Multichannel Seismic Research Articles

Dipping reflectors in the Norwegian Sea—ODP Leg 104 drilling results

The following is an extended abstract of a paper presented at the Marine Studies Group meeting ‘Structure of Continent-Ocean Boundaries’ , 11 December 1985. Ocean Drilling Program (ODP) Leg 104 successfully completed a number of deep drill holes on the Outer V0ring Plateau and the Vering Basin during July and August 1985 (Fig. 1; Eldholm, Thiede, Taylor et al. 1986). One of the principal objectives of the leg was to drill and sample a thick oceanward-dipping wedge of seismic reflectors, known to characterize much of the multichannel seismic (MCS) profile data recorded across ocean-continent transitions in the NE Atlantic north of 55°N. Elsewhere, these reflectors have previously been the target of deep drilling during Legs 48 and 81 of the International Phase of Ocean Drilling IPOD (Montadert, Roberts et al. 1979; Roberts, Schnitker et al. 1984). These efforts met with some success, sampling only the uppermost part of the sequence and identifying a series of tholeiitic lava flows. Over the Outer V~ring Plateau, a number of MCS profiles reveal a distinct change in seismic character at the base of the well-stratified dipping reflector sequence, where an irregular surface characterized by a band of low-frequency, high-amplitude reflectors occurs. This surface is referred to as K. Prior to Leg 104, therefore, relatively little was known regarding the variation in petrographic character of the flows at depth, their evolution and origin, and particularly the character of the material below the reflector sequence. A single deep drillhole (642E) successfully recovered a section through the

Structure of the Kerguelen Plateau province from Seasat altimetry and seismic reflection data

The Kerguelen Plateau in the southern Indian Ocean stands 2–4 km above the adjacent sea floor and is ∼2,500 km long (Fig. 1)1,2. Seasat provided a unique data set for deriving the free-air gravity field of the region (Fig. 2)3–5. Here we report the results of a new analysis of the plateau province's structure employing both Seasat and newly-acquired multichannel seismic (MCS) data6,7 for ground truth. The northern sector is characterized by volcanism and a sedimentary basin; the southern sector by a broad anticlinal arch, major faulting, and a sedimentary basin; and the eastern sector by an abyssal basin (Labuan) and bounding ridge (William's). The three sectors argue for a more complex tectonic evolution of the feature than has been previously proposed.

Marine Multichannel Seismology Workshop

The multichannel seismic (MCS) reflection technique, developed by the oil industry for petroleum exploration in sedimentary basins, has proven to be a powerful tool for imaging subsurface geology in a wide variety of tectonic settings at a scale suitable for detailed investigations of geological structures and processes. In the ocean basins, MCS studies have provided new insight into the tectonic history of rifted and convergent continental margins, the structure of the oceanic crust and midocean ridges, and the sedimentation history and paleoceanography of deep ocean basins. MCS techniques have thus developed into an important tool for marine geological and geophysical research.The National Science Foundation recently sponsored a Workshop on the Future of Academic Marine Multichannel Seismology in the United States, held in Boulder, Colo., on March 19–20, 1984, to review the current state of marine academic MCS in the United States and to make recommendations on the facilities and funding required to meet future scientific needs. The workshop, which was convened by Brian T.R. Lewis of the University of Washington, included 19 scientists representing the major U.S. oceanographic institutions with interests in marine seismic work. This article summarizes the major recommendations developed at this workshop, which have been included in a more comprehensive report entitled ‘A National Plan for Marine Multichannel Seismology,’ which has been submitted to the National Science Foundation for future publication.

Seismic Stratigraphy and Sedimentation of Magdalena Fan, Southern Colombian Basin, Caribbean Sea

Analysis of all available seismic data from the Magdalena Fan in the southern Colombian basin, Caribbean Sea, allows subdivision of the sedimentary section into six seismic sequences (units). Although sediments were deposited in the present-day Magdalena Fan region since about Late Cretaceous, terrigenous sedimentation became significant only in the late Cenozoic during deposition of the upper three sequences associated with the uplifts of the Andes. These upper three sequences comprise the Magdalena Fan proper. The uppermost sequence probably represents the last main phase of sedimentation subsequent to the major uplift of the Andes in the Pliocene. The morphologic and shallow acoustic (3.5 kHz) characteristics of this fan unit are: upper fan, 1:60-1:110 gradients, chann ls having well-developed levees, and several subbottom reflectors in all areas except in channels; middle fan, 1:110-1:200 gradients, numerous channels with very subdued levees, and several to few subbottom reflectors; lower fan, <1:250 gradients, small channels, relatively smooth sea floor, and few or no subbottom reflectors. The decrease in number of subbottom reflectors as well as in subbottom penetration downfan apparently results from increasing amounts of coarse-grained sediments. Features in the form of regular hyperbolic echoes and sediment waves are very common in the upper, middle, and to some extent the lower fan, although their heights gradually decrease downfan. On multichannel seismic (MCS) records, the upper fan exhibits conspicuous channel-levee development and coalesc ng wedge-shaped reflection patterns indicative of levee deposits. The middle fan is characterized by the presence of chaotic and discontinuous reflection patterns, which may have resulted from the presence of numerous channels, and hyperbolic features and sediment waves of the type recorded on 3.5 kHz records. The lower fan region has relatively flat, continuous reflections. Within the topmost seismic unit, several episodes of sedimentation can be inferred from MCS records. These episodes are probably related to uplifts in the source region, lowered sea levels, or shifting of the Magdalena River delta in space and time, subsequent to the major orogeny in middle Pliocene. A model of sedimentary processes proposed on the basis of the characteristics described visualizes a dominant role for channelized and overbank turbidity current deposition in the upper fan. The influence of these currents decreases and that of the unchannelized currents increases downfan. On the lower fan, deposition by unchannelized turbidity sheet flows forms the dominant mode of sedimentation. In addition, slumping on the back sides of oversteepened levees and along broad fronts of the continental slope, and other downslope mechanisms may also have influenced fan sedimentation significantly.

Cenozoic and late Mesozoic stratigraphy and subsidence history of the Norwegian margin

The stratigraphy of the passive continental margin off Norway is mapped, utilizing both Deep Sea Drilling Project (DSDP) data at five sites on the outer margin and multichannel seismic (MCS) data collected by Lamont-Doherty Geological Observatory. Distinct units in the acoustic stratigraphy are tied to biostratigraphic and lithostratigraphic units in the drill data and mapped throughout most of the margin. Stratigraphy of the pre–lower Eocene section not encountered in drilling is inferred from depositional patterns and correlation with the continental-shelf sequences, the stratigraphy of which has been developed independently from detailed surveys. The margin can be divided into three physiographic provinces: two broad shelf and slope regions separated by the Jan Mayen Fracture Zone, and the steep, narrow shelf and slope off the Lofoten Islands. Each experienced a separate Cenozoic subsidence history. The Voring Plateau is partly underlain by oceanic crust; it subsided nearly 1,000 m less than expected from thermal contraction, but it did so following the development of a relatively deep-water environment in the Paleocene. North of the Voring Plateau, the margin may have subsided a few hundred metres more than thermal studies had predicted. There is no evidence for a phase of extensional deformation associated with the inception of spreading. The youngest faulted level occurs at the Late Jurassic level, preceding sea-floor spreading by nearly 100 Ma, and deformation in the Cretaceous section is characterized by large uplift structures that may be of halokinetic origin. The physiographic provinces as well as the Cenozoic subsidence patterns along the margin appear to be related to the style and extent of Mesozoic basin development and tectonics along the margin.

Morphologic, acoustic, and sedimentologic characteristics of the Magdalena Fan

The Magdalena Fan can be divided into: upper fan—1:60–1:110 gradients, channels with well-developed levees, generally several subbottom reflectors on 3.5-kHz records, and fine-grained sediments; middle fan—1:110–1:200 gradients, channels with very subdued levees, several to few subbottom reflectors on 3.5-kHz records, and chaotic and discontinuous reflections on multichannel seismic (MCS) records; lower fan—<1:250 gradients, small channels and relatively smooth seafloor, generally coarsegrained sediments, few or no subbottom reflectors on 3.5-kHz records, and flat continuous reflections on MCS records. In addition to the turbidity currents, slumping along the continental slope and elsewhere also influenced sedimentation in the fan.

Morpho-acoustic and sedimentologic characteristics of the Indus Fan

The upper Indus Fan is characterized by an average 1∶500 gradient, chanels with 100 m high levees, several continuous subbottom reflectors on 3.5-kHz records, and generally fine-grained sediments. Multichannel seismics show the levee complexes typified by overlapping wedge-shaped reflection sets and channel axis by high-amplitude discontinuous reflections. The middle fan has 1∶500–1∶1000 gradients and channels with ≈20 m high levees. The lower fan has gradients less than 1∶1000, channels with 8–20 m high levees, few or no subbottom reflectors on 3.5-kHz records, and high sand content. Besides the dominant unchannelized turbidity currents, channelized and overbank flows also played a significant role in the sedimentation of the lower fan.

Stratigraphy and Structure of Southern Blake Plateau, Northern Florida Straits, and Northern Bahama Platform from Multichannel Seismic Reflection Data

Approximately 2,100 km of 24-fold multichannel seismic reflection data reveal much about the subsurface geology for a large part of the continental margin east of Florida. Discordance between the westward-dipping pre-breakup sediments and the eastward-sloping basement along the edge of the Blake Plateau is interpreted as an effect of a splinter of continental margin derived from the African plate by a spreading-center jump in the Middle Jurassic. Early rifting centered under the main part of the Blake Plateau became inactive, as a spreading-center jump shifted the active rift to east of the present Blake Escarpment along the Blake Spur magnetic anomaly. In the northern Florida Straits the data reveal that the breakup unconformity, underlain by Triassic-Lower Jurassic(?) arkosic volcaniclastics, extends from southern Florida to the western Bahama Banks. These volcaniclastics are associated with the rift-crust of intermediate nature formed just prior to and during breakup of the North American and African continental plates. Back-reef platform deposits of limestones, dolomites, and evaporites of Late Jurassic to Albian age extend from the Blake-Bahama Escarpment westward beneath Florida. These deposits formed what once was a megabank extending over a wider area than the present smaller isolated Bahama Banks. The formation of the Florida Straits and Bahama channels occurred during the Cenomanian transgression. Only on the present Bahama Banks and Florida platform did shallow-water carbonate deposition persist to maintain a shallow-bank environment. Evidence of recurring scour by current erosion is found in the Florida Straits. Erosional events apparently occurred in the middle Cenomanian, middle Paleocene, early-middle Eocene, and Eocene-Oligocene, which coincidentally are times of lower eustatic sea level according to Vail et al (1977a). This evidence of Florida current scour indicates that the current was present as far back as the Cenomanian. Major faulting appears to have dropped the Northeast Providence Channel relative to the western Bahamas after the Albian. Submarine erosion and bank buildup created the channels and smaller relief features like Great Abaco Knoll beginning in about the Cenomanian. A carbonate bank margin and reef complex was present along the Bahama Escarpment since the Middle Jurassic. Apparently these organic buildups seeded on originally shallow structural relief on oceanic basement created during the spreading-center jump to the position of the Blake Spur magnetic anomaly. The bank margin apparently has retreated at least 15 km from a Late Jurassic-earliest Cretaceous position now marked by a bench below the Au unconformity. Active faulting occurred along the Great Abaco fracture zone at least through the Late Cretaceous and perhaps into the Tertiary. These relatively young tectonic events, together with the post-Albian faults in Providence Channel, indicate interactions between the Atlantic and Caribbean plates and that extensions of faulting have taken place far to the northeast of Cuba and the Greater Antilles.

Two‐Ship Multichannel Seismic Experiments for deep crustal studies: Expanded spread and constant offset profiles

Expanding spread profiles (ESP's) are spatially localized, wide‐angle reflection/refraction surveys using two ships, a multichannel array, and Raydist ranging. The value of the data is primarily the fidelity with which the data are recorded and the multiplicity of coverage provided by the multichannel array. We feel that the ESP technique has several distinct advantages over other methods of deep crustal studies. These are (1) very accurate range and timing information, (2) fixed and well‐known receiver separation (100 m) for accurate phase velocity determination across the array, (3) because of accurate timing and range information, traces from different shots may be used to improve velocity determination (this is limited only by the persistence of the particular arrivals), (4) true amplitude recording that allows the use of sophisticated signal processing techniques and tightly constrains models of the observed data, (5) no difficulties with receiver coupling, (6) calibration of charge size changes through overlapping receiver positions, (7) a minimum amount of explosives is required because of the redundancy provided by the multichannel receiving array, (8) minimal deployment and recovery time coupled with low equipment loss rate, (9) ‘real‐time’ data monitoring and analysis to insure the quality of the recorded data, and (10) digital recording in a readily exchangeable standard format to facilitate the use of digital computers in reducing the vast amount of data acquired. Data of this quality in range, time, and amplitude are required for any detailed inversion or modeling scheme. Dense spatial sampling is required to avoid spatial aliasing, to demonstrate phase coherence between traces, and to observe amplitude variations over short distances. Constant offset profiles (COP's) are fixed range, wide‐angle reflection/refraction surveys using two ships, a multichannel array, and Raydist ranging. Using this technique, it is possible to develop a profile of refracting type interfaces which are normally absent from the reflection record. With earlier methods, these events are recorded only as ‘spot’ measurements, and lateral variation is only coarsely determined. Since constant offset profiles are multichannel array measurements at a specific range, variations in velocity, arrival time, and signal amplitude along the profile can be accurately determined. By combining these data with near‐vertical incidence multichannel data along the same track, a detailed survey of the deep ocean crust and particularly its lateral variability is possible. This experimental technique is the only method available for a detailed continuous mapping of lateral crustal variations from nonreflecting horizones.

The Case for Using Multichannel Seismic Refraction Equipment for Site Investigation

Research Article| January 01, 1978 The Case for Using Multichannel Seismic Refraction Equipment for Site Investigation S. J. BULLOCK S. J. BULLOCK S. J. Bullock is with R. F. Loxton, Hunting and Associates, Bramley, Johannesburg. Search for other works by this author on: GSW Google Scholar Environmental & Engineering Geoscience (1978) xv (1): 19–35. https://doi.org/10.2113/gseegeosci.xv.1.19 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation S. J. BULLOCK; The Case for Using Multichannel Seismic Refraction Equipment for Site Investigation. Environmental & Engineering Geoscience 1978;; xv (1): 19–35. doi: https://doi.org/10.2113/gseegeosci.xv.1.19 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract The seismic refraction method as applied to site investigation is frequently regarded by the professional engineer as useful, but of uncertain accuracy.This paper presents for the nongeophysicist the basic principles, applications and limitations of the method using both single and multichannel seismic units with either a hammer or explosive energy source. Many of the operational problems of obtaining good quality records are not well appreciated and neither are many of the pitfalls frequently encountered during the interpretation stage. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.