Shallow Seismic Reflection Data Research Articles

Unrecognized post-rifting deposition and uplift history of the Erlian Basin, Northern China: Evidence from seismic reflection data and apatite fission-track thermochronology

Early Cretaceous rift basins in Northeast Asia typically experienced varying degrees of post-rifting thermal subsidence and compressional deformation. However, previous workers believed that only a thin post-rifting sequence was developed in basins west of the Great Xing'an Range, making a proper understanding of the post-rift tectonic history in this part of Northeast Asia difficult. In this study, newly acquired shallow seismic reflection data were utilized to unravel the shallow deformation of the Erlian Basin, which to the west of the Great Xing'an Range contains a thin post-rifting sequence. Furthermore, we used apatite fission-track thermochronology to analyze the post-rifting deposition and uplift processes in the Erlian Basin, for which there are limited stratigraphic records. Our results show that the Erlian Basin experienced a typical post-rifting stage whose sequence from the Albian to the Campanian was not controlled by normal faults. The corresponding sedimentary thickness was up to 2 km, which contradicts the previously reported little or no post-rifting thermal subsidence. The current thin post-rifting sequence is the remaining stratum after 1.4–1.8 km of denudation at the end of the Erlian period. In addition, by further comparing the sedimentary thickness and time-lag in the onset of post-rifting to those of other basins in Northeast Asia, we found two driving mechanisms of rifting in Northeast Asia, which occurred simultaneously: the gravitational collapse and back-arc extension. Moreover, the basins in Northeast Asia experienced a gradual weakening in the compressional deformation from east to west during the Cenomanian to the Coniacian, and an intense compressional deformation in the east of Northeast Asia and large-scale vertical uplift in the west during the Late Cretaceous Campanian to the Eocene.

Near-surface diffractor detection at archaeological sites based on an interferometric workflow

Detecting small-size objects is a primary challenge at archaeological sites due to the high degree of heterogeneity present in the near surface. Although high-resolution reflection seismic imaging often delivers the target resolution of the subsurface in different near-surface settings, the standard processing for obtaining an image of the subsurface is not suitable to map local diffractors. This happens because shallow seismic-reflection data are often dominated by strong surface waves that might cover weaker diffractions and because traditional common-midpoint moveout corrections are only optimal for reflection events. We propose an approach for imaging subsurface objects using masked diffractions. These masked diffractions are first revealed by a combination of seismic interferometry and nonstationary adaptive subtraction, and then further enhanced through crosscoherence-based supervirtual interferometry. A diffraction image is then computed by a spatial summation of the revealed diffractions. We use the phase-weighted stack to enhance the coherent summation of weak diffraction signals. Using synthetic data, we show that our scheme is robust in locating diffractors from data dominated by strong Love waves. We test our method on field data acquired at an archaeological site. The resulting distribution of shallow diffractors agrees with the location of anomalous objects identified in the [Formula: see text] model obtained by elastic SH/Love full-waveform inversion using the same field data. The anomalous objects correspond to the position of a suspected burial, also detected in an independent magnetic survey and corings.

Left High and Dry: Deglaciation of Dogger Bank, North Sea, Recorded in Proglacial Lake Evolution

Reconstructions of palaeo-ice sheet retreat in response to climate warming using offshore archives can provide vital analogues for future ice-sheet behaviour. At the Last Glacial Maximum, Dogger Bank, in the southern North Sea, was covered by the Eurasian Ice Sheet. However, the maximum extent and behaviour of the ice sheet in the North Sea basin is poorly constrained. We reveal ice-marginal dynamics and maximum ice extent at Dogger Bank through sedimentological and stratigraphic investigation of glacial and proglacial lake sediments. We use a large, integrated subsurface dataset of shallow seismic reflection and geotechnical data collected during windfarm site investigation. For the first time, an ice stream is identified at Dogger Bank, based on preserved subglacial bedforms, eskers and meltwater channels. During ice-sheet advance, a terminal thrust-block moraine complex formed, whose crest runs approximately north-northeast to south-southwest. Subsequent ice stream shutdown caused stagnation of ice, and rapid retreat of the ice-sheet margin. The moraine complex, and outwash head from an adjacent ice-sheet lobe to the west, dammed a large (approximately 750 km2) proglacial lake. Subsequent sedimentation infilled the lake with 30 m of glacial outwash sediments. A lobate subaqueous fan formed at the ice-sheet margin, which thins towards the southeast with iceberg scours and ice-rafted debris at the base, and is onlapped by lake sediments calibrated to core as alternating clay and silt laminae, interpreted to be varves. The lake became isolated from the retreating ice-sheet margin, and ice-sheet retreat slowed. Sediment-laden meltwater was supplied to the ice-distal proglacial lake for c. 1500-2000 years. Subsequent ice-sheet retreat off Dogger Bank was more rapid due to the negative subglacial slope. The stepped retreat of rapid downwasting, slow retreat, and a final rapid phase off Dogger Bank occurred after the LGM at around 27 ka and before formation of a ribbon lake, dated previously to 23 ka and approximately 60 metres lower in elevation, formed to the north of Dogger Bank. The complicated stratigraphic architecture revealed through these data improves forecasting of ground conditions for turbine footings at Dogger Bank, an important step in the provision of clean, sustainable energy.

Seismic interferometry facilitating the imaging of shallow shear‐wave reflections hidden beneath surface waves

ABSTRACTHigh‐resolution reflection seismics is a powerful tool that can provide the required resolution for subsurface imaging and monitoring in urban settings. Shallow seismic reflection data acquired in soil‐covered sites are often contaminated by source‐coherent surface waves and other linear moveout noises (LMON) that might be caused by, e.g., anthropogenic sources or harmonic distortion in vibroseis data. In the case of shear‐wave seismic reflection data, such noises are particularly problematic as they overlap the useful shallow reflections. We have developed new schemes for suppressing such surface‐wave noise and LMON while still preserving shallow reflections, which are of great interest to high‐resolution near‐surface imaging. We do this by making use of two techniques. First, we make use of seismic interferometry to retrieve predominantly source‐coherent surface waves and LMON. We then adaptively subtract these dominant source‐coherent surface waves and LMON from the seismic data in a separate step. We illustrate our proposed method using synthetic and field data. We compare results from our method with results from frequency–wave‐number (f‐k) filtering. Using synthetic data, we show that our schemes are robust in separating shallow reflections from source‐coherent surface waves and LMON even when they share very similar velocity and frequency contents, whereas f‐k filtering might cause undesirable artefacts. Using a field shear‐wave reflection dataset characterised by overwhelming LMON, we show that the reflectors at a very shallow depth can be imaged because of significant suppression of the LMON due to the application of the scheme that we have developed.

Structural patterns of the Lake Erçek Basin, eastern Anatolia (Turkey): evidence from single-channel seismic interpretation

This study presents an analysis of the single-channel high-resolution shallow seismic reflection data from Lake Ercek, eastern Anatolia, to provide key information on the deformational elements, on the fault patterns and on the overall tectonic structure of the Lake Ercek Basin. High-resolution seismic data reveal major structural and deformational features, including N–S trending normal faults and W–E trending reverse faults bounding the Lake Ercek Basin, basement highs and folded structures along the marginal sections of the lake. The N–S trending normal faults asymmetrically control the steep western margin and the gentle eastern deltaic section, while the W–E trending reverse faults appear at the northern and southern margins. The N–S trending normal faults, half-graben structure, and the gradual thickening of sediments in the Ercek Basin toward the fault scarps strongly suggest an extensional tectonic regime resulting from an N–S compression. The Ercek Basin is an extension-controlled depocenter; it is a relatively undeformed and flat-lying deep Basin, forming a typical example of the half-graben structure. The N–S trending normal faults appear to be currently active and control the lake center and the E-delta section, resulting in subsidence in the lake floor. In the N- and S-margins of the lake, there is evidence of folding, faulting and accompanying block uplifting, suggesting a significant N–S compressional regime that results in the reverse faulting and basement highs along the marginal sections. The folding and faulting caused strong uplift of the basement blocks in the N- and S- margins, subsequently exposing the shelf and slope areas. The exposed areas are evident in the erosional unconformity of the surface of the basement highs and thinned sediments. The tilted basement strata and subsequent erosion over the basement block highs suggest prominent structural inversion, probably long before the formation of the lake. New high-resolution seismic data reveal the fault patterns and structural lineaments of the Lake Ercek and provide strong evidence for an ongoing extension and subsidence. The present study provides new structural insights that will support future tectonic and sedimentary studies and the development of strategies related to active earthquake faults and major seismic events in the region of Lake Ercek.

Quantitative imaging for civil engineering by joint full waveform inversion of surface-based GPR and shallow seismic reflection data

Full waveform inversion (FWI) of ground penetrating radar (GPR) and seismic data is a promising imaging tool for the detailed characterization of latent cavities and tunnels. In this study, surface-based GPR FWI and seismic FWI are combined to quantitatively image the geometry and geophysical attributes of the subsurface. These two FWI methods are integrated into a joint procedure and linked by cross-gradient functions which can provide structural constraints between the parameters. The elastic and electromagnetic properties of the medium (P-wave velocity, permittivity, and conductivity) are linked and simultaneously inverted in the joint FWI approach. This joint FWI approach is tested using three numerical examples. In the first two examples, target objects with various shapes are contained in both homogeneous and inhomogeneous media. In the third example, two sediment-filled cavities and a fractured area are analyzed in the calcarenite layers. The results show that the joint FWI approach can integrate the common structure of models and correct the model structures that are inaccurately reconstructed in the individual FWI methods, providing a comprehensive structural interpretation of the study areas.

Mesozoic and older rift basins on the SE Greenland Shelf offshore Ammassalik

AbstractSeismic reflection data and shallow cores from the SE Greenland margin show that rift basins formed by the mid- to Late Cretaceous in the offshore area near Ammassalik. Here termed the Ammassalik Basin, this contribution documents the area using reprocessed older shallow seismic reflection data together with a more recent, commercial deep seismic reflection profile. The data show that the basin is at least 4 km deep and may be regionally quite extensive. Interpretation of gravity anomaly data indicate that the basin potentially covers an area of nearly 100 000 km2. The sediments in the basins are at least of Cretaceous age, as indicated by a sample from just below the basalt cover that was dated as Albian. Dipping sediment layers in the basins indicate that older sediments are present. Comparison of the data to the conjugate Hatton margin where older basins are exposed beneath the volcanic cover shows similar stratigraphy of similar ages. Reconstructions of the position of the basin during the Permian–Triassic and Jurassic suggest that older sedimentary strata could also be possible. In contrast to the conjugate Hatton margin, possible older strata subcrop out below the seafloor along the shallow margin, providing a future opportunity to sample some of the oldest sediments to determine the onset of rifting between SE Greenland and the Hatton margin.

An example of extreme near-surface variability in shallow seismic reflection data

Variability of material properties in the shallow subsurface presents challenges for near-surface geophysical methods and exploration-scale applications. As the depth of investigation decreases, denser sampling is required, especially of the near offsets, to accurately characterize the shallow subsurface. We have developed a field data example using high-resolution shallow seismic reflection data to demonstrate how quickly near-surface properties can change over short distances and the effects on field data and processed sections. The addition of a relatively thin, 20 cm thick, low-velocity layer can lead to masked reflections and an inability to map shallow reflectors. Short receiver intervals, on the order of 10 cm, were necessary to identify the cause of the diminished data quality and would have gone unknown using larger, more conventional station spacing. Combined analysis of first arrivals, surface waves, and reflections aided in determining the effects and extent of a low-velocity layer that inhibited the identification and constructive stacking of the reflection from a shallow water table using normal-moveout-based processing methods. Our results also highlight the benefits of using unprocessed gathers to pragmatically guide processing and interpretation of seismic data.

The 3D Autojuggie: automating acquisition of 3D near‐surface seismic reflection data

ABSTRACTNew instrumentation, known as the 3D Autojuggie, has been developed that allows acquisition of three‐dimensional shallow seismic reflection data in a cost‐effective manner. The 3D Autojuggie is capable of simultaneously planting 220 geophones with 0.5‐m spacing in the inline and crossline directions in less than 1 minute. Likewise, all 220 geophones can be picked up in the same amount of time so that the receiver grid can be moved and redeployed on the order of a few minutes without disconnecting cables and seismographs. This significantly reduces the time required to roll geo‐phones and cables and the number of crew members needed for three‐dimensional high‐resolution seismic surveying. While efficiency of three‐dimensional surveying is demonstrated here, the use of the Autojuggie will be limited in rough terrain and in dry or rocky top soil.This study presents two field tests of the 3D Autojuggie. The first one was conducted over soil using conventional geophones with spikes, and the second one was conducted over hard pavement using geophones mounted on base plates. Each dataset is compared to corresponding walkaway lines of conventional manually placed control geophones. Data acquired using the 3D Autojuggie are directly comparable and nearly indistinguishable to hand‐planted geophones. Field experiments resulted in a minimum of triple the acquisition efficiency increase compared with conventional geophone deployment methods. This work demonstrates that the 3D Autojuggie can facilitate the acquisition of high‐resolution three‐dimensional seismic data, making possible routine ultra‐shallow seismic imaging.

S/N enhancement by means of array simulation for near surface seismic investigations

ABSTRACTWe present a procedure for enhancing the signal‐to‐noise ratio (S/N) of shallow seismic reflection data based on two different steps: 1) an acquisition step that requires the recording of closely spaced common source records with standard source and receiver equipment and 2) a processing step where weighted or un‐weighted source and receiver arrays are simulated on the basis of required needs for source related noise attenuation and depth penetration. The data acquisition can be carried out employing single source‐single geophone recordings, with a standard 24 or 48‐channel equipment. Simple energy sources such as weight drop or sledgehammer are considered. The design and application of the spatial filters in the processing phase is very flexible and can be tailored to the specific needs. In fact, the simulated source and/or receiver arrays can be time and/or space variant and can be weighted to provide the desired responses. Optimal weights can be determined by means of Chebyshev polynomials. Real data examples show the increase in the data quality in terms of better coherent noise attenuation and of enhanced depth penetration.

Regional hydrostratigraphy and insights into fluid flow through a clay aquitard from shallow seismic reflection data

In the Montérégie region, east of Montreal, Quebec, Canada, high-resolution compressional (P-) and shear (S-) wave reflection sections were obtained using a vibratory source and three-component (3C) landstreamer data acquisition system during field programs carried out in 2009 and 2010. The work was part of a strategy to improve the knowledge and understanding of groundwater resources within the glacial and postglacial sediments in the region. The >100 line-km of seismic profiling is an excellent example of hydrogeophysics, providing detailed information on the depth to bedrock, the architecture and stratigraphy, and physical properties of the overlying sediments on both the regional and local scales (hydrostratigraphic mapping), as well as insights into fluid (water and/or gas) transport through the surface aquitard. Correlations of porosity and shear-wave velocity in the region also allow some estimation of the variation in subsurface porosities for hydrogeological modeling.

Neogene Stratigraphy Of The Eskişehir Graben And The Investigation Of Coal Deposition By Seismic Reflection Method

The study area is located within the Eskisehir Graben, north of Agapinar village in the Sevinc district and east of Eskisehir City. Miocene deposits unconformably overlie the basement of Paleozoic metamorphites and Mesozoic ophiolites. The m1 series constituted by gravel, sandstone and claystone is observed at the base of Miocene deposits. The overlying m2 series, from bottom to top, represents a sequence of partly conglomerate, green claystone, coal, gray sandstone, dark gray to green siltstone, bituminous marl, claystone, coal, green claystone, sandstone and fine grained conglomerate. The m3 series of limestone and conglomerate is patchy and Pliocene deposits consisting of loose gravel, claystone overly them. The stratigraphical succession in the region covered by Neogene deposits has been investigated during surveys and reservoir drilling in licence areas belonging to MTA. High resolution shallow seismic reflection data were collected on two lines crossing each other considering field conditions (vegetated areas, irrigation channels, Porsuk River, railway line, etc.) and locations of boreholes made in the basin. These collected data were assessed in the Data Processing Center of the Seismic Research Division of the Geophysical Research Department (MTA),to produce the final sections of the seismic reflection. The data were then correlated with borehole data on or adjacent to the lines. The surface of reflection (yellow colored level) which could be continuously traced through all sections was deduced as the Paleozoic basement corresponded to ophiolites in borehole data. Reflection surfaces which are observed in areas, where the seismic basement deepened by multi-staged discontinuity surfaces of the seismic basement and rely especially on slopes of seismic basement, were determined as coal-claycontactzone. It was also revealed that, data obtained from drilling works continuing in the region correlate with geophysical interpretations made on seismic profiles.

An Improved Technique for Static Correction in a High Resolution Shallow Seismic Reflection Data using Difference in Reflection Times.

Shallow Seismic reflection survey was carried out in Zaria, located in the basement complex of central northern Nigeria, with the aim of characterising the granitic batholith. However, the effect of near surface material, which could make reflection events appear disjointed or deeper than they really are, could be enormous if not corrected for. This work sets out to correct for this events by making use of difference in reflection times estimated from initially known models of the subsurface. During the data acquisition the receivers where placed at 1 m interval, with a constant offset of 1 m. the common midpoint (CMP) method with a 12 fold coverage was employed. Previous refraction tomography model carried out in the area was used as a guide during the 2D velocity model generation, making use of the observed travel times. The observe time was used to generate the initial model, which was later corrected using the previous known subsurface model, thereby noting the difference in travel time induced by the near surface materials. The results obtained showed that the reflection events were more coherent and in their actual reflecting point on the statically corrected seismic sections, than the section without static correction because of the excess time from the near surface material with variable p wave velocity. This experiment has proved that difference in reflection travel times obtained from previous known model or borehole information in conjunction with the observed travel times serve as a better tool in applying static correction to seismic reflection data, than any previous known conventional methods that relies on a single technique. Key Words: Static correction, High resolution, Seismic reflection, Reflection times

The use of F-K filter in the elimination of artefacts from a shallow seismic reflection data in Zaria, Nigeria

Near surface high resolution 2-D seismic reflection survey was carried out along the Zaria batholith. Zaria area forms part of the Precambrian basement complex of north central Nigeria which comprises Precambrian rocks made up of granite, gneiss and Low-grade metasediments. The granitic Zaria batholith intruded the gneiss and metasediments which forms the country rock. Reflection signals from shallow seismic survey in this area are often marred by surface waves, refraction and multiple reflections. The processing of the seismic data if not well handled often leads to migration artefacts which could be misconstrued as a seismic event. The field procedure employed for the geophysical survey was the split spread mode. The geophones were fixed at an interval of 1m, and a constant offset of 0.5 m. The common depth point method (CDP) with 12 fold coverage was adopted for the survey.Several artefacts were noticed on the result of the seismic migrated section, when the fk filter was initially applied only on the pre-stacked seismic data. The filtering and the common midpoint stacking could not completely eliminate the seismic noise on the pre-stacked seismic data. However, when an fk filter was applied on the post-stacked seismic section, the remaining seismic noise that survives the initial processing flow was eliminated. When migration was carried out on the filtered post-stack seismic section, the resulting seismic section was free from artefacts, andshowed clear seismic events.KEYWORDS: f-k filter, artefacts, shallow seismic, reflection, migration. Zaria

Late Holocene chronology, origin, and evolution of the St. Bernard Shoals, Northern Gulf of Mexico, USA

Several shore-parallel marine sand bodies lie on the Louisiana continental shelf. They are Trinity Shoal, Ship Shoal, Outer Shoal, and the St. Bernard Shoals. These shoals mark the submerged positions of ancient shorelines associated with abandoned deltas. Three of these shoals are single elongate deposits. The fourth shoal, the St. Bernard Shoals, consists of a group of discrete sand bodies ranging in size from 44 to 0.05 km2, 25 km southeast of the Chandeleur Islands in 15–18 m of water. The St. Bernard Shoals are stratigraphically above the St. Bernard delta complex, which was active 2,500–1,800 years b.p. Understanding the evolution of the St. Bernard Shoals is necessary to reconstruct the Holocene chronology of the St. Bernard delta complex and the eastern Louisiana continental shelf. For this study, 47 vibracores and 400 km of shallow seismic reflection data collected in 1987 across the Louisiana shelf were analyzed. In June 2008, 384 km of higher-resolution seismic reflection data were acquired across the study area and appended to the preexisting datasets. Vibracores were integrated with seismic profiles to identify facies and their regional distribution. Our results demonstrate that the deltaic package stratigraphically below the St. Bernard Shoals is chronologically younger than the northern distributaries, but derived from the same trunk distributary channel (Bayou la Loutre). The river eventually bypassed the northern distributaries, and began to deposit sediment further onto the continental shelf. After abandonment, the overextended delta lobe was rapidly transgressed, creating a transgressive shoreline that eventually coalesced with earlier shorelines in the region to form the Chandeleur Islands. The St. Bernard Shoals formed by the reworking of the relict distributary deposits exposed on the inner to mid shelf during and subsequent to shoreface ravinement.

First seismic imaging results of tectonically complex structures at shallow depths beneath the northwest Canterbury Plains, New Zealand

Seismogenic structures underlie many regions of the vast Canterbury Plains on the South Island of New Zealand. Most of these structures are hidden beneath a layer of rapidly deposited Late Pleistocene sediments, the youth and thickness of which make the general application of conventional paleoseismological studies impractical. In an attempt to improve our understanding of potentially active structures in this region, we have acquired, processed and interpreted shallow seismic reflection data across the northwest Canterbury Plains. To separate the useful reflected signals from unusually high amplitude ambient and source-generated noise, we subjected the data to a specially tailored processing scheme that included time- and space-variant spectral balancing, custom static corrections and mutes, F-X deconvolution, DMO corrections and finite-difference migration. The final stacked and migrated seismic sections supply high-resolution images of the basement and overlying layered Cretaceous- to Quaternary-age supracrustal rocks that have been complexly faulted and folded. At one location, the uppermost Late Pleistocene layers appear to have been gently buckled.

Seismic images and rock properties of the very shallow structure of Campi Flegrei caldera (southern Italy)

In September 2001, an extensive active-seismic investigation (Serapis experiment) was carried out in the Gulfs of Naples and Pozzuoli, with the aim of investigating and reconstructing the shallow crustal structure of the Campi Flegrei caldera, and possibly identifying its feeding system at depth. The present study provides a joint analysis of the very shallow seismic reflection data and tomographic images based on the Serapis dataset. This is achieved by reflection seismic sections obtained by the 3D data gathering and through refined P-velocity images of the shallowest layer of Pozzuoli Gulf (z 3,500 m/s) correlates with units made up of agglomerate tuffs and interbedded lava, which form the southern edge of the caldera, which was probably formed following the two large ignimbritic eruptions that marked the evolutionary history of the area under study. The upper part of the anomaly that tends to split into two parallel arcs is correlated with dikes, volcanic mounds and hydrothermal alteration zones noted in previous shallow reflection seismic analyses. The depth of the transition between the upper and lower parts of the anomaly is characterized by an abrupt Vp increase on the one-dimensional (1D) profiles extracted from the 3D tomographic model and by the presence of a strong reflector located at about 0.6/0.7 s Two Way Time (TWT) on Common Mid Point gathers. The move-out velocity analysis and stack of the P–P and P–S reflections at the layer bottom allowed to estimate relatively high Vp/Vs values (3.7 ± 0.9). This hypothesis has been tested by a theoretical rock physical modeling of the Vp/Vs ratio as a function of porosity suggesting that the shallow layer is likely formed by incoherent, water saturated, volcanic and marine sediments that filled Pozzuoli Bay during the post-caldera activity.

High-resolution ultra-shallow subsurface imaging by integrating near-surface seismic reflection and ground-penetrating radar data in the depth domain

This study presents the integration of complementary near-surface geophysical methods for high-resolution ultra-shallow imaging of the subsurface. Shallow-seismic reflection (SSR) and ground-penetrating radar (GPR) data of approximately the same wavelength were collected along a coincident survey line. The top of the saturated zone was successfully imaged using both techniques, but no significant reflectors were imaged above the water table using SSR or below the water table using GPR. The SSR and GPR data sets were depth converted and merged into a single data set, resulting in a more complete image of the subsurface than either method can provide alone. Instead of having two sections representing depths of ∼ 0–5 m (GPR) and ∼ 4–30 m (SSR), the combined section includes the near-surface stratigraphy, water table, and bedrock, among other geologic features. It is important to note that this method of data merging is intended only for qualitative interpretation of layers of differing electrical and elastic properties. In the case shown here, the two geophysical methods image comparable features in depth, but they respond to different physical properties.

Bedrock imaging using post-critical shallow seismic reflection data

We acquired shallow seismic reflection data in an unusual range of offsets greater than critical distance, and examined the feasibility and consequences of processing these post-critical reflections using conventional seismic reflection procedures. The main motivations to develop this work were: (i) the reflections observed at larger offsets are sharper and stronger than at near offsets, and (ii) it is possible to select a broad optimum offset window, which will speed up the execution of a seismic reflection survey. In order to accomplish the proposed task, refraction data and elastic reflection modelling were integrated into processing of reflection data and their interpretation. The information from the refracted arrivals was used to image the shallower reflector (dry unconsolidated overburden), to estimate the interval velocity model above the bedrock, and to perform static correction. The elastic reflection modelling was decisive to perform the post-critical seismic reflection survey, and to check the reliability of our interpretation. The lithologic log information corroborated the feasibility of imaging subsurface structure from post-critical reflection data by conventional processing, provided additional velocity-depth information is integrated in the reflection processing.

Delineation of Faulting and Basin Geometry along a Seismic Reflection Transect in Urbanized San Bernardino Valley, California

Fourteen kilometers of continuous, shallow seismic reflection data acquired through the urbanized San Bernardino Valley, California, have revealed numerous faults between the San Jacinto and San Andreas faults as well as a complex pattern of downdropped and uplifted blocks. These data also indicate that the Loma Linda fault continues northeastward at least 4.5 km beyond its last mapped location on the southern edge of the valley and to within at least 2 km of downtown San Bernardino. Previously undetected faults within the valley northeast of the San Jacinto fault are also imaged, including the inferred western extension of the Banning fault and several unnamed faults. The Rialto-Colton fault is interpreted southwest of the San Jacinto fault. The seismic data image the top of the crystalline basement complex across 70% of the profile length and show that the basement has an overall dip of roughly 10° southwest between Perris Hill and the San Jacinto fault. Gravity and aeromagnetic data corroborate the interpreted location of the San Jacinto fault and better constrain the basin depth along the seismic profile to be as deep as 1.7 km. These data also corroborate other fault locations and the general dip of the basement surface. At least 1.2 km of apparent vertical displacement on the basement is observed across the San Jacinto fault at the profile location. The basin geometry delineated by these data was used to generate modeled ground motions that show peak horizontal amplifications of 2–3.5 above bedrock response in the 0.05- to 1.0-Hz frequency band, which is consistent with recorded earthquake data in the valley.