High-resolution Seismic Investigations Research Articles

Innovative land seismic investigations for CO2 geologic storage in Denmark

The increasing global interest in geologic carbon storage as a feasible way of reducing CO2 atmospheric levels requires extensive onshore high-resolution seismic investigations to characterize suitable storage sites, for example, close to major CO2 emitters. To partly address this challenge and to acquire quality data at shallower and greater depths in a cost- and time-effective approach, a tailored acquisition scheme is adopted and tested at a candidate site in Stenlille, Denmark. The survey aims to understand whether an anticline reservoir structure known to exist at a 1000 m depth can serve for long-term CO2 storage and presents the structural integrity for this purpose. The data are recorded using a combination of nodal arrays, spaced at 10 m in a fixed geometry, and a set of more closely (2 m) spaced digital recorders, mounted on a landstreamer, which was moved at each shot location. Two 12 t mini vibrators are used as seismic sources. The nodal and landstreamer data sets are compared and combined into a unique data set for reflection imaging purposes along five profiles with a total length of approximately 12 km. The seismic sections obtained using this tailored combination of different recorders provide images of the entire shallow and deeper structures with an unprecedented resolution at the different depth levels necessary to assess the full potential of the suggested CO2 storage reservoir. The results significantly increase the existing knowledge of the extent and structural closure of the reservoir as well as a possible fault, all of which are critical for future risk analysis and planning of the storage.

A high-resolution seismic investigation for shallow iron ore

A high-resolution shallow seismic investigation involving a 3D-3C, 3D-1C, 19 zero-offset ZVSP and a 3D SWD was undertaken in an effort to image a subsurface iron ore deposit. The same project area ...

High-Resolution Seismic Reflection Investigation of Subsidence and Sinkholes at an Abandoned Coal Mine Site in South Africa

The high-resolution seismic reflection method was conducted at an abandoned coal mine site located in the Benoni area, near the city of Johannesburg, South Africa, with the aim of investigating the subsidence- and sinkhole-related features that are commonly observed in the area. Like many old abandoned coal mine sites, the study area lacks adequate mine records that show the extent of the underground coal mining activities that took place in the area in the late 1930s. The preliminary geotechnical investigation conducted in the study area using percussion drilling had also confirmed the existence of underlying weathered dolomitic rocks that may give rise to dolomite-related sinkholes and subsidence. The seismic survey has revealed the occurrence of subsidence and sinkhole features in the area to be due to the collapse of the colliery roof into subsurface voids caused by coal extraction that took place at the depth between 7 and 13 m, as well as the dissolution cavities that exist within the underlying dolomite formation at the depth of about 50 m below the surface. The size of the coal-mining-related voids varies between 10 and 15 m and corresponds to the size of the room and pillar mining technique employed in the coal extraction, while the size of the dissolution cavities that exist within the underlying dolomite formation varies from 2 to 10 m.

Subseafloor Investigation of Sediments at Southern Tauranga Harbour, New Zealand, before Capital Dredging

ABSTRACT Jorat, M.E.; Moon, V.G.; Hepp, D.A.; Kreiter, S.; de Lange, W.P.; Feldmann, S., and Morz, T., 2017. Subseafloor investigation of sediments at southern Tauranga Harbour, New Zealand, before capital dredging. The Port of Tauranga plays a key role in New Zealand's export–import industry, and capital dredging commenced in October 2015 to extend the shipping channels to accommodate larger container vessels. This study investigated two-dimensional and three-dimensional subsurface estuarine sediment stratigraphy to predict the sedimentological conditions encountered during dredging operations to ensure that appropriate dredging methodologies were used to minimise the generation of turbidity. Eight cone penetration tests (CPTs), 14 core descriptions, and a high-resolution seismic investigation in Stella Passage (the main shipping channel of Tauranga Harbour) provided the basis for this research. Six major units comprise the stratigraphy; in ascending order they are lower pumiceous sand and silt (UNIT6), ...

Sedimentary evolution of Lake Van (Eastern Turkey) reconstructed from high-resolution seismic investigations

This paper presents results of a multi-channel seismic reflection survey at Lake Van and provides constraints on the sedimentary evolution of the lake. The geophysical data of the lake confirm the existence of three physiographic provinces: a shelf, a slope, and a deep, relatively flat basin. The most prominent features identified on the shelf and slope are clinoforms, submerged channels, as well as closely spaced lake floor depressions, reflecting a highly variable lake-level history. The morphological depressions are interpreted as resulting from subaquatic erosion by channelized, sediment-laden currents into horizontally bedded fan sediments. Submerged channels on the eastern shelf are interpreted as meandering-slope channels, probably as a consequence of a lake-level fall that exposed the shelf area. Clinoforms on the Eastern fan may represent relict deltas formed during stationary or slightly rising lake-level intervals. Merging subsurface imaging interpretation with morphological studies of exposed sediments reveals lake-level fluctuations of several hundreds of meters during the past ca. ~550 ka. The lake has three prominent basins (Tatvan, Deveboynu, and the Northern basin) separated by basement ridges (e.g., the Northern ridge). The seismic units in the Tatvan and Northern basins are dominated by alternations of well-stratified and chaotic reflections, while the Deveboynu basin subsurface consists mainly of chaotic units. The chaotic seismic facies are interpreted as mass-flow deposits, probably triggered by earthquakes and/or rapid lake-level fluctuations. The moderate-to-high-amplitude, well-stratified facies seen in the deeper parts of the basins are interpreted as lacustrine deposits intercalated with tephra layers. The occurrence of a clinoform in the deepest part of the lake suggests a major flooding stage of Lake Van more than ~400 ka ago. Seismic profiles from the deepest part of the lake basin show remarkably uniform and continuous stratigraphic units without any major erosional feature following the flooding event, indicating that the lake was never completely dry afterward and therefore significantly older than previously suggested.

Active faulting and neotectonics in the Baelo Claudia area, Campo de Gibraltar (southern Spain)

The Strait of Gibraltar area is part of the western Eurasian-African convergence zone characterized by a complex pattern of deformation, including thrusting and folding and active normal faulting. Generally, the area is of low-seismicity; only some minor earthquakes have been recorded in the last hundred years. Archaeoseismological data evidences earthquake destruction occurring twice during Roman times. A better neotectonic framework and knowledge on the paleostress evolution of the Strait of Gibraltar area is necessary to find the local sources for those events and to establish an understanding of the recent deformation.Paleoseismic evidence for one moderate earthquake event around 6000-5000 BP along the normal Carrizales Fault is described in this paper. Off-shore high-resolution seismic investigations, structural and paleostress data, high-resolution GPR and geoelectrical resistivity measurements, outcrop investigations and trenching studies are discussed. The data reveal that active faulting takes place along N–S trending normal faults. Hence, N–S directed normal faults in the area are claimed as local candidates for moderate earthquake activity. Return periods of moderate earthquakes in the order of at least 2000–2500years in the study area may have to be taken into account. Structural data, such a paleostress data and joints are presented and a deformation history for the Strait of Gibraltar area in southern Spain is developed in this study.

Detailed images of the shallow Alpine Fault Zone, New Zealand, determined from narrow-azimuth 3D seismic reflection data

Previous high-resolution seismic reflection investigations of active faults have been based on 2D profiles. Unfortunately, 2D data may be contaminated by out-of-the-plane reflections and diffractions that may be difficult to identify and eliminate. Although full 3D seismic reflection methods allow out-of-the-plane events to be recognized and provide superior resolution to 2D methods, they are only rarely applied in environmental and engineering studies because of high costs. A narrow-azimuth 3D acquisition and processing strategy is introduced to produce a high-resolution seismic reflection volume centered on the Alpine Fault Zone (New Zealand). The shallow 3D images reveal late Quaternary deformation structures associated with this major transpressional plate-boundary fault. The relatively inexpensive narrow-azimuth 3D acquisition pattern consisting of inline source and receiver lines was easily implemented in the field to provide 2- by [Formula: see text] CMP coverage over an approximately 500- by [Formula: see text] area.The narrow-azimuth acquisition strategy was well suited for resolving complex structures within the fault zone. Challenges in processing the data were amplified by the effects of strong velocity heterogeneity in the near surface and the presence of complex dipping, diffracted, and truncated events. A carefully tailored processing scheme including surface-consistent deconvolution, refraction static corrections, noise reduction, dip moveout (DMO) corrections, and 3D depth migration greatly improved the appearance of the final stacks. The 3D images reveal strong reflections from the faulted and folded late Pleistocene erosional basement surface. A steeply dipping planar main (dominant) fault strand can be inferred from the geometry and truncations of the overlying postglacial sediments. The 3D images reveal that the average apparent vertical displacement [Formula: see text] of the basement surface across the dominant fault strand at this location is somewhat less than that estimated from a pilot 2D seismic reflection profile, suggesting that the provisional dip-slip rate based on the 2D data is a maximum.

High resolution seismic investigation in salt mining context

The high resolution seismic (HRS) reflection survey was conducted on an experimental site in eastern France. Three profiles were designed in order to check the detectability of salt-mining cavities at depth range of 110–180 m. The detection and location of cavities on the HRS images are based on indirect symptoms corresponding to cavity-induced perturbations of lower markers. In most cases, the cavities were detected and the accuracy of estimation of horizontal extensions of the cavities was surprisingly good (of about 10 m). The accuracy of vertical positioning is difficult to establish when the detection is performed indirectly and the only information that could be derived is that a cavity is beyond the masked geological interface. The detection ability depends on the size of the cavity in regard to wavelength and depth. These factors are included in AN coefficient which was used in order to evaluate the detectability of a given cavity.

High resolution seismic investigations for the determination of water flow directions during sulphur deposits exploitation

This paper shows the results of seismic research on some problems of sulphur in-situ mining. The difficulties during exploitation were mainly connected with the lack of detailed information on the structure and tectonics of deposit and overburden strata. This kind of information could be obtained using the High Resolution Seismic method.

A high-resolution seismic CHIRP investigation of active normal faulting across Lake Tahoe Basin, California-Nevada

We measured extension rates across Lake Tahoe Basin for the last 60 k.y. based on measured displacement of offset marker surfaces across three active faults beneath Lake Tahoe. Seismic chirp imaging with submeter accuracy, together with detailed multibeam and light detection and ranging (LIDAR)-derived bathymetry, was used to measure fault offset, thickness of earthquake-derived colluvial wedges, depth of wave-cut paleoterraces, and other geomorphic features. An analysis of these features provides refined estimates of extension rates and new information on Holocene faulting, and places Tahoe Basin deformation into the larger context of Walker Lane and Basin and Range tectonics. Measured offset marker surfaces include submerged wave-cut paleoterraces of Tioga age (19.2 ± 1.8 ka), McKinney Bay slide deposits (ca. 60 ka), and a winnowed boulder surface of Tahoe age (ca. 62 ka). Estimated vertical offset rates across submerged geomorphic surfaces are 0.43-0.81 mm/yr for the West Tahoe fault, 0.35-0.60 mm/yr for the Stateline-North Tahoe fault, and 0.12-0.30 mm/yr for the Incline Village fault. These offset rates indicate a combined east-west extension rate across Lake Tahoe Basin, assuming 60° fault dips, of 0.52-0.99 mm/yr. This estimate, when combined with the Genoa fault-slip rate, yields an extension rate consistent with the magnitude of the extension deficit across Carson Valley and Lake Tahoe Basin derived from global positioning system (GPS) velocities. The Stateline-North Tahoe, Incline Village, and West Tahoe faults all show evidence for individual Holocene earthquake events as recorded by either colluvial wedge deposits or offset fan-delta stratigraphy.

Detailed seismic imaging of a chemical munition dumpsite in the Bornholm Basin, south-western Baltic

Very high resolution seismic investigations were carried out over a munition dumpsite in the Bornholm Basin, south-western Baltic Sea. After WW2 over 32,000 tons of chemical weapons were dumped in this area. The aim of the investigations was to image the internal structure of the dumpsite and to identify possible natural hazards with regard to the dumped war material. Two geophysical surveys were carried out in the summers of 2006 and 2007. During the surveys, acoustic sources with a distinct frequency spectrum (parametric echosounder, sparker, boomer) were deployed simultaneously. This approach proved very fruitful as it allowed to study the sediments in a wide depth range and in the highest possible detail. In total, seven seismic–stratigraphic units have been identified, related to different stages in the Holocene and late glacial history. Water depths range roughly between 96 m in the north and 70 m in the south where the basin shallows and older glacial sediments are outcropping. The seafloor topography at the dumpsite is generally quite flat, locally marked by small pit-like features that are probably related to the dumping impact. Small-scale fluid escape features are present throughout the dumpsite area. Large-scale doming occurs locally and is possibly linked to the migration of deep thermogenic gas. A large number of buried objects were identified on the acoustic data. Their burial depth is generally less than 1 m. The data confirm the wide variety of dumped war material, ranging from bombs and shells to encasements and containers. The distribution of the objects seems to be very heterogeneous, with locally high concentrations and areas of low object density. Four different shipwrecks were identified but their relation to the dumped warfare is not clear.

Ultra-high-resolution marine 2D–3D seismic investigation of the Liman Tepe/Karantina Island archaeological site (Urla/Turkey)

2D and 3D high-resolution seismic investigations were performed on submerged coastal archaeological sites at Iskele and near to Karantina Island in the Bay of Izmir in western Turkey. Tectonic subsidence of the coastline has submerged a number of archaeological features associated with an important Early Bronze Archaic settlement (Liman Tepe) and the classical Ionian city of Clazomenae. Seismic surveys were focused on imaging of an Archaic harbour structure and other submerged Hellenistic and Roman architectural features. Seismic data were acquired with the SEAMAP-3D ultra-high-resolution 3D marine seismic acquisition system developed for detailed archaeological site investigation. A 2D reconnaissance survey was performed over a 2 km 2 area around Karantina Island to evaluate the seismic penetrability and to locate sites for further 3D investigation. This survey predominantly revealed marine sediment layers covering the local bedrock, which is characterized by scattering of seismic energy showing its rocky nature. Two ultra-high-resolution 3D seismic surveys were performed. The first covered a 350 m × 30 m area in the modern harbour targeting a prominent Archaic harbour structure. The second was acquired across a 120 m × 40 m area on the southeast shore of the Karantina Island close to a Roman architectural feature. The 3D surveys were acquired with nominal line spacings of 1 m, using a 8 × 4 pseudo-rigid hydrophone array and a Boomer source firing at 3 Hz shot frequency. Automated processing of the seismic data using a portable Linux cluster provided stacked 3D seismic volumes with 25 cm × 25 cm bin size on-site. The 3D seismic survey of the harbour clearly imaged the submerged Archaic structure and the underlying sediment sequence. The seismic time slices reveal two seismic anomalies (2–3 m in diameter) in the harbour basin sediments. The 3D surveys southeast of Karantina identified a thicker marine sediment sequence overlying steeply dipping bedrock reflectors. The sediment sequence records the rapid accumulation and progradation of the coastline following the construction the Alexander causeway linking the mainland with the island in 334 B.C.

Sismoestratigrafia de alta resolução no lago Juparanã, Linhares (ES - Brasil) como base para estudos sobre a sedimentação e tectônica quartenária

O estudo sismoestratigrafico de alta resolucao do lago Juparana, o maior do sistema de lagos do baixo vale do rio Doce (Espirito Santo, Sudeste do Brasil), foi realizado, fornecendo informacoes para discutir sobre a origem e evolucao deste sistema de lagos ao longo do Quaternario. Duas sismossequencias foram identificadas. A sismossequencia mais antiga e caracterizada pelo padrao transparente de reflexao (reflection free ), tendo sido associada a depositos aluviais holocenicos. A sismossequencia mais recente e caracterizada por um padrao paralelo de reflexoes sismicas e foi associada a fase lacustre instalada apos a barragem do antigo ''vale Juparana''. O mapa de isopacas da sismossequencia mais nova documenta uma distribuicao irregular dos depositos lacustres em tres depocentros distintos, com limites relativamente abruptos, sugerindo um possivel controle neotectonico na evolucao do lago.

High-resolution mapping of shallow gas accumulations and gas seeps in San Simón Bay (Ría de Vigo, NW Spain). Some quantitative data

San Simon Bay in the innermost part of the Ria de Vigo is characterized by an abundance of very shallow gas accumulations and methane seeps. During the expeditions of April–June–September 2004 within the Spanish-funded Gs2G project, detailed very high-resolution seismic and field investigations were carried out to study the shallow gas and the seeps. Direct gas fluxes also were measured from bubble streams. For the first time, the surface area and gas front depth of a shallow gas field has been mapped and quantified in the inner bay of Ria de Vigo. This field overlaps spatially with the distribution of Holocene mud within the bay. Seismic data show 3.6 km2 affected by acoustic turbidity but this surface can be extended up to 9.5 km2 of San Simon’s muddy subtidal area. Mounded turbidity superimposed on the main gas field has been mapped and characterized as anthropogenically (mussel rafts) mediated gas accumulations. Different acoustic anomalies have been identified and interpreted as being due to gas escapes from the present seabed sediment. The very high resolution of the seismic data makes it possible to identify a new type of seep, here named ‘acoustic smoke.’ A direct relationship can be observed between the gas front of accumulations and escape features, both acoustic seeps and pockmarks. The methane flux has been estimated from the subtidal environment in San Simon based on detected acoustic targets and direct measurements of current bubble flow. The total estimated methane flux from the seabed into the water column ranges from 10.1 to 48.8 t/year, and into the atmosphere from 7.0 to 34.2 t/year. The intertidal San Simon environment is also actively venting methane, as indicated by the presence of bubbling during high tide and white patches of Beggiatoa sp.

Geomorphological and sedimentological evolution of a lake basin under strong volcano-tectonic influence: The seismic record of Lago Calafquén (south-central Chile)

The Chilean Lake District is a geodynamically active area located at the foot of the Andean cordillera and permanently influenced by the Chilean active continental margin. The study area is prone to steady mass movements that are triggered by active volcanism, strong earthquakes and climatic forces. On the glacial-born lake Lago Calafquén high-resolution seismic investigation was performed to select suitable coring sites for sedimentary analyses, to revise the existing bathymetry and to integrate the lake bottom topography into the geomorphological context of the surroundings. The sub-bottom profiling reveals a complex structure of the lake floor, which results in a division into an eastern and a central sub-basin. The eastern basin encompasses the deep basin plain which is a depositional area for dense suspension-loaded underflows. The morphology of the basement of the central sub-basin is affected by large landslides. Large rock-slope failures of debuttressed sidewalls fill the primary lake floor of the southern side covered by more than 40 m of sediment. A high-resolution seismic analysis of the sedimentary infill reveals deposits and structures of large mass movements and turbidites, obviously triggered by strong earthquakes. Incidents related to rebound effects by isostatic uplift are also considered to provoke remarkable magnitudes that trigger mass movement processes in the central sub-basin. Furthermore, volcanic impact frequently interrupts normal lacustrine sedimentation of the sediment succession by deposition of tephra layers. The serialized geodynamic impacts on Lago Calafquén cause severe damage to the sedimentary archive. Thus, suitable coring sites for palaeoenvironmental reconstruction should be selected carefully.

Shallow architecture of the Wadi Araba fault (Dead Sea Transform) from high-resolution seismic investigations

In a high-resolution small-scale seismic experiment we investigated the shallow structure of the Wadi Araba fault (WAF), the principal fault strand of the Dead Sea Transform System between the Gulf of Aqaba/Eilat and the Dead Sea. The experiment consisted of 8 sub-parallel 1 km long seismic lines crossing the WAF. The recording station spacing was 5 m and the source point distance was 20 m. The first break tomography yields insight into the fault structure down to a depth of about 200 m. The velocity structure varies from one section to the other which were 1 to 2 km apart, but destinct velocity variations along the fault are visible between several profiles. The reflection seismic images show positive flower structures and indications for different sedimentary layers at the two sides of the main fault. Often the superficial sedimentary layers are bent upward close to the WAF. Our results indicate that this section of the fault (at shallow depths) is characterized by a transpressional regime. We detected a 100 to 300 m wide heterogeneous zone of deformed and displaced material which, however, is not characterized by low seismic velocities at a larger scale. At greater depth the geophysical images indicate a blocked cross-fault structure. The structure revealed, fault cores not wider than 10 m, are consistent with scaling from wear mechanics and with the low loading to healing ratio anticipated for the fault.

Very high resolution seismic investigation of the sedimentary infill of the Leucate lagoon (Aude and Pyrénées-Orientales – SE France)

Abstract A very high resolution seismic investigation has for the first time allowed the imaging of the sedimentary infill of a Mediterranean lagoon. The Leucate lagoon is part of a lagoon system located along the shore of the western Gulf of Lions, from the Rhone delta to the Spanish border. These lagoons are separated and protected from the sea by sandy barriers, also called lidos, which are the result of a process of shore regularization by waves. The seismic data, obtained by using a boomer-Seistec, coupled with lithological and radiocarbon data [Martin, 1978] previously collected from cores, have revealed three main sedimentary formations : The basal formation represents the substratum of the lagoon and the middle and upper formations its infill. The basal formation displays a very uniform seismic facies with reflectors almost constantly dipping towards the sea. It comprises conglomeratic sediments and is interpreted as a progradating fluvio-deltaïc formation. Its upper surface is erosional and is locally deeply incised. In the shallowest parts of the lagoon, where the basal formation almost emerges, its upper part is reworked by modern processes into multiple cut-and-fill structures. The middle formation overlaps the basal formation, and constitutes the main depositional unit of the lagoon fill (up to 20 ms twtt thick). The seismic facies are variable and correspond to sand and clay sediments deposited under fluvio-lagoonal to lagoonal and marine conditions. The upper formation represents the upper part of the infill. It rests above the middle formation through a conformable surface, locally slightly erosional, and overlaps the basal formation along the western rim and in the shallowest parts of the basin. The thickness of this upper formation does not exceed 3mst wtt. It mainly consists of clay sediments of lagoonal origin. The main characteristic of this upper formation is a thin sole of very dense sand at the base. This bed is also a remarkable seismic reflector, and is interpreted as resulting from the maximum marine flooding of the system. This occurs before the beginning of the barrier construction, and the progressive closure of the lagoon. Another remarkable aspect of the upper formation is the simultaneity of its basal part with the lido construction. In this upper unit, the seismic data allow the imaging of the lateral passing between the planar bedded sediments of the infill, with the sigmoidal beds representing washover fans that construct the lagoonal side of the lido. The uppermost part of the formation represents the final and present-day stage of the lagoonal infill since the final closure by the barrier. Dating, performed on cored sediments, allow the sediments of the basal formation to be assigned to the Middle Pleistocene (with no more precision). The erosion of the top of the basal formation is interpreted as fluvial incision during the last sea level fall. The lagoon infilling is of Holocene age and comprises two stages : the first and main stage corresponds to fluvio- to marino-lagoonal sedimentation, and occurred before 6,000 B.P. The second corresponds to the recent to modern infilling that began around 4,000 B.P. with the construction of the lido and the closure of the lagoon. The two stages are clearly separated by a period of maximum marine flooding. The thickness of the lagoon fill is relatively limited, probably no more than 20 m.

AP- and SH-wave Seismic Reflection Investigation of the Kentucky Bend Scarp in the New Madrid Seismic Zone

An on-going high-resolution seismic reflection investigation, integrating P - and SH -wave reflection techniques, is being conducted in the central New Madrid seismic zone to describe the geometry and style of the geologic structure associated with a NW-SE fault at the Kentucky Bend scarp (KBFS). To date, approximately 1.9 km of P -wave data and 0.9 km of SH -wave data have been collected. Two perpendicular six-fold P -wave reflection surveys have been performed obliquely to the axis of the KBFS to image the style and azimuth of structure associated with the deeper Cretaceous (427 m) and Paleozoic (533 m) reflection horizons. The P -wave data also served as reconnaissance for siting the target areas imaged by the near-surface SH -wave technique. A high-angle reverse fault has been successfully imaged 540 m in front (northeast) of the present scarp topography using P - and SH -wave techniques, indicating a broad zone of near-surface deformation. The fault, along with north-dipping reflectors, has been traced from the top of the Paleozoic bedrock into the water-saturated, unconsolidated sand and gravel of the Quaternary alluvium. It is interpreted to be a northeast-vergent fault, striking approximately N30°W. This interpretation is consistent with the surface expression of the scarp and the nature of the contemporary seismicity. The fault is also spatially consistent with historic reports of the two waterfalls created in the Mississippi River after the great 1811–1812 earthquakes, suggesting that the KBFS represents part of the surface rupture associated with the events.

An interdisciplinary approach to the evaluation of physical parameters of shallow marine sediments

Abstract A large number of high-resolution seismic investigations and in situ geotechnical measurements were carried out in the southern Baltic, offshore Bornholm. The geotechnical measurements, carried out with Ifremer’s Geotechnical Module and Kiel’s GISP system, provided information on cone resistance, induced pore pressure and density of the surficial sediments. The seismic data were subjected to different processing techniques, including detrending and multiple quotient division, which made it possible to deduce the reflection coefficients and impedance contrasts at subbottom interfaces. The different geotechnical and seismic data sets were carefully integrated. There is a good correlation between the in situ results and the interpreted seismic units. Strong variations in geotechnical plots correspond with the reflectors identified on processed seismic sections. The general trend of calculated impedances agrees well with the lithological interpretation. However, it would appear that the inversion method does not always result in quantitatively correct values of the sediment parameters. Further development and refining of this method will undoubtedly provide better estimates of the in situ bulk properties of the shallow marine sediments.

Latest Quaternary Outer Shelf and Slope Deposits, Northern Gulf of Mexico, USA: Industry Research Consortium

A consortium of oil companies is undertaking a multiyear shallow coring and high-resolution seismic investigation of the outer shelf-slope deposits of the northern Gulf of Mexico. The program's objective is to reconstruct the depositional, seismic stratigraphic, biostratigraphic, isotopic, and organic geochemical record of the latest Quaternary of a portion of the Gulf margin. Results will serve as an analog to deeper hydrocarbon reservoirs and to help in understanding sedimentation with glacial-interglacial sea-level changes. Nearly continuous cores up to 850 ft long from Main Pass Blocks 303, 242, 288 and Viosca Knoll Block 774 are being described and analyzed. Main Pass 303 core samples Holocene to Oxygen Isotope Stage 6 deposits and as old or older strata are expected to have been penetrated in the other boreholes. Three or four sea level cycles are represented. Seismic acquisition is planned for late 1990. The outer shelf and slope of the northern Gulf is an area of active growth faulting, high sedimentation rate, diapirism, and slumping. Recent work, including this study, shows that near-surface stratigraphy consists of coarse clastics alternating with carbonate-rich mud. Sands and gravels are inferred to have been deposited mostly during lowstands and rising sea level in deltas and valley-fillmore » systems. Delta clinoforms on high-resolution seismic may represent shelf-margin wedges, prograding wedges, or highstand systems tracts. Highstand deposits are characterized largely by thin, continuous intervals of bioturbated muds containing abundant diagenetic carbonate and pyrite. Precise dating and correlation of outer shelf-upper slope facies to magnitude and direction of sea level change is intended during the project.« less