Pre-drilling reflection survey of the Black Forest, SW Germany

Summary. The unified seismic exploration program, consisting of 345 km of deep reflection profiling, a 200 km refraction profile, an expanding spread profile and near-surface high resolution reflection meaasurements, revealed a strongly differentiated crust beneath the Black Forest. The highly reflective lower crust contains numerous horizontal and dipping reflectors at depths of 13-14 km down to the crust-mantle boundary (Moho). The Moho appears as a flat horizontal first order discontinuity at a relatively shallow level of 25–27 km above a transparent upper mantle. From modelling of synthetic near-vertical and wide-angle seismograms using the reflectivity method the lower crust is supposed to be composed of laminae with an average thickness of about 100 m and velocity differences of greater than 10% increasing from top to bottom. The upper crust is characterised by mostly dipping reflectors, associated with bivergent underthrusting and accretion tectonics of Variscan age and with extensional faults of Mesozoic age. A bright spot at 9.5 km depth is characterised by low velocity material suggesting a fluid trap. It appears on all of the three profiles in the centre of the intersection region. The upper crust seems to be decoupled from the lowest crust by a relatively transparent zone which is' also identified as a low-velocity zone. This low velocity channel is situated directly above the laminated lower crust. The laminae in the Rhinegraben area are displaced vertically to greater depths indicating an origin before Tertiary rift formation and a subsidence of the whole graben wedge.     

Seismic reflection measurements behind the Hikurangi convergent margin, southern North Island, New Zealand.

Summary. The active Australian-Pacific plate boundary passes through New Zealand. In the north, the Pacific plate subducts beneath the Australian plate with an accretionary wedge forming the eastern continental (Hikurangi) margin of the North Island. The structure of the region behind the Hikurangi margin changes from the extensional back-arc basin under central North Island to a postulated crustal downwarp under the southern North Island. A 100 km long multichannel seismic reflection profile was recorded across the region of crustal downwarp. The data show discontinuous coherent reflectors dipping westwards at the east end of the profile, and east dipping reflectors at the west end, from depths of 9 to 15 s two way time. Simple hand migration of these events indicate that the east dipping reflectors, interpreted as the base of the Australian plate crust, abut against the west dipping reflectors which are interpreted as marking the top of the subducted Pacific plate. Detailed earthquake hypocentre locations in the area show a dipping zone of high seismicity, the top of which coincides closely with the west dipping events, thus supporting this interpretation.     

The Hatton Bank continental margin—I. Shallow structure from two-ship expanding spread seismic profiles

Summary. The Hatton Bank passive continental margin exhibits thick seaward dipping reflector sequences which consist of basalts extruded during rifting between Greenland and Rockall Plateau. Multichannel seismic reflection profiling across the margin reveals three reflector wedges with a maximum thickness near 7 km, extending from beneath the upper continental slope to the deep ocean basin. We present results of the velocity structure within the dipping reflector sequences at eight locations across the margin, interpreted by synthetic seismogram modelling a set of multichannel expanding spread profiles parallel to the margin. At the top of some reflector sequences, we observe a series of 100 m thick high- and low-velocity zones, which are interpreted as basalt flows alternating with sediments or weathered and rubble layers. At the profile locations, the base of the dipping reflectors correlates with P -wave velocities near 6.5 km s⁻¹. However, elsewhere the reflectors appear to extend significantly deeper than the inferred 6.5 km s⁻¹ velocity contour, indicating that the velocity structure may not be controlled solely by lithological boundaries but also by metamorphic effects. Shear-waves were observed on two lines, permitting the calculation of Poisson's ratio. The decrease in Poisson's ratio from 0.28 to near 0.25 in the upper 5 km of crust may also indicate the effect of metamorphism on seismic properties, or alternatively may be explained by crack closure under load.     

Wave Propagation In A Vertical Transversely Isotropic Medium: Field Experiment and Model Study

Summary. The continent-ocean transition adjacent to Hatton Bank was studied using a dense grid of single-ship and two-ship multichannel seismic profiles. The interpretation of the explosive expanding spread profiles (ESPs) which were shot as part of this survey are discussed here in detail. Extensive seaward dipping reflectors are developed in the upper crust across the entire margin. These seaward dipping reflectors continue northwards on the Faeroes and Vøring margins, where they have been shown to be caused by basaltic lavas, as well as on the conjugate margin of East Greenland. The dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension. The ESPs show clear seismic arrivals out to ranges of 100 km. Wide-angle Moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals. The determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms. The results from the ESPs show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7.3–7.4 km s⁻¹. This lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the Moho. The lower crustal lens of high-velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas, now seen as dipping reflectors on the margin.     

Joint traveltime inversion of wide-angle seismic data and a deep reflection profile from the central North Sea

We report results from the Seismic Wide-Angle and Broadband Survey carried out over the Mid North Sea High. This paper focuses on integrating the information from a conventional deep multichannel reflection profile and a coincident wide-angle profile obtained by recording the same shots on a set of ocean bottom hydrophones (OBH). To achieve this integration, a new traveltime inversion scheme was developed (reported elsewhere) that was used to invert traveltime information from both the wide-angle OBH records and the reflection profile simultaneously. Results from the inversion were evaluated by producing synthetic seismograms from the final inversion model and comparing them with the observed wide-angle data, and an excellent match was obtained. It was possible to fine-tune velocities in less well-resolved parts of the model by considering the critical distance for the Moho reflection. The seismic velocity model was checked for compatibility with the gravity field, and used to migrate and depth-convert the reflection profile. The unreflective upper crust is characterized by a high velocity gradient, whilst the highly reflective lower crust is associated with a low velocity gradient. At the base of the crust there are several subhorizontal reflectors, a few kilometres apart in depth, and correlatable laterally for several tens of kilometres. These reflectors are interpreted as representing a strike section through northward-dipping reflectors at the base of the crust, identified on orthogonal profiles by Freeman et al. (1988) as being slivers of subducted and imbricated oceanic crust, relics of the mid-Palaeozoic Iapetus Ocean.     

A deep seismic profile of 800 km length recorded in southern Queensland, Australia

Summary. In 1984, the Australian Bureau of Mineral Resources and the Geological Survey of Queensland recorded a regional seismic reflection profile of over 800 km length from the eastern part of the Eromanga Basin to the Beenleigh Block east of the Clarence Moreton Basin. A relatively transparent upper crustal basement with an underlying, more reflective lower crust is characteristic of much of the region. Prominent westerly dipping reflectors occur well below the sediments of the eastern margin of the Clarence Moreton Basin and the adjacent Beenleigh Block, and provide some of the most interesting features of the entire survey. A wide angle reflection/refraction survey of 192 km length and an expanding reflection spread of 25 km length were recorded across the Nebine Ridge. The only clear deep reflectors are interpreted as P-to-SV or SV-to-P converted reflections from a mid-crustal boundary at a depth of about 17 km. The combined Nebine Ridge data provide well-constrained P and S wave velocity models of the upper crust, and suggest a crustal structure quite different from that beneath the adjacent Mesozoic basins.     

Upper mantle structure beneath the Mid-Atlantic Ridge north of the Azores based on observations of compressional wave

Summary. Four seismic refraction profiles have been interpreted which serve to indicate the structure of the lithosphere near the Mid-Atlantic Ridge close to the Azores. An east–west profile which crosses the ridge axis yields a crustal structure. Although energy is propagated across the ridge axis within the crust the axial region marks a clear barrier to propagation within the mantle. A profile parallel to the axis (4 my isochron) shows, below a 7.6 km/s layer, a low-velocity zone underlain by an 8.3 km/s refractor 9 km below the sea bed. On profies normal to the ridge axis higher velocities, which are observed on lines shot towards the ridge, can be attributed to this refractor if it has a dip of several degrees away from the ridge. On another profile parallel to the axis (9 my isochron) a velocity of about 8.3 km/s is only found to exist much deeper at about 30 km depth. These observations are interpreted in the light of seismic refraction results recently obtained by Lewis & Snydsman and of quantitative petrological models, such as that of Bottinga & Allègre. A velocity model based on Bottinga & Allègre's model allows us to understand our results qualitatively. In particular the two 8.3 km/s refractors at 9 and 30 km depth correspond to two different residual peridotite layers. The upper layer contains 1.5–2 wt per cent water and as the lithosphere moves away from the ridge axis the temperature in this layer becomes low enough to start hydration reactions. These cause the low-velocity zone observed at 4 my and the total disappearance of the shallow level refractor before 9 my.     

博鳌地区沙坝-泻湖沉积及探地雷达的应用

通过钻孔资料和探地雷达（GPR）反射剖面，揭示了海南岛东部博螯地区晚更新世以来沙坝-泻湖体系沉积序列和内部反射结构，探地雷达（GPR）给出了有关沙体厚度，岩性特征，潜水面深度，海水侵入范围以及沙体含水性等重要参数，是海岸带环境调查的有效手段。通过和钻孔资料的对比，在玉带滩海岸沙坝上部层序中识别出两套具有区域意义的反射波组，南岗村沙洲浅滩识别出一套反射波组，综合分析表明，博螯地区沙坝-泻湖体系晚更新世以来随海平面的上升经历了基岩侵蚀→河流，河漫滩相→泻湖→海岸沙坝，半封闭泻湖，河口湾的演化过程。     

Crustal structure beneath exposed accreted terranes of Southern Alaska

Summary. The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8–9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors.     

Geochemical characterization of loess-paleosol sequences: Comparison between the upper reaches of the Hanjiang and Weihe river valleys,China

This paper aims to compare the geochemical characteristics of loess-paleosol sequences in the upper reaches of the Hanjiang and Weihe river valleyswhich are located in the semi-humid temperate zone and humid subtropical zonerespectively. The Mituosi(MTS) profile in the upper reaches of the Hanjiang River valley and the Yaohecun(YHC) profile in the Weihe River valley were selected for this comparative research. The stratigraphic characteristicscompositionchemical weathering intensityleaching rates of Ca and Namobility of major elementsand transport features of Na and Fe were analyzed with respect to depth and compared between the two profiles. This study reached the following conclusions.(1) The composition of the loess-paleosol sequences in two regions are quite similar to the average composition of the upper continental crust(UCC)indicating that the loess in the two regions came from multiple sources and was mixed well. Thereforethe loess in the two regions is considered aeolian loess.(2) Compared with the loess-paleosol sequence in the Weihe River valleythe loess-paleosol sequence in the upper reaches of the Hanjiang River valley features a darker color; a higher chemical index of alteration(CIA) value; higher leaching rates of Na and Ca; higher migration ratio(relative to K) of AlSiMgand Na; and lower migration ratio of Fe and Ca. This evidence indicates that the loess-paleosol sequence in the humid subtropical environment experienced stronger chemical weathering intensity than the loess-paleosol sequence in the semi-humid temperate zone.(3) Both the YHC profile and MTS profile record a period of climate deterioration at 6000–5000 a BP. The period punctuated the mid-Holocene Climatic Optimum(8500–3100 a BP) in the study area.     

The crustal structure beneath the northwest fjords, Iceland, from receiver functions and surface waves

Five broad-band seismic stations were operated in the northwest fjords area of Iceland from 1996 to 1998 as part of the Iceland Hotspot project. The structures of the upper 35  km or so beneath these stations were determined by the modelling and joint inversion of receiver functions and regional surface wave phase velocities. More than 40 teleseismic events and a few regional events containing high-quality surface wave trains were used. Although the middle period passband of the seismograms is corrupted by oceanic microseismic noise, which hinders the interpretation of structural details, the inversions reveal the overall features. Many profiles obtained exhibit large velocity gradients in the upper 5  km or so, smaller zero gradients below this, and, at ~23  km depth, a zone 2–4  km thick with higher velocity gradients. The two shallower intervals are fairly consistent with the 'upper' and 'lower' crust, defined by Flovenz (1980 ). The deep zone of enhanced velocity gradient seems to correspond to the sharp reflector first reported by Bjarnason et al . (1993 ) and identified by them as the 'Moho'. However, this type of structure is not ubiquitous beneath the northwest fjords area. The distinctiveness of the three intervals is variable, and in some cases a structure with velocity gradient increasing smoothly with depth is observed. We term these two end-members structures of the first and second types respectively. Structures of the second type correlate with older areas. Substantial variation in fundamental structure is to be expected in Iceland because of the great geological heterogeneity there.     

Modelling and inversion of single-ended refraction data from the shot gathers of multifold deep seismic reflection profiling—an approach for deriving the shallow velocity structure

A multifold crustal-scale deep seismic near-vertical reflection profile generates a large number of single-ended shot gathers, which provide redundant data sets because of overlapping coverage of the shallow refractors. We present an approach for deriving the shallow velocity structure by modelling and inversion of single-ended seismic refraction first arrival traveltime data. We apply this method to a data set acquired with a 12-km long spread with 100 m spacing of shots and receivers, of the Neoproterozoic Marwar basin in the NW Indian shield. The approach is shown to be quite successful for delineating the shallow refractor depths, steep dips and velocities, even in the absence of regular reverse refraction profiles. The study reveals two-layered sedimentary formations, Malani volcanics and a complicated basement configuration of the Marwar basin, and provides a measure of resolution and uncertainty of the estimated model parameters. A seismic section of the near-trace gather is found to be qualitatively consistent with the derived structural features of the basin. The relative highs and lows, observed in the Bouguer gravity profile, further corroborate the derived velocity model. The present approach can be especially useful in offshore areas and elsewhere, where the single-ended multifold seismic profiles are the only available data sets.     

The Central Australian seismic experiment, 1985: preliminary results

Summary. The major objective of the Central Australian seismic experiment is to investigate the structural evolution of the Arunta Block and the Ngalia and Amadeus Basins. A regional north-south reflection line of 420 km length from the Northern Arunta Province to the southern part of the Amadeus Basin was recorded in 1985. The most significant basement features are prominent bands of reflectors from beneath the Northern Arunta Province and the Ngalia Basin at times of between 4 and 10 s that dip towards the north. Deep crustal features south of the Ngalia Basin are less clear except in the Redbank Zone. Bands of deep reflectors similar to those observed in the north occur at times of between 5 and 10 s beneath the southern part of the Amadeus Basin. Additional seismic profiling included a reflection line of 40 km length recorded across the northern margin of the Redbank Zone, three expanding spread reflection profiles and a tomographic experiment. An east-west seismic refraction profile of 400 km length was recorded within the Arunta Block, and suggests an average crustal thickness of 55 km.     

Deep crustal geoelectric structure beneath the Northumberland Basin

Summary. In terms of lateral variations in conductivity structure, the southern Southern Uplands and Northumberland Basin are characterized by a region of attenuated vertical magnetic fields with small spatial gradients reflecting the presence of a substantial conducting zone. Five magnetotelluric data sets from the region have been analysed to provide accurate and unbiased estimates of the impedance tensor. The response data are used to investigate the deep geoelectric crustal structure of the region. Three appropriate sets of response data have been subjected to two construction algorithms for 1-D inversion. The geoelectric profiles recovered identify a deep crustal conducting zone underlying the Northumberland Basin. The zone, modelled as a layered structure, dips steeply from mid-crustal depths underneath the Northumberland Basin to lower crustal depths to the NW. The structure thus correlates, in location and geometry, with a deep crustal reflecting wedge detected offshore by a deep seismic reflection profile.     

Preliminary deep reflection studies in the Arunta Block, Central Australia

Summary. In 1985, near-vertical incidence reflection profiling was carried out across the Arunta Block in Central Australia. This region consists of exposed Proterozoic metasediments, granites and granulites. There is usually a limited sedimentary coverage generated by deep weathering. The seismic sections for the deep crust are markedly different from those previously recorded in Eastern Australia where there is extensive sedimentary cover. One of the striking features is the presence of energy with frequencies as high as 100 Hz at two-way times of 5-6 s. Reflections are found throughout the crust, and there is no zone that can be characterised as non-reflective. The strongest reflectors commonly lie in the intervals around 4-6 s and 8–11 s and display significant dip. Individual shot records show fairly rapid variations in amplitude and waveform within a reflection band and the correlation between records from adjacent shots can also be somewhat limited. Such features are not well suited to the application of standard processing techniques designed for subhorizontal structures, and call into question the utility of conventional stacking. The character of the reflections changes markedly with varying frequency which suggests that they arise by interference phenomena, probably associated with laterally varying lamellar structures.     

Reflectivity of the crystalline crust: hypotheses and tests

Summary. The nature of reflectors within the crystalline basement remains the subject of inference except where reflections have been traced directly to outcrop. Geological models of basement reflectors need to be developed which incorporate geophysical constraints obtained from measurements on seismograms, but most geological information still comes from speculative interpretations of seismic experiments run in different regimes. Pronounced lower-crustal reflectivity, detected worldwide, is ascribed in various geological hypotheses to primary lithologic layering, to ductile strain banding, or to trapped fluids. A BIRPS deep crustal profile across the Atlantic continental margin suggests that the observed reflectivity is not related in any simple way to the amount of extensional strain undergone. Study of worldwide crustal profiles shows that exposed high-pressure terranes are not as reflective as in situ lower crust at high pressure, suggesting either that these granulite terranes are not representative of the lower crust or that physical properties, possibly the presence of fluids or thermally controlled ductile strain banding, are more likely responsible for observed reflectivity than are simple lithologic boundaries. The argument for the importance of physical properties in causing observed lower-crustal reflectivity is strengthened by an observed negative correlation between depth to the reflective lower crust and regional surface heat-flow.     

The PUMA experiment west of Lewis, U.K.

Summary. In August-September 1984 a 165 km long, reversed, wide-angle, seismic line was shot along part of the BIRPS WINCH deep seismic reflection profile west of Lewis in the Outer Hebrides. The experiment was recorded using a new type of sea-bottom receiver, the PUMA (Pull-Up Multichannel Array). This consists of an 1100 m long array of hydrophones connected to a sea-bottom recording package. Airgun and explosive shots recorded by the PUMA provide a densely sampled record section, allowing even low amplitude arrivals to be traced across the section. An initial, 1-D interpretation of the data reveals a crustal thickness of 27 km, and confirms that a band of reflectors seen at around 8.6 s TWT on the normal incidence data, and interpreted as the 'reflection Monol, coincide with the Moho determined by wide-angle reflections and refractions.     

Crustal structure from an OBS survey of the Nootka fault zone off western Canada

Summary. The Nootka fault zone is the boundary between the small Explorer and Juan de Fuca plates which are situated between the America and Pacific plates off western Canada. To investigate the crustal structure in the region, three explosive/large airgun refraction lines were shot into three ocean bottom seismometers (OBSs) with three-component geophone assemblies. In this phase of the study, P -wave velocity—depth models are interpreted by comparison of the travel time and amplitude characteristics of the observed data with theoretical seismograms computed using a WKBJ algorithm. The interpretation gives relatively consistent results for the upper crust. However, the structure of the lower crust is significantly different among the various profiles. Upper mantle velocities range from 7.5 to 8.3 kms⁻¹ and the sub-bottom crustal thickness vanes from 6.4 to 11 km. Nevertheless, these seismic models are consistent in general terms with oceanic crustal models represented by ophiolite complexes. Some aspects of the differences among profiles can be explained by consideration of a recent tectonic model for the development of the fault zone. This requires, within a 1 Myr time interval, variations in the process of crustal formation at the ridge, crustal 'maturing', or both. The abnormally thick crust near a spreading centre may result in part from the complex interaction of the Juan de Fuca and Explorer plates with the larger and older America and Pacific plates. Upper mantle velocity variations are consistent with the concept of velocity anisotropy. The different record sections show that seismic energy is attenuated for ray paths traversing the Nootka fault zone.     

Axial Intrusion Zone beneath the Median Valley of the Mid-Atlantic Ridge at 37°N Detected by Explosion Seismology

Summary. Four seismic refraction lines, three of which had shots every 250 m, were shot across, along and parallel to the median valley of the Mid-Atlantic Ridge at 37° N. A method has been developed for calculating the effect on the travel times of the rough sea-floor relief beneath the profiles and has been used to correct all the travel times for this effect. Most arrivals were from a main refractor of apparent velocity 5·4 to 6·3 km s⁻¹; only beyond 35 km were faster arrivals observed from an 8·09 ± 36 km s⁻¹ refractor. The main refractor corresponds in depth, at least approximately, to the top of Layer 3 of the ocean basins but its velocity is significantly less than normal for Layer 3, perhaps due to dip. A study of time residuals along two profiles across the median valley indicates the presence of a 2 to 3 km wide low velocity zone (about 3·2 km s⁻¹) beneath the median valley floor. This zone extends over the upper 2·5 km of the crust and is believed to represent a zone of intrusion through which magma passes on its way to the sea floor.     

The application of GPR to barrier—lagoon sedimentation study in Boao of Hainan Island

Sedimentary successions and internal structure of the coastal barrier-lagoon system of Boao, eastern Hainan Island were studied through utilizing data from test holes and trenches and ground-penetrating-radar (GPR) profiles. During late Pleistocene, fluvial and delta plains developed over an unevenly eroded bedrock during low sea level stand, followed by the formation of littoral and lagoon facies and defined coastal barrier-lagoon-estuary system during the post-glacial uppermost Pleistocene-lower Holocene eustatic rise of the sea level, and the upper Holocene high stand. GPR results show that Yudaitan, a sandy coastal bar backed by a low-laying land (shoal) just east of the active lagoon, is a continuous, parallel and slightly-wavy reflectors indicating homogeneous sandy or sandy gravel sediments, and inclined reflectors partly caused by progradation and accumulation of beach sand and gravel. Quasi-continuous, hummocky and chaotic reflectors from the shoal of Nangang village correspond to mixed accumulation of sands and clay. This research indicates the GPR is a non-intrusive, rapid, and economical method for high-resolution profiling of subsurface sediments in sandy gravelly coast.