Fault Plane Reflections Research Articles

Seismic reflection imaging of the Juan de Fuca plate from ridge to trench: New constraints on the distribution of faulting and evolution of the crust prior to subduction

AbstractWe present prestack time‐migrated multichannel seismic images along two cross‐plate transects from the Juan de Fuca (JdF) Ridge to the Cascadia deformation front (DF) offshore Oregon and Washington from which we characterize crustal structure, distribution and extent of faults across the plate interior as the crust ages and near the DF in response to subduction bending. Within the plate interior, we observe numerous small offset faults in the sediment section beginning 50–70 km from the ridge axis with sparse fault plane reflections confined to the upper crust. Plate bending due to sediment loading and subduction initiates at ~120–150 km and ~65–80 km seaward of the DF, respectively, and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity. Most bend faulting deformation occurs within 40 km from the DF; on the Oregon transect, bright fault plane reflections that extend through the crust and 6–7 km into the mantle are observed. If attributed to serpentinization, ~0.12–0.92 wt % water within the uppermost 6 km of the mantle is estimated. On the Washington transect, bending faults are confined to the sediment section and upper‐middle crust. The regional difference in subduction bend‐faulting and potential hydration of the JdF plate is inconsistent with the spatial distribution of intermediate‐depth intraslab seismicity at Cascadia. A series of distinctive, ridgeward dipping (20°–40°) lower crustal reflections are imaged in ~6–8 Ma crust along both transects and are interpreted as ductile shear zones formed within the ridge's accretionary zone in response to temporal variations in mantle upwelling, possibly associated with previously recognized plate reorganizations at 8.5 Ma and 5.9 Ma.

Fluid accumulation along the Costa Rica subduction thrust and development of the seismogenic zone

AbstractIn 2011 we acquired an 11 × 55 km, 3‐D seismic reflection volume across the Costa Rica margin, NW of the Osa Peninsula, to accurately image the subduction thrust in 3‐D, to examine fault zone properties, and to infer the hydrogeology that controls fluid accumulation along the thrust. Following processing to remove water column multiples, noise, and acquisition artifacts, we constructed a 3‐D seismic velocity model for Kirchhoff prestack depth migration imaging. Images of the plate boundary thrust show high‐reflection amplitudes underneath the middle to lower slope that we attribute to fluid‐rich, poorly drained portions of the subduction thrust. At ~ 5 km subseafloor, beneath the upper slope, the plate interface abruptly becomes weakly reflective, which we interpret as a transition to a well‐drained subduction thrust. Mineral dehydration during diagenesis may also diminish at 5 km subseafloor to reduce fluid production and contribute to the downdip change from high to low amplitude. There is also a layered fabric and systems of both thrust and normal faults within the overriding plate that form a “plumbing system.” Faults commonly have fault plane reflections and are presumably fluid charged. The faults and layered fabric form three compartmentalized hydrogeologic zones: (1) a shallow NE dipping zone beneath the slope, (2) a steeply SW dipping zone beneath the shelf slope break, and (3) a NE dipping zone beneath the shelf. The more direct pathway in the middle zone drains the subduction thrust more efficiently and contributes to reduced fluid pressure, elevates effective stress, and creates greater potential for unstable coseismic slip.

Spectral element modelling of fault-plane reflections arising from fluid pressure distributions

The presence of fault-plane reflections in seismic images, besides indicating the locations of faults, offers a possible source of information on the properties of these poorly understood zones. To better understand the physical mechanism giving rise to fault-plane reflections in compacting sedimentary basins, we numerically model the full elastic wavefield via the spectral element method (SEM) for several different fault models. Using well log data from the South Eugene Island field, offshore Louisiana, we derive empirical relationships between the elastic parameters (e.g. P-wave velocity and density) and the effective–stress along both normal compaction and unloading paths. These empirical relationships guide the numerical modelling and allow the investigation of how differences in fluid pressure modify the elastic wavefield. We choose to simulate the elastic wave equation via SEM since irregular model geometries can be accommodated and slip boundary conditions at an interface, such as a fault or fracture, are implemented naturally. The method we employ for including a slip interface retains the desirable qualities of SEM in that it is explicit in time and, therefore, does not require the inversion of a large matrix. We perform a complete numerical study by forward modelling seismic shot gathers over a faulted earth model using SEM followed by seismic processing of the simulated data. With this procedure, we construct post-stack time-migrated images of the kind that are routinely interpreted in the seismic exploration industry. We dip filter the seismic images to highlight the fault-plane reflections prior to making amplitude maps along the fault plane. With these amplitude maps, we compare the reflectivity from the different fault models to diagnose which physical mechanism contributes most to observed fault reflectivity. To lend physical meaning to the properties of a locally weak fault zone characterized as a slip interface, we propose an equivalent-layer model under the assumption of weak scattering. This allows us to use the empirical relationships between density, velocity and effective stress from the South Eugene Island field to relate a slip interface to an amount of excess pore-pressure in a fault zone.

Modeling sideswipe in 2D oceanic seismic surveys from sonar data: Application to the Mariana arc

In two-dimensional (2D) marine seismic-reflection surveys, out-of-plane rough seafloor bathymetry can cause multiple ocean-bottom reflections that complicate the interpretation of shallow reflections. Although migration corrects for the in-plane position of reflectors, it cannot resolve the inherent ambiguity in their out-of-plane positions. We show how swath bathymetry, routinely collected in many such surveys, can be used to model out-of-plane seafloor reflections and prevent their misinterpretation as subsurface geology. We use both raw and gridded multi-beam bathymetry data to build images that represent seafloor reflections in migrated seismic data. Comparison of these images to the seismic sections reveals whether suspicious features are out-of-plane water bottom reflections or subsurface reflections. Multi-channel seismic surveys across the Marianas intra-oceanic arc system provide examples where rough seafloor topography produced reflections that were initially misinterpreted. We use our seafloor reflection modeling (SRM) approach to help distinguish a possible landslide from a volcanic cone, to help distinguish real from apparent fault-plane reflections bounding a sediment-filled basin, and to verify that a possible magma chamber reflection results from sub-surface structure, not seafloor sideswipe.

Location and geometry of the Wellington Fault (New Zealand) defined by detailed three‐dimensional georadar data

Earthquakes with surface‐wave magnitudes of 7.3–7.9 are estimated to be associated with the rupture of the Wellington Fault at relatively regular intervals of 500–770 years. The last such earthquake probably happened between AD 1510 and 1660. Along its southern segment, the Wellington Fault passes through Wellington, New Zealand's capital, and the densely populated Hutt Valley. It is considered to be a highly hazardous structure. To map the shallow geometry of the Wellington Fault, we have collected 3‐D ground‐penetrating radar (georadar) data at two sites along the fault in the Hutt Valley. At one site, the first ever georadar fault plane reflections from an active strike‐slip fault are observed. They coincide with conspicuous diffractions generated by abrupt truncations of structures against the fault plane. These georadar data provide the most vivid shallow images of any active fault surveyed to date. At the second site, apparent offsets of lineaments in the sedimentary sections on either side of the fault are consistent with ∼20 m of dextral displacement estimated from the offset of a nearby terrace riser. The dips and minimum depth extents of the primary zones of fault displacement at the two sites are 55–75°SE and ∼20 m and 72–84°SE and ∼12 m, respectively. Although the georadar‐defined zones of faulting are a few meters wide, prominent reflection fabrics suggest that shearing, fracturing, and crushing extend for several tens of meters on either side of the fault.

Dipping San Andreas and Hayward faults revealed beneath San Francisco Bay, California

The San Francisco Bay area is crossed by several right-lateral strike-slip faults of the San Andreas fault zone. Fault-plane reflections reveal that two of these faults, the San Andreas and Hayward, dip toward each other below seismogenic depths at 60° and 70°, respectively, and persist to the base of the crust. Previously, a horizontal detachment linking the two faults in the lower crust beneath San Francisco Bay was proposed. The only near-vertical-incidence reflection data available prior to the most recent experiment in 1997 were recorded parallel to the major fault structures. When the new reflection data recorded orthogonal to the faults are compared with the older data, the highest amplitude reflections show clear variations in moveout with recording azimuth. In addition, reflection times consistently increase with distance from the faults. If the reflectors were horizontal, reflection moveout would be independent of azimuth, and reflection times would be independent of distance from the faults. The best-fit solution from three-dimensional traveltime modeling is a pair of high-angle dipping surfaces. The close correspondence of these dipping structures with the San Andreas and Hayward faults leads us to conclude that they are the faults beneath seismogenic depths. If the faults retain their observed dips, they would converge into a single zone in the upper mantle ~45 km beneath the surface, although we can only observe them in the crust.

Using Core Properties and Seismic Reflectivity to Estimate Pore Pressure in an Active Decollement Fault: ABSTRACT

In the decollement zone of the Barbados accretionary prism, a 3-D seismic image exhibits patchy high-amplitude negative polarity reflections, which have been attributed to large overpressures confined to the fault zone. We collected laboratory P-wave velocity and porosity vs. pore pressure data, using core samples from and adjacent to the decollement zone at ODP Site 948. Logs constrain density and velocity through the decollement zone at Site 948. We use these data to calibrate the reflectivity of the fault zone to pore pressure through waveform and amplitude models of the fault plane reflections. Modeling of the positive polarity Site 948 reflection indicates that it can be explained by a lithologic boundary coincident with the decollement, without anomalous fault properties. By contrast, the dominantly-negative polarity waveform of the reflection [approx]2 km arcward (beneath Site 947) is best modeled by inserting a 16-19 m thick zone of extremely low impedance into the Site 948 impedance structure, with a gradational return to [open quotes]normal[close quotes] impedance just above the positive boundary. Relative amplitudes in this reflection indicate a larger impedance contrast than can be accounted for at sub-lithostatic fluid pressure, based on the core properties data. We conclude that lithostatic pore pressure withmore » attendant hydraulic dilation of the fault zone is required to generate the negative-polarity reflections. Mapping of these reflections thus delineates zones of elevated fluid content and zero effective stress in the fault zone.« less

Evidence and mechanisms for forearc extension at the accretionary Costa Rica Convergent Margin

Seismic reflection data across the upper trench slope off the Nicoya Peninsula, Costa Rica, reveal a wide zone of nearly trench‐parallel normal faults. Although work in the last decade has shown that normal faults are present at many convergent margins, most examples (e.g., Japan, Peru‐Chile, and Guatemala) have been associated with margins experiencing subduction erosion or non‐accretion. In contrast, extension in the Costa Rica study area apparently is coeval with frontal accretion and underplating. The normal faults across the Costa Rica forearc are striking in seismic section due to the well‐layered, 2‐km‐thick upper slope apron. Fault plane reflections and reflector terminations show that the faults extend through the sedimentary apron and apparently into the underlying accretionary prism, indicating a deep‐seated deformation process. The zone of extension is from the midslope area to within 10 km of the shelf edge, a minimum width of about 20 km; the estimated extension across the zone is at least 1.5 to 3 km. Within the apron section, spacing between the faults is generally 200–500 m, and nominal fault dip is 20°–40° and predominantly landward. Activity on the normal faults appears to have occurred over a significant period of time based on increased displacement with depth and on fault‐controlled sedimentary thickening. At least some of the faults may be presently active; shallow reflectors and possibly the seafloor are displaced by faulting. Contemporary sediment accretion is documented by the same seismic reflection profiles showing offscraping and underplating near the toe of the wedge and out‐of‐sequence thrusting primarily below the midslope area. The consistent landward normal fault dip may be influenced by structural anisotropy in the prism and possible extensional reactivation of earlier thrust faults associated with accretion processes. With the available data it is not possible to conclusively determine the cause of the stress field leading to the upper prism and apron extension. However, the three most likely causes are underplating, changes in basal shear stress, or a brief episode of subduction erosion.

Deep seismic profile of the Amazonian Craton (northern Brazil)

Deep seismic profiling of the Amazonian craton (northern Brazil) reveals previously unknown deep structural features buried beneath a thin, relatively flat Phanerozoic sedimentary cover (Amazon basin). The seismic signature of the Precambrian crust is characterized by a transparent shallow basement down to 8–9 km followed by a highly reflective middle and lower crust. Middle and lower crustal events are dominated by abundant dipping, asymmetric arcuate reflections and diffractions, the shallowest of which occur beneath a structural arch (Jutaí arch). Although wide angle refraction data are unavailable, a decrease in the number of coherent reflections at about 15 s two‐way travel time (45 km) and small drops in amplitude and increases in average frequency are interpreted to mark the Moho. The deep Amazonian seismic line is located near the westernmost boundary of the Amazonian craton, imaging the Rondonian province, one of the youngest geochronological provinces of the craton (middle Proterozoic). To the south, the Rondonian basement crops out, revealing schists, gneisses, granulites, and important volumes of anorogenic granites. The prominent, laterally extensive arcuate reflections observed within the middle and lower crust may be related to this anorogenic magmatic activity, perhaps corresponding to layered intrusive complexes. On the other hand, intrusion of mantle derived material during an early Paleozoic rifting, as well as the Mesozoic episodes of mafic intrusion may also have contributed to this seismic signature. Similar seismic signatures found in eastern Australia and on COCORP profiles of the central U.S. craton suggest that such cratonic terranes may have undergone similar tectonic evolution. In the upper crust, probable fault plane reflections associated with ENE‐WSW reverse faults of Jurassic‐Early Cretaceous age may sole into the top of the reflective zone, suggesting that the lower limit of the transparent upper crust may represent a rheological boundary.

Capricorn and Northern Tasman Basins: Structure and Depositional Systems

The Bureau of Mineral Resources completed a major marine multi-channel seismic reflection survey (MCS) off southeast Queensland in December 1989. The research cruise, using RV 'Rig Seismic: was designed to investigate the structure, stratigraphy and petroleum resource potential of offshore basins along this sector of the Australian continental margin. About half of the 2900 km of seismic data acquired were recorded over the Capricorn Basin and margins of the northern Tasman Basin. The survey included seismic ties to the only deep offshore wells in the region, Aquarius-1 and Capricorn-lA, located in the northern Capricorn Basin. It was the first MCS survey shot in the region since 1974. Sonobuoy refraction, gravity, magnetic and bathymetric data were also collected. The Capricorn Basin evolved in the Late Cretaceous as a failed rift arm at the northern end of the Tasman rift system. From the seismic data, basin development in the Capricorn Basin is seen as typically comprising 0.5 ? 1.0 s twt of syn-rift continental/Irestricted marine deposits (Late Cretaceous-early Palaeogene) overlain by about 1.5 s twt of Eocene-Recent, mainly marine, post-rift sediments. Though basement structures generally follow a north-northwest trend, the structural pattern is complex, comprising a mainly discontinuous series of rift basins of various geometries. Complex structure is also indicated by gravity mapping. Shallow-dipping horizons that extend well into 'basement' may be fault-plane reflections from deep extensional structures, but could also be indicative of large wedges of old (Cretaceous+) sediments or volcaniclastics. In the deep-water southeastern part of the basin, at least 3.6 s twt (4 km) of Late Cretaceous ? Palaeogene section is associated with major half-graben development. A mid-Eocene compressional or transpressional event has reactivated basement structures and produced minor faulting and folding in the section. The event, which also produced significant uplift and erosion, is attributed to intraplate stress resulting from major global plate reorganization that occurred at about 43 Ma. Volcanism was active in the southern Capricorn Basin during Late Oligocene. Volcanic edifices several hundred metres high and 3 to 5 km across are exposed on the seafloor. In the Tasman Basin, a 3-km thick pile of Cainozoic post-breakup sediments overlies oceanic basement and highly extended continental crust at the base of the continental slope to the southeast of Fraser Island. A similar thickness of sediment is present at abyssal depths along the northern transform margin. Large-scale mass movements have taken place on the basin's steep and rugged marginal slopes.

Otway Basin Onshore Seismic Acquisition: Less Field Effort = Better Data Quality

Within the onshore Otway Basin, the PEL 27 Joint Venture has acquired excellent quality Vibraseis data using a 240-channel system with 120-fold coverage, short (15 m) group and receiver array intervals, only 6 geophones/station, and only one sweep/VP. Not only can layers within the Crayfish Subgroup (previously difficult to image) be mapped with higher confidence, but also the high-resolution data have preserved the hitherto rarely recognised reflections from fault planes.Provided far offset requirements are met, the most important controls on data quality are high fold, short group interval and short arrays. Optimal sweep duration is important also. A reduction from 12 to 6 geophones/station had no significant effect on data quality. Source effort (time/km) was reduced by 44% compared with the previous year’s seismic survey, resulting in a 24% increase in km/recording hour.Significant improvement in imaging fault-plane reflections has been achieved using finite-difference migration with a small step time of 4 ms (instead of the conventional 20 ms) and an interpolated sample rate of 2 ms (instead of 4 ms). These parameters accurately migrated reflections from fault-planes with dips to at least 50°. Conventional migration parameters, if applied to this steep dip/trace data, would migrate accurately only those events with dips less than 10°.

Subsurface imaging of the Garlock Fault, Cantil Valley, California

Imaging of the Garlock fault from seismic reflection data tests tectonic models for the Mojave Desert region of southern California. Models developed from geologic and geodetic evidence of fault movement rates disagree on whether the Garlock fault should dip north or south. Such models are further at odds with focal mechanism and dislocation analyses consistent with a vertical strike‐slip fault. Our analysis demonstrates a nonvertical, southward dip for the Garlock fault. Consortium for Continental Reflection Profiling Mojave line 5 collected seismic reflection data across the Garlock in Cantil Valley, where the 3‐km‐deep Cantil pull‐apart basin has developed between the southwest and east branches of the fault. We analyze the reflection data for evidence of the fault's structure to 5 km depths. Garlock fault plane reflections on unprocessed shot gathers merge with the first arrival as the source progresses to the surface trace of the east branch, explicitly tracing the Garlock fault as a seismic reflector from depth to the mapped contemporary fault trace. The apparent velocity of these reflections gives a 37°±10° south dip for the east branch of the fault. A prestack Kirchhoff sum migration images the east branch reflector with a 45° south dip. Our migration process fully accounts for the bending of seismic rays through the strong lateral velocity variations in Cantil basin. The south dip of the Garlock's east branch together with basement steps that decrease to the south suggest that Cantil basin developed from a 0.4–1 mm/yr component of extension normal to the fault. Development of the basin by detachment provides a mechanism to widen it as left‐lateral motion lengthens it, allowing the pull‐apart to maintain the globally constant length‐to‐width ratio of 3. Detachment by the Garlock at Cantil Valley similarly requires 0.4–1 mm/yr of dextral strike slip on a northwest striking fault such as the Helendale that cuts the Mojave between the Garlock and Pinto Mountain faults. Such motion is consistent with regional tectonic models.

Seismic reflections from normal faults in the northern North Sea

Abstract Fault-plane reflections from gravity-driven, listric faults in thick sedimentary sequences are well known. However, reflections from normal faults associated with crustal extension are much rarer and less well studied. We discuss a number of examples of such reflections, present on commercial seismic reflection data from the northern North Sea. Some of these examples have previously been described as reflected refractions, but we show that this requires unrealistic velocities within the sedimentary column. Instead, we consider it more likely that these events are true fault-plane reflections. Depth migration and image-ray migration are used to determine the probable geometry of the faults in both cross section and map view. Estimates of the true geological dip remain sensitive to the velocity structure used, particularly that applied to the Triassic half-graben fill. Despite this uncertainty, the depth-migrations show that these faults are composed of two near-linear segments in cross section. The upper parts (in the Jurassic and Upper Triassic section) dip at 40–50°, but the deeper parts (Triassic hangingwall against basement in the footwall) typically dip at 30–35°. Occasionally, reflections indicate that faults continue into the basement with a 30–35° dip. We interpret this fault geometry as originating from two phases of fault movement. Triassic extension caused rotation of sets of initially steep planar faults. Following Late-Triassic-to-Middle-Jurassic thermal subsidence, renewed extension in the Late Jurassic caused the faults to cut up through the overburden at a steeper angle. Without recognition of a two-phase extension history, such faults might be mistakenly interpreted as forming with a listric geometry.

Regional Seismic Reflection Profile from Railroad Valley to Lake Valley, East-Central Nevada, Reveals a Variety of Structural Styles Beneath Neogene Basins

Two seismic reflection lines that compose a 90-km east-west profile at approximately 38{degree}25{prime}N latitude, east-central Nevada, help define the structure beneath Railroad Valley, White River Valley, the southern Egan Range, Cave Valley, Muleshoe Valley, the southern Schell Creek Range, and Lake Valley, Preliminary seismic interpretations are being integrated with ongoing geologic mapping, gravity, and magnetic studies and with drill-hole data along this transect. In the Grant Canyon oil field of Railroad Valley, a gently west-dipping normal fault appears to have controlled the development of the Neogene basin. The fault is clearly defined by fault-plane reflections and by terminations of east-dipping reflections from Tertiary and Paleozoic strata that have rotated toward the fault; the fault projects to nearby outcrops of a major low-angle extensional fault mapped in the Grant Range to the east. White River Valley at this latitude consists of three east-dipping half-grabens and two intervening basement highs. Two half-grabens in the western part of the valley are bounded by west-dipping faults with intermediate to steep dips. East-dipping reflections in the southern Egan Range correspond to a homoclinal Paleozoic panel overlain by a veneer of Late Cretaceous and early Tertiary rocks. The north end of Muleshoe Valley yields a narrowmore » sag basin pattern between the southern Schell Creek Range and Dutch John Mountain, with no well-defined bounding faults. Lake Valley, on the east end of the profile, is a broad, complex basin containing normal faults with opposing dips. The progressive steepening of westerly dips in basin-fill beneath the west side of the basin suggests the presence of a major east-dipping listric fault.« less

Thick‐skinned deformation observed on deep seismic reflection profiles in western Argentina

One of the first deep seismic reflection profiles in South America has resulted from reprocessing nearly 90 km of 96 channel Vibroseis data that were originally collected by YPF (the Argentine National Oil Company) across the Sierra de la Huerta in the Sierras Pampeanas of western Argentina. Reprocessed sections show the following features in the upper 20 km of crust: (1) prominent layering within Carboniferous(?) to Quaternary strata; and (2) a complex suite of faint reflections which are interpreted to define both extensional and compressional faults. The Mesozoic strata are bounded by one or more west‐dipping normal faults. One or two east‐dipping thrust faults shallow with depth, a geometry similar to the Rocky Mountain foreland of the western U. S. A. One thrust has a clear surface trace. Although lacking evidence of surface scarps or clear fault plane reflections, panels of constant dip domain seen in the seismic data justify the application of fault‐bend folding techniques to suggest that a second, deeper fault becomes nearly flat at 15–20 km. Cumulative Cenozoic offset on the thrusts beneath the Sierra de la Huerta is about 13 km. A well‐located earthquake hypocenter lies within the footwall block of a major thrust and is presumed to represent susidiary thrust motion along a secondary high‐angle fault. Gravity models suggest that the thrust ramps may nucleate at an anomalous dense mass in the middle or lower crust. Interpretation of lower crustal structure is hampered by the lack of clear reflections, presumably due to small‐scale and low‐amplitude impedance contrast variations of lithologic units at those depths. Consideration of recorrelation limitations and signal penetration constraints provide likely rationale for the scarcity of clear reflections in the basement rocks of the region, but low reflectance of mafic metamorphic rocks remains a possible explanation.

Structure of the Nankai Trough Accretionary Zone from multichannel seismic reflection data

New multichannel seismic reflection data collected over the Nankai Trough image the accretionary complex in two areas: the International Program of Ocean Drilling leg 87 transect area (western area) and the region of upcoming Ocean Drilling Program leg 131 (eastern area). The incoming Shikoku Basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites. The basin section is overlain by a trench wedge that is 12–16 km wide and 350–750 m thick at the thrust front. Accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge. The zone in the western area is 4.5 km wide and is characterized by “kink” folds; the zone in the eastern area is only 2.5 km wide and does not exhibit such folds. The frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section. The overthrusting sediments form fault‐bend folds over these ramps. Thrust spacing at the toe of the slope is 1.5–2.5 km. The second thrust cuts up from an inferred décollement within the Shikoku Basin sedimentary section. In the eastern area, a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6.3 km seaward of the thrust front and underlies the entire protothrust zone. This reflector continues with reversed polarity under the accretionary complex and is at the level of the basal décollement. The underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism. The reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface. This decrease in velocity and density may indicate that the décollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism. The reflections from the first and second thrusts are also reversed polarity, possibly indicating that they also are pathways of fluid expulsion. The critical wedge taper of the western area is greater than that of the eastern area, an observation that is consistent with the existence of an overpressured décollement in the eastern area.

The Moine Thrust in the BIRPS data set

A second generation of BIRPS profiles combine with older data to form a loose (10 by 40 km spacing) grid over the offshore projection of the Moine Thrust. These profiles show clear reflections that arguably represent fault plane reflections from the Moine and related Caledonian thrust structures in the location suggested by some earlier workers. Reflectors predominate at two structural levels: 8-10 km (2.5-3.5 sTWT) and 13-16 km (4.0-5.5 sTWT) depths. The deeper set appears to define an antiformal stack of thrusts or duplex with possible continuation to deeper levels. By analogy with onshore sections and after subsequent extension is restored, this deeper set correlates with the Moine Thrust onshore. The shallower set aligns with normal faults that bound half grabens filled with Devonian and Permo-Triassic rocks. This set correlates with the Naver Thrust after similar restoration. Normal faulting appears to have reactivated some of the thrusts, increasing their reflectivity. Detailed stacking- and interval-velocity studies indicate that an increase of about 1-2km s −1 occurs across the reflections here associated with the two fault zones. The overall reflection pattern is consistent with a complex stack of thrust sheets, but lack of resolution prevents a complete structural analysis.

Offset‐dependent mis‐tie analysis at seismic line intersections

Offset‐dependent line‐intersection displays can be of significant direct interpretive value. On our data set from a proprietary location, use of these displays permits a refined structural interpretation and improves the usefulness of amplitude and amplitude‐versus‐offset displays for direct hydrocarbon detection. The degree of data reproducibility at each intersection may be exposed as a function of offset through the use of prestack and partial‐range stack splice and difference displays. At a set of marine line intersections, after only cursory processing, the range of residual amplitudes found by subtracting strike traces from dip traces is from −20 dB to +6 dB relative to the input. The average is about −8 dB. The poorest line intersection ties occur below the edges of deep bright spots, where velocity anomalies leading to time mis‐ties greater than 10 ms can occur. Shallow near‐surface velocity anomalies, out‐of‐plane arrivals including fault plane reflections, and, occasionally, water reverberation variations contribute locally to the data mis‐ties. Location errors are ubiquitous but of secondary importance, and random noise is more than 20 dB down. A simple and inexpensive routine splice display of line intersection data can be of significant value for quality control, for processing evaluation, and for interpretation. Issues of data reliability, such as screening of amplitude‐versus‐offset (AVO) anomalies by shallow gas and transmission‐path problems for large CMP offsets, as well as other problems in operational and research geophysics, may be addressed by analysis of line intersections in the offset domain.