2 km within 40 m.y. of formation and decreases in Vp from 6.8 to 6.5 km/s within 80 m.y.; both the mantle and layer 3 are statistically anisotropic. Dredge lithologies consist predominantly of serpentinized ultramafics and mafic igneous rocks ranging from basalt to gabbro, the gabbro often showing evidence of fractionation. Metamorphism of mafic rocks from zeolite to amphibolite facies grade is common. Velocities in oceanic serpentinites and basalts are generally lower than layer 3 refraction velocities. Unaltered gabbros have compressional wave velocities of approximately 7.0 km/s, which is high for layer 3, together with shear wave velocities Vs of 3.8 km/s and values of Poisson's ratio σ of 0.30. Metabasites containing hornblende and plagioclase have values of Vp = 6.8 km/s, Vs = 3.8 km/s, and σ = 0.28, in good agreement with those of layer 3. On the basis of petrology and velocity it is suggested that layer 3 is composed of hornblende metagabbro underlain by normal gabbro. In a model consistent with geophysical observations of heat flow, seismicity, gravity, and seismic structure at the ridge it is proposed that layer 2 and the upper levels of layer 3 form near the median valley but that deeper levels of layer 3 thicken for 40 m.y. by intermittent offridge intrusion fed from the underlying anomalous mantle. Ophiolites in such a model represent segments of thin immature ridge crest obducted onto continental margins during subduction of a spreading ridge.

Profiler-Sonobuoy Measurements in Colombia and Venezuela Basins, Caribbean Sea

Interval velocities, refraction velocities, and layer depths from 38 airgun-sonobuoy profiles made along a west-east traverse of the Colombia and Venezuela basins at about 15°N are presented in a schematic structure section, which includes details from single-channel reflection-profiler records converted to depths. The seismic data are correlated with earlier reflection and refraction surveys and with the results of JOIDES drilling. Two prominent subbottom reflectors, Horizon A^Prime and Horizon B^Prime, are traced across each basin. Interval and refraction velocities in the material beneath B^Prime in the Venezuela basin range between 3.5 and 5.7 km/sec, but most are in the range 4.5 to 5.3 km/sec. These values are in close agreement with published laboratory measurements of sonic velocities made on DSDP cores of basalts from Horizon B^Prime. It is therefore unlikely that the material below B^Prime can contain large amounts of sediment unless the sediment has a velocity near that of basalt. The discontinuous reflectors below B^Prime are thought to be thin layers of interbedded ash or sediment. It is argued that the Venezuela basin has normal oceanic that has been depressed about 3 km by the addition of flood basalts, and that the velocity structure of typical Caribbean crust (6.2 on 7.3 km/sec material) does not exist in the Colombia and Venezuela basins, except as arbitrary divisions in a scattering of crustal and basement velocities that range between 4.0 and 7.5 km/sec, as they do in other ocean basins. It is suggested that the upper interface of primordial basement coincides with the deepest reflector below B^Prime. Therefore, it is likely that the Late Cretaceous basalts and dolerites drilled at the depth of Horizon B^Prime do not represent primordial crust, but reflect the last major igneous event in the history of development of the Caribbean Sea.

Velocities, elastic moduli and weathering-age relations for pacific layer 2 basalts

Compressional (V p) and shear (V s) wave velocities have been measured to 10 kb in 32 cores of basalt from 14 Pacific sites of the Deep Sea Drilling Project. Both V pandV s show wide ranges (3.70to6.38km/sec forV pand1.77to3.40km/sec forV sat0.5kb) which are linearly related to density and sea floor age, confirming earlier findings by Christensen and Salisbury of decreasing velocity with progressive submarine weathering based on studies of basalts from five sites in the Atlantic. Combined Pacific and Atlantic data give rates of decreasing velocity of −1.89and−1.35km/sec per100my forV pandV s respectively. New analyses of oceanic seismic refraction data indicate a decrease in layer 2 velocities with age similar to that observed in the laboratory, suggesting that weathering penetrates to several hundred meters in many regions and is largely responsible for the extreme range and variability of layer 2 refraction velocities.

Progressive Weathering of Submarine Basalt with Age: Further Evidence of Sea-Floor Spreading

Densities of layer 2 basalt recovered during the Deep Sea Drilling Project have been found to decrease steadily with age, a finding ascribed to progressive submarine weathering in the context of sea-floor spreading. The least-squares solution for 52 density measurements gives a rate of decrease in density of Δρ/Δ t = −0.46 × 10 −2 g per cm 3 m.y. = −16 percent per 100 m.y., which is in excellent agreement with earlier estimates based on observed chemical depletion rates of dredged oceanic basalt. Weathering of sea-floor basalt, should it penetrate to any considerable depth in layer 2, will decrease layer 2 seismic refraction velocities, act as a source of geothermal heat, and substantially influence the chemistry of sea water and the overlying column of sediment.

Seismic-wave velocities in anisotropic ice: A comparison of measured and calculated values in and around the deep drill hole at Byrd Station, Antarctica

Ultrasonic (28 kHz) velocity logging was carried out to a depth of 1550 meters in the 2164-meter drill hole at Byrd station, Antarctica, in 1969–1970. Smoothed traveltimes from the logger chart were calibrated by taking spot measurements with a calibrated oscilloscope. As a result, velocities are accurate within about ±10 m/sec throughout most of the log. Velocities at depths of <400 meters show an excellent correlation with density. At all depths the correlation with velocities calculated from fabric diagrams is also good, although the calculated velocities show substantially more scatter than those observed. An explanation for a previously unexplained shear-wave arrival observed in refraction shooting has been found. Extrapolating velocities to the bottom of the ice on the basis of fabrics leads to a mean vertical velocity below the compaction zone of 3915 m/sec, which is in satisfactory agreement with estimates from wide-angle reflection shooting at the surface but is about 2½% higher than those commonly used in calculating ice thickness from reflection shooting. A very slow downward increase in the velocity calculated for horizontally propagating waves (0.02 sec−1) has been found to be consistent with both the effect of pressure on the solid ice and the results of detailed examination of refraction velocities. The velocities to a depth of 1000 meters appear to be very closely similar at two sites about 20 km apart near old and new Byrd stations.

A NONLINEAR LEAST‐SQUARES METHOD FOR SEISMIC REFRACTION MAPPING—PART I: ALGORITHM AND PROCEDURE

An iterative inversion method (Reframap) based on the kinematic properties of critically refracted waves is developed. The method is based on ray tracing and assumes homogeneous and isotropic media and ray paths confined to a vertical plane through each source‐detector pair. Unlike the earlier Profile or Time‐Term Methods, no restrictions are imposed on interface topography except that it be continuous almost everywhere (in the mathematical sense). As in the preexisting methods, more observations than unknowns are assumed. The algorithm and procedure, on which the Reframap Method is based, generate apparent dips for each source detector pair at the noncritical interfaces from the slope of a least‐squares line approximation to the interface functional in the neighborhood of each refraction point. In turn, the dip and path along the critical refractor is, at every iteration, pairwise approximated by a line through the critical refracting points. The incidence angles are computed recursively by Snell’s law. The solution of the overdetermined, nonlinear multiple refractor time‐distance system of simultaneous equations is sought by Marquardt’s algorithm for least‐squares estimation of critical refractor velocity and vertical thickness under each element.