Seismic Reflection Measurements Research Articles

Miocene tectonic activity at the boundary between NE Pannonian and NW Transylvanian basins (Romania): Insight from new seismic data

AbstractSituated at the junction between the Eastern Carpathians Mountains, the Pannonian and the Transylvanian basins, the Baia Mare region (Romania) has a complex geological history that witnessed the interaction among the three main tectonic provinces. Here, we report results from new seismic reflection measurements that provide modern information about the subsurface geological structure. The integrated analysis of the newly acquired and vintage seismic reflection data from the study area reveals details about the architecture of the Palaeogene and the Neogene deposits at the contact between the northeastern Pannonian and northwestern Transylvanian basins. In particular, it unveils a fault zone that most probably controlled the tectonic evolution of the eastern Pannonian and Transylvanian basins. A better understanding of the crustal structure and tectonic features in the study area is a first step into evaluating the geothermal potential of the region.

The Trendermarsch polder (North Frisia, Germany) - Geophysical and geoarchaeological investigations of an anthropogenic medieval coastal landscape and its vulnerability against natural hazards

Since medieval times, North Frisia (Schleswig-Holstein, Germany) has experienced large-scale natural and man-made geomorphological changes. First, Frisian settlers turned the coastal marshes and fenlands into arable land but also increased the region's vulnerability to flooding. Then major storm surges drowned wide areas in 1362 CE and 1634 CE and many (geo-)archives that could provide knowledge about medieval man-environment interactions and the overall appearance of the coastal landscape at that time were irretrievably lost.To better understand the natural palaeogeographical evolution and human intervention with the coastal environment, our research focused on the Trendermarsch polder (Nordstrand), that is one of few sites still reflecting the cultivated marsh landscape of the 13th to 14th cent. AD.Fieldwork comprised DEM analysis, geophysical prospection by electrical resistivity tomography (ERT), seismic reflection measurements, Direct Push (DP)-EC logging and coring to obtain high-resolution stratigraphic data. Sedimentary, geochemical and microfaunal palaeoenvironmental parameter (PEP) analyses of sediment samples allowed to calibrate geophysical and DP-EC results. Radiocarbon dating, archaeological age estimations and historical reports provided a geochronological framework.Results confirm the 12th cent. AD age of the Trendermarsch polder but also reveal distinct phases of channel incisions, that are likely related to extreme events. These tidal channels appeared to be a distinct gateway for storm surges and considerably influenced medieval to early modern land reclamation and cultivation measures. The Trendermarsch therefore sets an impressive example for the interplay of natural processes and man-made geomorphological changes and their consequences for a coastal landscape.

Automated facies identification by Direct Push‐based sensing methods (CPT, HPT) and multivariate linear discriminant analysis to decipher geomorphological changes and storm surge impact on a medieval coastal landscape

AbstractInad1362, a major storm surge drowned wide areas of cultivated medieval marshland along the north‐western coast of Germany and turned them into tidal flats. This study presents a new methodological approach for the reconstruction of changing coastal landscapes developed from a study site in the Wadden Sea of North Frisia. Initially, we deciphered long‐term as well as event‐related short‐term geomorphological changes, using a geoscientific standard approach of vibracoring, analyses of sedimentary, geochemical and microfaunal palaeoenvironmental parameters and radiocarbon dating. In a next step, Direct Push (DP)‐based Cone Penetration Testing (CPT) and the Hydraulic Profiling Tool (HPT) were applied at vibracore locations to obtainin situhigh‐resolution stratigraphic data. In a last step, multivariate linear discriminant analysis (LDA) was successfully applied to efficiently identify different sedimentary facies (e.g., fossil marsh or tidal flat deposits) from the CPT and HPT test dataset, to map the facies' lateral distribution, also in comparison to reflection seismic measurements and test their potential to interpolate the borehole and CPT/HPT data. The training dataset acquired for the key site from coring and DP sensing finally allows an automated facies classification of CPT/HPT data obtained elsewhere within the study area. The new methodological approach allowed a detailed reconstruction of the local coastal landscape development in the interplay of natural marsh formation, medieval land reclamation and storm surge‐related land losses.

A jó, a rossz és a csúf? – avagy a szénhidrogén-kutatás múltja, jelene és jövője a Kárpát-medencében – Szemle

A magyarországi szénhidrogén-kutatás története immár több mint egy évszázados távlatot ölel fel. A kezdeti egyszerű eszközökkel, felszíni térképek alapján történő kutatásból mára – követve és alkalmazva a nemzetközi trendeket és technikákat – egy sok tudományágat felölelő, komplex iparággá nőtte ki magát a honi szénhidrogén-ipar is. A kutatási szempontból folyamatosan “érő” Pannon medencében időről-időre kerültek bevezetésre újabb kutatási technikák, történtek jelentős felfedezések, melyek jelzik a szakemberek folyamatos fejlődését és sikereit. Cikkünkben nem célunk minden kutatási eredményt vagy alkalmazott technológiai újítást kronológiai sorrendben bemutatni, azonban azokat, melyek megítélésünk szerint jelentősen hozzájárultak a kutatás mindenkori eredményességéhez, mint mérföldköveket szeretnénk ismertetni. Felsorolni is nehéz, hogy hány jelentős esemény történt e bő évszázad alatt, de minden korszaknak voltak, vannak, és várhatólag lesznek olyan nagy jelentőségű technológiai újításai és felfedezései, amelyek nélkül a további fejlődés nem, vagy sokkal rögösebb úton ment volna, mehet végbe. Elég csak az Eötvös-ingára, a 2D majd – jóval később – a 3D szeizmikus mérések rutinszerű alkalmazására, a számítógépek kutatásban való térhódítására, vagy a mai komplex geológiai modellekre gondolni, hogy a jelentősebb eredményeket, mint például a hajdúszoboszlói, nagylengyeli, algyői, vagy a közelmúltban felfedezett mezőket ne is említsük.
 Múlt, jelen, jövő. Előbbi kettőről konkrét tudás, tényanyag áll rendelkezésre, függetlenül attól, hogy jó vagy rossz dolgokról kell beszélnünk. A jövő azonban nehéz kérdés. Jelentős kihívásokkal küszködik nemcsak a hazai, de a globális olajipar is. Egy érett szénhidrogén-medence kutatása nemcsak a komplex technológiai kihívást igénylő, többnyire éppen csak a gazdaságosság határán mozgó kisméretű reménybeli mezők miatt van nehéz helyzetben. A globalizáció miatt „rövidre zárt” ellátási láncok, a nagymértékű gazdasági függőség, az Európai Unió által választott energiapolitika összességében olyan méretű kihívást jelent, amellyel egyre nehezebb a kutatásban részt vevő cégeknek és szakembereknek megbirkózniuk. E kihívásokra is megpróbálunk néhány lehetséges kutatás-technikai megoldást felvázolni, amelyek közül némelyik talán tényleg elvezet oda, hogy a jövő mégse legyen olyan csúf.

Investigating the Norse Harbour of Igaliku (Southern Greenland) Using an Integrated System of Side-Scan Sonar and High-Resolution Reflection Seismics

This study presents the results of a marine geophysical survey performed in the Igaliku fjord in southern Greenland in order to understand the harbour setting of the former Norse settlement Garðar (modern Igaliku). The aims of the survey were (a) to reconstruct the former coastline during the first centuries of the Norse settlement period (c. 11/12th centuries) and (b) to search for archaeological remains on the seabed connected to maritime traffic and trade. In order to approach these goals, we used an integrated marine survey system consisting of a side-scan sonar and a reflection seismic system. The system was designed for lightweight transport, allowing measurements in areas that are logistically difficult to access. The side-scan sonar data revealed no remains of clear archaeological origin. Bathymetric data from seismic seabed reflection and additional Differential GPS height measurements yielded a high-resolution bathymetric map. Based on estimates of Holocene relative sea level change, our bathymetry model was used to reconstruct the shift of the high and low-water line since the early Norse period. The reconstructed coastline shows that a small island, which hosts the ruins of a tentative Norse warehouse at the mouth of the present harbour, was connected to the shore at low tide during the early Norse period. In addition, reflection seismics and side-scan sonar images reveal a sheltered inlet with steep slopes on one side of the island, which may have functioned as a landing bridge used to load ships. We also show that the loss of fertile land due to sea level rise until the end of the Norse settlement was insignificant compared to the available fertile land in the Igaliku fjord and is thus not the reason for the collapse of the colony.

Physical Properties and Structure of the Near Subsurface in the Săcel Area, Romania, from Shallow Seismic Measurements

Results are presented for shallow seismic reflection measurements performed southwest of Săcel village in Romania for the purpose of obtaining information about the geological structure in the near subsurface. The P-wave and S-wave velocity distributions were also obtained below the soil surface. The measurements were performed along a nearly linear profile on the top of an elongated hill. Most of the shot gathers were characterized by a good signal-to-noise ratio. A depth-converted migrated section was obtained after the processing of shot gathers, on which an image of sedimentary deposits with various thicknesses, separated by shallow faults until a depth of about 80 m, were observed. The P-wave and S-wave velocity-depth models for two segments were of considerable interest for a geotechnical study proposed for the construction of a windmill park. The two- and three-layered P-wave velocity-depth models were comparable until depths of about 10 m after first-arrival traveltime inversions. The lateral variations in the subsurface geological structure and lithology reflected the variations in the P-wave velocity values from both models. The S-wave velocity-depth models for comparable depth intervals were similar to those from the P-wave velocity-depth models. Reliable S-wave velocity distributions were obtained after inversion of fundamental-mode and higher-mode surface waves.

Shallow Seismic Reflection Investigation of Unstable Sedimentary Deposits in the Dăneasa Area, Romania

Results of shallow seismic reflection measurements, performed over an area with unstable sedimentary formations in the near subsurface, are presented. The investigated area is located on the Olt Valley, in the vicinity of the Dăneasa village, Romania, crossed by a railway line and a national road on which the traffic is very intense. Data were collected along a linear profile, with a length of 94 m, perpendicular to the railway line and the national road using a fixed spread of 48 vertical-component geophones with the sources rolling along the acquisition line. The surface waves generated by the intense traffic on the national road interfered with the seismic arrivals seen on most of shot gathers, especially with those seen on the traces from receiver locations greater than 48 m. The interrupted continuity of two shallow reflectors, R1 and R2, seen on the depth-converted seismic section at depth values shallower than 30 m, could be interpreted as an effect of the presence of two steep failure planes deveIoped into the ground. A P-wave velocity-depth model down to a depth of 20 m after first-arrival traveltime tomography was then created, on which four sedimentary layers with variable thicknesses and constant velocities, separated by the interfaces L1, L2 and L3, were interpreted. By comparing the depth-converted seismic section with the P-wave velocity-depth model, the reflectors R1 and R2 on the seismic section correspond pretty well with the interfaces L2 and L3 from the velocity-depth model.

Robust estimation of vertical symmetry axis models via joint migration inversion: Including multiples in anisotropic parameter estimation

Because the earth is predominately anisotropic, the anisotropy of the medium needs to be included in seismic imaging to avoid mispositioning of reflectors and unfocused images. Deriving accurate anisotropic velocities from the seismic reflection measurements is a highly nonlinear and ambiguous process. To mitigate the nonlinearity and trade-offs between parameters, we have included anisotropy in the so-called joint migration inversion (JMI) method, in which we limit ourselves to the case of transverse isotropy with a vertical symmetry axis. The JMI method is based on strictly separating the scattering effects in the data from the propagation effects. The scattering information is encoded in the reflectivity operators, whereas the phase information is encoded in the propagation operators. This strict separation enables the method to be more robust, in that it can appropriately handle a wide range of starting models, even when the differences in traveltimes are more than a half cycle away. The method also uses internal multiples in estimating reflectivities and anisotropic velocities. Including internal multiples in inversion not only reduces the crosstalk in the final image, but it can also reduce the trade-off between the anisotropic parameters because internal multiples usually have more of an imprint of the subsurface parameters compared with primaries. The inverse problem is parameterized in terms of a reflectivity, vertical velocity, horizontal velocity, and a fixed [Formula: see text] value. The method is demonstrated on several synthetic models and a marine data set from the North Sea. Our results indicate that using JMI for anisotropic inversion makes the inversion robust in terms of using highly erroneous initial models. Moreover, internal multiples can contain valuable information on the subsurface parameters, which can help to reduce the trade-off between anisotropic parameters in inversion.

Seafloor seismic acquisition using autonomous underwater vehicles

ABSTRACTAlthough narrow‐azimuth towed‐streamer data provide good image quality for structural interpretation, it is generally accepted that for wide‐azimuth marine surveys seabed receivers deliver superior seismic reflection measurements and seismically derived reservoir attributes. However, seabed surveys are not widely used due to the higher acquisition costs when compared to streamer acquisition. In recent years, there have been significant engineering efforts to automate receiver deployment and retrieval in order to minimize the cost differential and conduct cost‐efficient seabed receiver seismic surveys. These engineering efforts include industrially engineered nodes, nodes on a rope deployment schemes and even robotic nodes, which swim to and from the deployment location. This move to automation is inevitable, leading to robotization of seismic data acquisition for exploration and development activities in the oil and gas industry. We are developing a robotic‐based technology, which utilizes autonomous underwater vehicles as seismic sensors without the need of using a remotely operated vehicle for deployment and retrieval. In this paper, we describe the autonomous underwater vehicle evolution throughout the project years from initial heavy and bulky nodes to fully autonomous light and flexible underwater receivers. Results obtained from two field pilot tests using different generations of autonomous underwater vehicles indicate that the seismic coupling, and navigation based on underwater acoustics are very reliable and robust.

Near‐surface fault detection using high‐resolution shear wave reflection seismics at the CO2CRC Otway Project site, Australia

AbstractHigh‐resolution, near‐surface, shear wave reflection seismic measurements were carried out in November 2013 at the CO2CRC Otway Project site, Victoria, Australia, with the aim to determine whether and, if so, where deeper faults reach the near subsurface. From a previous P wave 3‐D reflection seismic data set that was concentrated on a reservoir at 2 km depth, we can only interpret faults up to 400 m below sea level. For the future monitoring in the overburden of the CO2 reservoir it is important to know whether and how the faults continue in the subsurface. We prove that two regional fault zones do in fact reach the surface instead of dying out at depth. Individual first‐break signatures in the shot gathers along the profiles support this interpretation. However, this finding does not imply perforce communication between the reservoir and the surface in the framework of CO2 injection. The shear wave seismic sections are complementary to existing P wave volumes. They image with high resolution (better than 3 m vertically) different tectonic structures. Similar structures also outcrop on the southern coast of the Otway Basin. Both the seismic and the outcrops evidence the complex youngest structural history of the area.

Basal characteristics of the main sticky spot on the ice plain of Whillans Ice Stream, Antarctica

Understanding the processes that affect streaming ice flow and the mass balance of glaciers and ice sheets requires sound knowledge of their subglacial environments. Previous studies have shown that an extensive deformable subglacial sediment layer favors fast ice-stream flow. However, areas of high basal drag, termed sticky spots, are of particular interest because they inhibit the fast flow of the overriding ice. The stick-slip behavior of Whillans Ice Stream (WIS) is perhaps the most conspicuous manifestation of a subglacial sticky spot. We present new ice-thickness and seismic-reflection measurements collected over the main sticky spot in the ice plain of WIS, allowing us to elucidate its role in the behavior of the ice stream. Ice-thickness and surface-elevation data show that the sticky spot occupies a subglacial topographic high. Water flow in response to the hydrological potential gradient will be routed around the sticky spot if effective pressures are similar on the sticky spot and elsewhere. The seismic experiment imaged a laterally continuous basal layer approximately 6 m thick, having compressional wave velocities of greater than 1800 m s−1 and density greater than 1800 kg m−3, indicative of a till layer that is stiffer than corresponding till beneath well-lubricated parts of the ice stream. This layer likely continues to deform under the higher shear stress of the sticky spot, and some water may be pumped up onto the sticky spot during motion events.

Finite-difference modelling to evaluate seismic P-wave and shear-wave field data

Abstract. High-resolution reflection seismic methods are an established non-destructive tool for engineering tasks. In the near surface, shear-wave reflection seismic measurements usually offer a higher spatial resolution in the same effective signal frequency spectrum than P-wave data, but data quality varies more strongly. To discuss the causes of these differences, we investigated a P-wave and a SH-wave seismic reflection profile measured at the same location on the island of Föhr, Germany and applied seismic reflection processing to the field data as well as finite-difference modelling of the seismic wave field. The simulations calculated were adapted to the acquisition field geometry, comprising 2 m receiver distance (1 m for SH wave) and 4 m shot distance along the 1.5 km long P-wave and 800 m long SH-wave profiles. A Ricker wavelet and the use of absorbing frames were first-order model parameters. The petrophysical parameters to populate the structural models down to 400 m depth were taken from borehole data, VSP (vertical seismic profile) measurements and cross-plot relations. The simulation of the P-wave wave-field was based on interpretation of the P-wave depth section that included a priori information from boreholes and airborne electromagnetics. Velocities for 14 layers in the model were derived from the analysis of five nearby VSPs (vP =1600–2300 m s-1). Synthetic shot data were compared with the field data and seismic sections were created. Major features like direct wave and reflections are imaged. We reproduce the mayor reflectors in the depth section of the field data, e.g. a prominent till layer and several deep reflectors. The SH-wave model was adapted accordingly but only led to minor correlation with the field data and produced a higher signal-to-noise ratio. Therefore, we suggest to consider for future simulations additional features like intrinsic damping, thin layering, or a near-surface weathering layer. These may lead to a better understanding of key parameters determining the data quality of near-surface shear-wave seismic measurements.

3D seismic survey at the Millennium uranium deposit, Saskatchewan, Canada: Mapping depth to basement and imaging post-Athabasca structure near the orebody

Three-dimensional seismic reflection measurements have been used to assist mine planning at the Millennium uranium deposit, Canada. The deposit is located within the crystalline basement, separated from the overlying Athabasca Basin sediments by an unconformity potentially associated with significant fluid flow. The primary objective of the [Formula: see text] survey was to image the unconformity and possible post-Athabasca deformation structures in and around the deposit. Clear unconformity reflections are observed within most of the survey area, although there are amplitude variations due to complex geology, including intense hydrothermal clay alteration around the deposit. Finite-difference modeling indicates that the wide-angle character of the unconformity reflections is due to a gradual velocity increase at the unconformity. The reflections are obscured by large time delays, due to Quaternary sediments covering the area, making refraction static corrections crucial. The seismic interpretation shows large variations in the unconformity depth (from approximately 430 to 650 m), indicating a pronounced basement depression that coincides with a gravity low. Reflections from the unconformity are vague within the depression, especially in the vicinity of the deposit. Although the orebody is not directly visible in the seismic image, there is a lack of reflectivity coincident with the alteration surrounding the mineralization. We also observed reflections which likely originate at the contact between the altered and fresh basement rock located beneath the deposit. The seismic data further indicate post-Athabasca faults in the vicinity of the orebody. Based on the initial seismic interpretation, the depth of the crown pillar was adjusted and the mine infrastructure moved away from areas interpreted to be affected by the intense hydrothermal alteration surrounding the deposit. The capability to image the unconformity, post-Athabasca structure, and hydrothermal alteration also highlights the potential use of seismic surveys in uranium exploration.

Fore-arc mantle wedge seismicity under northeast New Zealand

Relocated upper mantle seismicity, using a 3D velocity model, delineates three tightly clustered (about 12 km diameter) zones of seismicity that occur in the mantle wedge under the fore-arc region of the northern Hikurangi margin, North Island, New Zealand. These clusters extend from the subducting plate at about 55 km depth up to about 35 km, and coincide with strong seismic reflectivity recorded active source seismic reflection measurements, suggesting a fluids association. They have maximum magnitudes of about Mw 4.5. The clusters correspond closely with a change from low to high Vp and Qp, and with average Vp/Vs that is interpreted to be caused by the transition from serpentinised to normal peridotite. The location of this transition appears to correspond to the locus of dehydration of peridotite, suggesting that the seismicity is caused by dehydration embrittlement. The release of these fluids where the serpentinised mantle wedge reaches the locus of dehydration may provide a mechanism for transferring additional fluids or fluid fluxed partial melt into the adjacent back-arc basin and facilitate the generation of voluminous magmas that are erupted there. The occurrence of serpentinite in the fore-arc mantle under northeastern North Island may influence the shallow extent of interplate coupling under the region. The location and spacing of the clusters is consistent with thermally driven diapiric or fluid upwellings, but they also show a spatial relationship with proposed lithospheric fractures and associated volcanic seamounts on the subducting Hikurangi plateau.

Backarc basin oceanic crust and uppermost mantle seismic velocity structure of the Shikoku Basin, south of Japan

Seismic refraction and reflection measurements were made along three profiles across the Shikoku Basin, one of the three backarc basins on the Philippine Sea plate. The P-wave velocity models show the presence of a very large horizontal irregularity related to the seamounts on the extinct spreading center, transition zones from the Shikoku Basin to Kyushu-Palau Ridge, and from the Shikoku Basin to the Izu-Ogasawara Island arc. However, the other areas were found to have features similar to those of a normal oceanic crustal model composed of oceanic Layer 2 with a high velocity gradient and Layer 3 with a small velocity gradient. The Shikoku backarc basin oceanic crust was found to be characterized by a thinner igneous crust, especially in Layer 3, and the azimuthal anisotropy in the uppermost mantle expected from the seafloor spreading was not detected.

Bayesian evidence computation for model selection in non-linear geoacoustic inference problems

This paper applies a general Bayesian inference approach, based on Bayesian evidence computation, to geoacoustic inversion of interface-wave dispersion data. Quantitative model selection is carried out by computing the evidence (normalizing constants) for several model parameterizations using annealed importance sampling. The resulting posterior probability density estimate is compared to estimates obtained from Metropolis-Hastings sampling to ensure consistent results. The approach is applied to invert interface-wave dispersion data collected on the Scotian Shelf, off the east coast of Canada for the sediment shear-wave velocity profile. Results are consistent with previous work on these data but extend the analysis to a rigorous approach including model selection and uncertainty analysis. The results are also consistent with core samples and seismic reflection measurements carried out in the area.

Seismic layer 2A variations in the Lucky Strike segment at the Mid‐Atlantic Ridge from reflection measurements

The SISMOMAR experiment carried out seismic measurements of the slow spreading Lucky Strike segment of the Mid‐Atlantic Ridge, located approximately 300 km south of the Azores platform. We present results from a segment scale reflection study of seismic layer 2A that is centered on the Lucky Strike volcano and covers the central 54 km of the 70 km long segment and extends 30 km on both sides of the ridge axis. Our study allows new conclusions about the role of tectonic, magmatic and hydrothermal processes in shaping the upper crustal structure of the Lucky Strike segment. The seismic reflection measurements show an almost constant layer 2A two‐way time within the median valley and an abrupt layer 2A two‐way time decrease off‐axis. This two‐way time decrease is caused by a layer 2A velocity increase on the order of 1 km/s. The uniform two‐way time in the median valley results from a constant thickness layer 2A, which could be caused by the existence of a porosity threshold linked to the lithologic lava/dike boundary. The regularity of the layer 2A/2B interface within the median valley indicates that layer 2A is built entirely inside the median valley. The off‐axis velocity increase is consistent with hydrothermal alteration and the aging of the crust.

Integrated inversion of TEM and seismic data facilitated by high penetration depths of a segmented receiver setup

ABSTRACTResearch aimed at improving and developing methods for noise reduction in the transient electromagnetic method (TEM) has resulted in an alternative strategy for performing TEM measurements called the segmented receiver coil setup. The measurement strategy involves the simultaneous use of two receiver coils and the resulting TEM data sets have demonstrated signal‐to‐noise ratio improvements of up to a factor of 25 when compared to the traditional setup. This significant improvement has opened up new opportunities for deeper penetration and hence an enhanced integrated inversion with seismic data sets. A case study is presented in which the segmented receiver coil setup is employed in the western part of Denmark. Previous geophysical investigations performed in the same area include multi‐channel reflection seismic measurements and SkyTEM measurements performed with an early version of the SkyTEM instrument having a limited depth of investigation. Deep structures recognized in the seismic data therefore remained unresolved in the SkyTEM data. Notably the elevation of the highly conductive Palaeogene clay, expected to be encountered at approximately 300 m depth in the area, has not been determined by the use of SkyTEM data. With the increased signal‐to‐noise ratio of the segmented receiver coil setup it is possible to resolve resistivity changes to greater depth and thus to achieve an enhanced integrated inversion together with the seismic data. The geological setting between depths of 200–300 m, which is effectively only mapped two‐dimensionally along seismic lines, can be mapped three‐dimensionally using the segmented receiver coil setup. In order to obtain the most reliable geological information, from the TEM data individual soundings are inverted in 1D utilizing the seismic data as a priori information thereby optimizing every single inversion model setup to the local sedimentary stratification. Ultimately, the larger penetration depth is the key to an improved geological understanding of the study area, because the integrated interpretation of seismic and TEM data sets yields valuable lithological and structural information that cannot be resolved by either data type alone.

Geoacoustic Inversion for the New Jersey Shelf: 3-D Sediment Model

A 3-D model of sediment sound speed for a 90-km <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area on the New Jersey shelf was constructed by application of a geoacoustic inversion technique. This approach is based on a combination of seismic reflection measurements and a perturbative inversion scheme using horizontal wave number estimates. In a two-step process, seismic reflection measurements were used to identify depths at which discontinuities in the sound-speed profile (SSP) likely occur. Then, the perturbative inversion algorithm made use of this <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a priori</i> information by employing qualitative regularization, an optimal method for addressing stability and uniqueness issues associated with solving the ill-posed inversion problem that provides for resolution of the layered seabed structure.

Depth, age and dynamic topography of oceanic lithosphere beneath heavily sedimented Atlantic margins

Our understanding of the thermal structure of oceanic lithosphere is based upon observations of water-loaded subsidence and plate age. These observations are predominantly from areas with sparse sedimentary cover, which are unperturbed by crustal thickening and by regional epeirogeny. Few such measurements are available from old oceanic lithosphere where sediment thickness is large. In order to address this omission, we have analyzed a legacy dataset of seismic reflection and wide-angle profiles from the margins of the Atlantic Ocean. Our aims are twofold. First, we determine the isostatically corrected age–depth variation of old oceanic lithosphere adjacent to continental margins and compare it with existing trends. Secondly, we isolate residual depth anomalies and compare them with dynamic topographic predictions made from long-wavelength gravity anomalies. Within the sedimentary pile, we have used a suite of coincident seismic reflection and wide-angle measurements to create a method for converting two-way travel time into depth and sediment density. We also correct for changes in oceanic crustal thickness. On the oldest portions of the Atlantic Ocean, the corrected age–depth variation is consistent with results from the Pacific Ocean, which suggests that the oceanic plate has a thickness of 90–95 km. Positive and negative deviations from the global age–depth trend are common and can be as large as ± 1 km. They mostly (but not always) correlate with long-wavelength gravity anomalies if the admittance is ~ 30 mGal km − 1 . We propose that these deviations are primarily generated by convective circulation within the upper mantle.