High-resolution Seismic Survey Research Articles

Lacustrine sedimentary records of brutal events in Lake Le Bourget (Northwestern Alps-Southern Jura) [ Enregistrements d'événements brutaux dans la sédimentation lacustre du Lac du Bourget (Alpes occidentales-Jura méridional).

Lake Le Bourget is a glacial lake, situated in a tectonically active region. The Wûrm glaciers scoured out an elongated overdeepened basin of more than 300 m below present lake level and the lake basin accumulated almost 150 m of post-glacial to Holocene sediments. The sediment infill of the basin was studied by means of two high resolution seismic surveys, in 1991 and in 1993. Additionally, a combined side scan sonar and bathymetry survey provided a detailed image of the lake floor topography. The sediment series clearly illustrate the evolution of the depositional environment with changing climate since the last déglaciation. The well-stratified character of the infill is in some places disturbed by faulting and sediment instabilities. A southern fault zone can be related to the sub-lacustrine continuation of a well-known basement fault. A northern fault zone is characterized by listric faulting, affecting the sub-lacustrine Rhône fan. The sediment instability deposits occur along the lake borders and near the delta regions, as the result of thick sediment accumulation on the sub-lacustrine slopes, prone to re-mobilisation, and the occurence of earthquakes triggering the sediment slides. At present, lake floor sediment deformation by recent to historical earthquakes is visible in the form of rectilinear scarps and fluid expulsion craters.

Palaeoceanographic information derived from acoustic surveys of glaciated continental margins: examples from eastern Canada

Abstract Much palaeoceanographic information can be gleaned from seismic, sidescan and bathymetric data on glaciated continental shelves. High-resolution seismic reflection surveys of regional ice terminus features define palaeo-ice margins, datable using interfingering ice-proximal glacimarine deposits. Isopach maps show differences in regional accumulation rates of sediments during deglaciation phases and help locate positions of former glacifluvial discharge outlets. Maps of ice-distal glacimarine muds suggest that surface currents carrying buoyant plumes of glacial flour may extend >100 km from an ice margin. Transport and melt of icebergs controlled by circulation of deeper outer shelf and slope currents and episodic storms can result in a glacial influence >1000 km from the ice margin, but most iceberg-rafted sedimentation occurs within a few hundred kilometres of calving. The direction of these palaeo-mid-depth currents may be obtained from statistical analysis of relict iceberg scour patterns and from sediment dispersal patterns. The geometry of glacimarine deposits commonly suggests that there was water column stratification with associated weak near-seafloor currents. Comparison of the shape of glacimarine and post-glacial sediment packages shows that in many places the tidal current intensity has increased through the Holocene, reflecting sea-level change on the shelf.

20th-century glacial-marine sedimentation in Vitus Lake, Bering Glacier, Alaska, U. S. A.

Vitus Lake, the ice-marginal basin at the southeastern edge of Bering Glacier, Alaska, U.S.A., is a site of modern, rapid, glacial-marine sedimentation. Rather than being a fresh-water lake, Vitus Lake is a tidally influenced, marine to brackish embayment connected to the Pacifie Ocean by an inlet, the Seal River. Vitus Lake consists of five deep bedrock basins, separated by interbasinal highs. Glacial erosion has cut these basins as much as 250 m below sea level. High-resolution seismic reflection surveys conducted in 1991 and 1993 of four of Vitus Lake’s basins reveal a complex, variable three-component acoustic stratigraphy. Although not fully sampled, the stratigraphy is inferred to be primarily glacial-marine units of (1) basal contorted and deformed glacial-marine and glacial sediments deposited by basal ice-contact processes and submarine mass-wasting; (2) acoustically well-stratified glacial-marine sediment, which unconformably overlies the basal unit and which grades upward into (3) acoustically transparent or nearly transparent glacial-marine sediment. Maximum thicknesses of conformable glacial-marine sediment exceed 100 m. All of the acoustically transparent and stratified deposits in Vitus Lake are modern in age, having accumulated between 1967 and 1993. The basins where these three-part sequences of “present-day” glacial-marine sediment are accumulating are themselves cut into older sequences of stratified glacial and glacial-marine deposits. These older units outcrop on the islands in Vitus Lake.In 1967, as the result of a major surge, glacier ice completely filled all five basins. Subsequent terminus retreat, which continued through August 1993, exposed these basins, providing new locations for glacial-marine sediment accumulation. A correlation of sediment thicknesses measured from seismic profiles at specific locations within the basins, with the year that each location became ice-free, shows that the sediment accumulation at some locations exceeds 10 m year−1.

20th-century glacial-marine sedimentation in Vitus Lake, Bering Glacier, Alaska, U. S. A.

Vitus Lake, the ice-marginal basin at the southeastern edge of Bering Glacier, Alaska, U.S.A., is a site of modern, rapid, glacial-marine sedimentation. Rather than being a fresh-water lake, Vitus Lake is a tidally influenced, marine to brackish embayment connected to the Pacifie Ocean by an inlet, the Seal River. Vitus Lake consists of five deep bedrock basins, separated by interbasinal highs. Glacial erosion has cut these basins as much as 250 m below sea level. High-resolution seismic reflection surveys conducted in 1991 and 1993 of four of Vitus Lake’s basins reveal a complex, variable three-component acoustic stratigraphy. Although not fully sampled, the stratigraphy is inferred to be primarily glacial-marine units of (1) basal contorted and deformed glacial-marine and glacial sediments deposited by basal ice-contact processes and submarine mass-wasting; (2) acoustically well-stratified glacial-marine sediment, which unconformably overlies the basal unit and which grades upward into (3) acoustically transparent or nearly transparent glacial-marine sediment. Maximum thicknesses of conformable glacial-marine sediment exceed 100 m. All of the acoustically transparent and stratified deposits in Vitus Lake are modern in age, having accumulated between 1967 and 1993. The basins where these three-part sequences of “present-day” glacial-marine sediment are accumulating are themselves cut into older sequences of stratified glacial and glacial-marine deposits. These older units outcrop on the islands in Vitus Lake.In 1967, as the result of a major surge, glacier ice completely filled all five basins. Subsequent terminus retreat, which continued through August 1993, exposed these basins, providing new locations for glacial-marine sediment accumulation. A correlation of sediment thicknesses measured from seismic profiles at specific locations within the basins, with the year that each location became ice-free, shows that the sediment accumulation at some locations exceeds 10 m year−1.

Refined field static corrections in near‐surface reflection profiling across rugged terrain

Deriving effective static (elevation and weathering) corrections is more difficult for near‐surface high resolution seismic reflection surveys than for deeper profiling investigations; reflections tend to be less continuous and frequencies much higher in the former. In his three‐part tutorial series on static corrections (TLE, January‐March 1993), Dave Marsden provides a comprehensive account of problems associated with computing land‐based static corrections and proposes various methods for resolving them. In particular, he stresses the importance of using intermediate floating datums in regions of floating topography in order to keep field static corrections as small as possible before the calculation and application of NMO corrections. Furthermore, he suggests a running average of receiver elevations over the active spread length as an appropriate intermediate floating datum. After application of NMO, further time shifts to bring the data from the floating datums to final flat datums may be applied; th...

Seismic reflection profiles across the "Mine Series" in the Noranda camp of the Abitibi belt, eastern Canada

The Abitibi–Grenville Lithoprobe project completed a regional (line 21) and a high-resolution (line 21-1) seismic survey in the Noranda Central Volcanic Complex of the Blake River Group, Abitibi, Quebec. Line 21 provides a regional framework in which the Archean crust is divided into three layers, two of which are discussed here: the uppermost layer, which corresponds to the Blake River Group, is the least reflective, and lies above 4 s (12 km), and the mid-crustal layer, which is composed of a complex pattern of generally east-northeast-dipping reflectors and lies between 4 and 8 s. Within the regional data, the Mine Series of the Central Volcanic Complex is imaged as a semitransparent series of reflectors overlying a highly reflective east-facing structure interpreted as the subvolcanic Flavrian pluton. The high-resolution data (line 21-1) were collected in the vicinity of the Ansil mine. The seismic images in this region can be controlled by surface geology and extensive drill-hole data, and the project was designed to test the applicability of seismic reflection profiling in providing structural and stratigraphic information for use in mineral exploration: shallow-dipping reflectors correlate well with lithological variations or contacts in the volcanic sequence; strong subhorizontal reflectors correspond to diorite and gabbro dykes and sills; several abrupt lateral changes in the reflectivity coincide with known intrusive contacts such as the Lac Dufault pluton.

Expulsion of Shallow Gas in the Skagerrak—Evidence from Sub-bottom Profiling, Seismic, Hydroacoustical and Geochemical Data

A combined high resolution seismic, sub-bottom profiling, and multi-beam echo-sounding survey in the Skagerrak (Danish sector of the North Sea) together with gas analyses at a station along the profile exhibit the expulsion of gas (mainly methane) and the presence of gas-charged sediments at shallow depth. The echo-soundings yield detailed insight into the distribution and shape of typical sea-floor features associated with gas seepage, such as pockmarks. The pockmarks reach dimensions of 800 m in length, 300 m in width, and 15 m in depth, with the long axis running parallel to the slope of the Norwegian Trench. Processing of the multi-channel high resolution seismic data and the digitally recorded sub-bottom profiler signals indicate an internal compressional velocity of about 1050 m s-1 within the gas-charged sediments reaching from the sea-floor to a sub-bottom depth of about 23 m. Using the lateral distribution and thickness of the gas-charged sediments in conjunction with a mean concentration of gas of 3000 ppb, the present amount of trapped gas is estimated to be 6·45 × 1011 g CH4. The flux of methane through the sea-bed into the water column appears to be 7·2 × 1010 g CH4 per year. To explain the small difference in size between the methane pool in near-surface sediments and the annual flux through the sea-bed, a constantly high supply of methane from leaking hydrocarbon reservoirs at greater depths has to be active.

Late Wisconsinan and Holocene depositional history, northeastern Gulf of Mexico

High-resolution seismic surveys and sedimentologic studies in the northeastern Gulf of Mexico reveal multiple late Quaternary fluvial erosion surfaces. The Apalachicola River, largest river in the northeastern Gulf, incised a network of large channels which today lie buried 5–40 m beneath the present-day inner shelf and the estuary of the modern river. An abundance of coastal and estuarine borehole data enables the seismic records to be quantified to yield paleohydrologic information. The paleohydrologic data show that the Apalachicola was a river of significantly higher discharge before the late Holocene, prior to migrating eastward to its present location. Available evidence indicates that the Apalachicola Delta began prograding in its present location at least 2000 years ago. Much of the near-surface sedimentologic and geomorphologic features of the inner shelf of the northeastern Gulf of Mexico, including perhaps the Florida Middle Ground reef tract, are the result of earlier episodes of migration and delta-building by the Apalachicola River.

Holocene coastal depositional sequences on a tectonically active setting: southeastern Tauranga Harbour, New Zealand

More than 70 cores, a high-resolution seismic survey, and SCUBA observations provide the basis for the interpretation of depositional environments that accumulated during the Quaternary in the southeastern Tauranga Harbour area of the North Island, New Zealand. Three lithofacies comprise this sequence; in ascending order they are pumiceous sand and gravel, shelly mud and shelly sand. The pumiceous sand is interpreted as fluvial and fan deposits of Pleistocene to early Holocene age with a radiometric date of 9420 ± 100 yr BP near the top of the unit. The shelly mud represents low-energy estuarine deposition of essentially normal marine salinity in a valley-like setting. This unit dates at 8100 ± 80 yr BP. The extensive overlying shelly sand thickens seaward and represents wave-dominated shoreface conditions much like the present nearshore environment. Radiometric dating of samples within the present harbor are all between 6000 and 7000 yr BP and those seaward of the spit to Mt. Maunganui are less than 3370 ± 100 yr BP. The barrier spit that has attached to the volcanic headland began accumulating about 4000–5000 years ago.

High heat flow anomalies over salt structures on the Texas Continental Slope, Gulf of Mexico

We made 74 closely spaced (< 2 km apart) heat flow measurements around and over two salt structures on the Texas continental slope, Gulf of Mexico. The values outlined the shape of the heat flow anomalies over both structures. Based on a preceding high resolution seismic survey, we interpreted these structures to be a cylindrical plug and a salt tongue extending from the crest of a wall‐shaped feeder. The heat flow observations clearly reflect differences between the two features and are consistent with the prior structural interpretation. The values over the salt plug are nearly all greater than 70 mW/m². The measurements over the salt tongue have a sharp heat flow peak of 90 mW/m² associated with the presumed feeder and rather uniform values around 60 mW/m² over the remainder. The variation of heat flow over both structures is smooth and shows no apparent scatter. Heat flow values off these features are uniformly low, around 30 mW/m². Thermal effects from bottom water temperature fluctuation, slope sedimentation, diapiric movement of the salt body, and pore fluid migration appear unable to provide a satisfactory explanation for the observations. However, thickness variations of a highly conductive salt body can easily account for the heat flow anomalies. We suggest that modeling of the conductive anomaly should provide substantial constraints on the bottom geometry of the salt.

High resolution seismic for resolving coal seam structure in difficult terrain

This paper describes a high-resolution seismic survey in the Southern Coalfields of New South Wales over rugged terrain in areas yet to be mined. The aim of the survey was to identify faults in the working seam, which are very disruptive to longwall mining operations. Rapidly changing topography with deep gorges, swamps and dense bush create access and environmental difficulties for seismic operations. For the first time, attempts have been made to acquire seismic data across difficult gorge and swamp country. The techniques used were successfully adapted from previous surveys carried out in less difficult terrain. Current indications are that there will be minimal impact on the environment. The data collection and processing parameters developed enabled good coherent data to be obtained across deeply incised terrain. Faults with various throws have been interpreted.

High-resolution seismic reflection survey of a massive sulfide orebody in Sardinia, Italy — An experiment

A first attempt to investigate the application of the high-resolution seismic reflection technique in massive ore exploration was carried out in Sardinia (Italy). A mixed sulfide mineralized orebody (pyrite and sphalerite) within Cambrian carbonate rocks in the Campo Pisano mining area (southwest Sardinia) was investigated. The seismic profile data, obtained using a Geometrics ES-2420 seismograph, were subsequently processed on a Digital VAX/VMS computer with the CogniSeis software seismic package DISCO. On the basis of this analysis, it is possible to delineate with the seismic reflection technique, the contact surfaces between the mineralized orebody and surrounding rock, on condition that an accurate selection of the acquisition parameters and appropriate data processing be made.

Geotechnical properties of soft clays from the Witch Ground Basin, central North Sea

Abstract The Witch Ground Basin to the northeast of Peterhead is occupied by Late to Postglacial clays which reach a maximum thickness of over forty metres. High resolution seismic surveys have shown that the Sediments can be divided into two stratigraphic units on the basis of their acoustic signatures: the upper Witch Ground Formation shows in its lower part a finely detailed pattern of laterally continuous multiple reflectors, whereas the lower Swatchway and Coal Pit Formations present a disorganised signature in which few reflectors are continuous. Profiles of bulk density, water content and undrained shear strength have been obtained from two continuously sampled, adjacent boreholes which penetrated the sediments to a depth of forty metres. These show that the deposits of the Witch Ground Basin comprise a single geotechnical unit with the properties expected of a fine grained sediment of glaciomarine origin. The principal geotechnical properties change gradationally as a result of changes in composition and from selfweight compression. The different seismic signatures on which the basin stratigraphy is based are not reflected by major geotechnical changes within the sediment, but may well result from secondary features of the geotechnical profile.

An evaluation of seismic and borehole data available from onshore and offshore site investigations of relict glaciated areas

Abstract Using late Quaternary ice marginal sites from the North Sea and Iceland a comparison of the sedimentary information available from subaerial and submarine site investigations was made. This was with regard to palaeo-environmental reconstructions of ice marginal Sedimentation processes. A detailed high resolution seismic survey in-the North Sea was compared to a similar subaerial site in Iceland. Submarine cores were compared with logged sections available from cliffs. These comparisons highlighted the differences in resolution of the structural information available and also the problems of applying existing modeis of glacimarine Sedimentation to offshore sites. This has raised questions concerning the accuracy of reconstructing palaeo-environments and processes in offshore sites presently under investigation. The implications of this for engineering projects in offshore areas requiring very precise site information about relict glaciated areas are significant.

Acoustical and geological characteristics of near-surface sediments, upper continental slope of northern Gulf of Mexico

The continental slope off Texas and Louisiana is the most structurally and sedimentologically complex area in the Gulf of Mexico. This 120,000 km2 area is dominated by diapiric tectonics, resulting in numerous faults, oil and gas seeps, and other geological phenomena that affect near-surface sediments. Bottom gradients range from 0 to 20°. High-resolution seismic surveys, foundation borings, and drop cores reveal several mass-movement features, acoustic wipeout zones, sea floor erosion, faults, hardgrounds, bioherms, reefs, and outcrops. Recent data indicate that most geological activity takes place during relative sea level lowering and low stands.

Stratigraphic mapping of sedimentary formations in southern Ontario by ground electromagnetic methods

Multifrequency and transient electromagnetic (EM) soundings were done at several locations in southern Ontario to evaluate the possibility of stratigraphic mapping of gently dipping sedimentary formations by ground EM techniques. The possibility of detecting structural features such as folds, faults, and grabens in the formations was also explored. The two EM techniques were used to map a buried river valley near the village of Copetown, about 75 km southwest of Toronto. The valley, buried under fairly resistive glacial till and dolomite formations was mapped at depths ranging from 100 to 200 m. The interpreted shape and depth of the valley agreed well with information from a high resolution seismic reflection survey and data from wells penetrating the bedrock. Transient EM (TEM) data, using square transmitter loops with side dimensions comparable to the transmitter‐receiver separation for the multifrequency system, was found to be less affected by the presence of shallow conductors and lateral inhomogeneities than the multifrequency data, and had greater depth of investigation. TEM soundings were made on five additional profiles in the area. Lithologic logs from old oil and gas wells located near the profiles provided information for comparison with EM sounding interpretations. The depths to various dolomite, shale, and limestone formations interpreted from EM data agreed well with drillhole information. The EM soundings also detected structures in the Paleozoic formations such as faults and folds that were not previously known. The soundings confirmed the existence of a graben at one site at a depth of 100 m, which was postulated from logs from a cluster of closely spaced drillholes by geologists. The survey results indicated that EM sounding methods can be used for stratigraphic mapping in areas where detailed geological information is unavailable either because the bedrock is concealed by overburden, or when drillholes are sparsely distributed.

Solution‐collapse depressions and suspensates in the limnocrenic lake of banyoles (ne Spain)

AbstractA high resolution seismic reflection survey in the Banyoles limnocrenic solution lake allowed penetration of dense suspensates occupying cone‐like bottom depressions of different size. The depressions result from the dissolution and collapse of underlying Eocene calcareous and gypsiferous materials over which lacustrine sediments of varying thicknesses have accumulated.The suspensates occupying the depressions present three main types of seismic signatures: stratified, semistratified, and transparent. The densities of the suspensates and the water depths of their tops, which fluctuate continuously, vary from one depression to another. The maximum seismically recorded suspensate thickness is 44 m.Morphological and structural features, seismic characters, and variable degrees of hydraulic activity, point to the existence of different stages of maturity in the lake bottom depressions. This work brings new insight on the dynamics and evolution of limnocrenic solution lakes.

Side-Scan Sonar and Submersible Observations: New Techniques for Gleaning More Information from Sea-Floor Outcrops: ABSTRACT

Advances in high resolution side-scan sonar imaging technology are so effective at imaging sea-floor geology that they have greatly improved the efficiency of a bottom sampling program The traditional sea-floor geology methodology of shooting a high-resolution seismic survey and sampling along the seismic grid was considered successful if outcrops were sampled on 20% of the attempts. A submersible was used sparingly because of the inability to consistently locate sea-floor outcrops. Side-scan sonar images have increased the sampling success ratio to 70-95% and allow the cost-effective use of a submersible even in areas of sparse sea-floor outcrops. In offshore basins this new technology has been used in consolidated and semiconsolidated rock terranes. When combined with observations from a two-man submersible, SCUBA traverses, seismic data, and traditional sea-floor bottom sampling techniques, enough data are provided to develop an integrated sea-floor geologic interpretation. On individual prospects, side-scan sonar has aided the establishment of critical dip in poor seismic data areas, located seeps and tar mounds, and determined erosional breaching of a prospect. Over a mature producing field, side-scan sonar has influenced the search for field extension by documenting the orientation and location of critical trapping cross faults. These relatively inexpensive techniques can providemore » critical data in any marine basin where rocks crop out on the sea floor.« less

A High-Resolution Seismic Survey of the Holocene Section Offshore Part of the Dutch Mesotidal Barrier Shoreline: ABSTRACT

A High-Resolution Seismic Survey of the Holocene Section Offshore Part of the Dutch Mesotidal Barrier Shoreline: ABSTRACT

A reversal of throw and change of trend on the Wellington Fault in Wellington Harbour

Abstract A high-resolution seismic survey of the northwestern side of Wellington Harbour shows that the most recent trace of the predominantly dextral strike-slip Wellington Fault crosses the coastline in the vicinity of the Thorndon interisland ferry wharf and lies 250–400 m from shore off Kaiwharawhara. Between Ngauranga and Petone, recent dislocation can not be detected with certainty beneath shallow, almost seismically “opaque”, possibly methane-rich layers. The recent trace in Wellington Harbour cuts sediments deposited after the postglacial rise of sea level. It has a trend that is similar to the recent trace at Petone and about 10° clockwise from the recent trace behind Wellington City. The change in trend occurs close to the Thorndon wharf. An unusual, but not unique, feature is that the recent trace in the harbour is upthrown on the southeastern (harbour) side, whereas long-term movement has uplifted the northwestern (landward) side. Recent fault dislocation is defined by a scarp about 1.5 m high...