Crustal Reflection Data Research Articles

2018 Southeast Lachlan Seismic Survey: New Heights

Six hundred and twenty nine kilometres of deep crustal reflection data were collected for the Southeast Lachlan 2D seismic survey during March to April 2018. The purpose of the survey was to image ...

Enhancing crustal reflection data through curvelet denoising

Suppression of incoherent noise, which is present in the seismic signal and may often lead to ambiguous interpretation, is a key step in processing associated with crustal reflection data. In this paper, we make use of the parsimonious representation of seismic data in the curvelet domain to perform the noise attenuation while preserving the coherent energy and its amplitude information. Curvelets are a recently developed mathematical transform that has as one of its properties minimal overlap between seismic signal and noise in the transform domain, thereby facilitating signal-noise separation. The problem is cast as an inverse problem and the results are obtained by updating the solution at each iteration. We demonstrate the effectiveness of this procedure at removing noise on both synthetic shot gathers and a synthetic stacked seismic section. We then apply curvelet denoising to deep crustal seismic reflection data where the signal-to-noise ratio is low. The reflection data were recorded along Lithoprobe's SNORCLE Line 1 across Paleoproterozoic-Archean domains in Canada's Northwest Territories. After initial processing, we apply the iterative curvelet denoising to both pre-stack shot gathers and post-stack data. Ground roll, random noise and much of the anomalous vertical energy is removed from the pre-stack shot gathers, to the extent that crustal reflections, including those from the Moho, are clearly seen on individual gathers. Denoised stacked data show a series of dipping reflections in the lower crust that extend into the Moho. The Moho itself is relatively flat and characterized by a sharp, narrow band of reflections. Comparing the results for the stacked data with those from F-X deconvolution, curvelet denoising outperforms the latter by attenuating incoherent noise with minimal harm to the signal. Because curvelet denoising retains amplitude information, it provides opportunities for further studies of seismic sections through attribute analyses. Curvelet denoising provides an important new tool in the processing toolbox for crustal seismic reflection data.

Crustal‐scale prestack depth imaging of the 1994 and 1999 LARSE surveys

ABSTRACTWhile seismic imaging for crustal and mantle structures has traditionally relied on surface wave and refraction data, the use of reflection data for crustal‐scale targets has been largely limited to the common midpoint (CMP) stack techniques. The rapid increase in the number of seismograph array deployments in recent years in crustal and mantle seismology has reached a level such that a re‐examination of the imaging techniques is becoming necessary. In this paper we show the advantage of prestack depth imaging for crustal reflection studies, based on data from two reflection surveys of the Los Angeles Regional Seismic Experiment (LARSE) to map faults and crustal‐scale structures. Our analysis indicates that the quality of the previous images of these surveys is limited by the CMP stack technique. For comparison, we present here depth images of the same LARSE data using wave equation prestack depth imaging and a tomographic velocity model based on first arrivals of the LARSE surveys and local earthquakes. Our new images are considerably improved over previous images in terms of resolution and reflector continuity. The new images show reflectors throughout the crust and suggest truncations in the Moho associated with the San Andreas Fault. A series of bright reflector segments, which are associated with the San Gabriel and San Andreas faults have been identified and might represent reflections from the fault zones. Our results suggest that the presence of high noise level, strong lateral velocity heterogeneity and wide angle geometry argue for, rather than against, the use of prestack depth imaging over the simple CMP stack techniques. As demonstrated in this study, it is now viable to conduct prestack depth imaging of crustal reflection data using a velocity model based on earthquake first arrivals thanks to the dense acquisition deployment.

Skeletons and fractals — a statistical approach to deep crustal seismic data processing and interpretation

Coherent seismic reflection events are identified using seismic skeletonization, an automatic pattern recognition technique in which waveforms are parameterized such that the attributes of each event are identified and stored in a relational database, or ‘event file’. This new approach to seismic interpretation allows statistical analyses of the attributes including estimation of scaling laws for reflectivity observed on deep seismic data, and development of new filtering techniques based on characteristics such as dip and reflection event length. Applications of the technique to deep crustal reflection data sets from the Alberta Basement Transect of the LITHOPROBE project assist interpretation in areas that lack surface outcrop or other means of relating reflections from the deep crust to the surface.

Identification and interpretation of azimuthally varying crustal reflectivity with an example from the southern Canadian Cordillera

A new approach to seismic interpretation, seismic skeletonization, is used to analyze crustal reflection data by identifying numbers of reflection events, their lengths, their energies, and their dips, with respect to azimuths of recorded profiles. Application of the method to crustal reflection data in the southern Canadian Cordillera demonstrates strong directional dependence of crustal reflectivity, with persistent trends at 025–040° (205–220°), 085–095° (265–275°), 115–125° (295–305°), and 140–160° (320–340°). These trends are visible in both the upper and lower crust and may be related to regional structural trends. Further applications of the technique, particularly in different tectonic regions, may allow categorization of seismic patterns and cataloguing of signatures for complex reflection data in a manner similar to seismic stratigraphy and structural patterns in stratified rocks.

Crustal kinematics and deep structure of the northern Rhine Graben, Germany

Surface and subsurface geological data, unpublished information gleaned from industrial seismic surveys, and results from a deep reflection seismic profile across the Tertiary northern Rhine Graben indicate that crustal extension in excess of 6 km was achieved by asymmetric detachment of the upper crust at a depth of about 15 km. Comparison of a simple geometric‐kinematic cross‐sectional model and its depth extrapolation with the DEKORP‐9‐N deep crustal reflection data suggests that extension below the graben was achieved by slip along a gently westward dipping shear zone which truncated and offset lower crustal planar fabrics and which continues into the uppermost mantle. The planar lower crustal reflectors probably originated mainly during late Variscan (Permocarboniferous) crustal extension and/or magmatic underplating.

LITHOPROBE: a scientific update-new images of the continental crust

LITHOPROBE, Canada's national earth sciences research program which integrates geophysical, geological and geochemical investigations in a collaborative effort among scientists from universities, government and industry to extend and relate surface geology to structures at depth, is up and running. The Lithoprobe Secretariat is established at the University of British Columbia. The Lithoprobe Seismic Processing Facility (LSPF) for research reprocessing of Lithoprobe and related data is installed at the University of Calgary. Access by the Canadian research community will be available through remote site connections. All scientific aspects of the program are in operation and plans for the future are in hand. Some of the spectacular new images of continental crustal structures that have been obtained and their tectonic significance are highlighted in this summary. Individual contributions of specific studies by Lithoprobe scientists are included in this volume.Along the eastern portion of the Southern Cordillera Transect, 270 km of crustal reflection data acquired in 1985 and subsequent magnetotelluric results define the westward extent of autochthonous North American basement (truncated by the crustal scale east-dipping Slocan Lake fault zone), the thickening of Purcell Proterozoic supracrustal rocks by folding and thrusting and a major compressional shear zone underlying the Valhalla gneiss complex. In 1988, Seismic profiling was extended westward across the allochthonous terranes and plutonic complexes to the west coast. About 900 km of 60-fold, 18 s data were recorded by an industry contract crew; initial processing by contract is in progress.In our Appalachian transect, Lithoprobe East, more than 3000 km of marine crustal reflection data have been acquired around Newfoundland and in the Gulf of St. Lawrence. Three deep crustal blocks, each of which exhibits a characteristic Seismic signature, can be traced through the region. Intracrustal reflectors on the Seismic line run across the continental margin northeast of Newfoundland help determine the nature of extensional tectonics and associated sedimentary basin evolution on the rifted margin.The GLIMPCE Transect has enabled a glimpse of the mid-continent crust by recording marine reflection data in the Great Lakes. The sections from Lake Superior reveal — 10 km of sediments overlying a 20 km thick sequence of strongly reflective volcanics and inter-flow sediments, perhaps representing the greatest vertical extent of intra-continental rift deposits on Earth.The original Archean/Early Proterozoic crust is greatly thinned beneath the rift basin. In the Lake Huron/Georgian Bay profile, the Grenville front is imaged as the westernmost reflection of a spectacular 70 km wide band of southeast-dipping reflections that clearly truncates prominent sub-horizontal crustal reflections of the Superior granite-rhyolite terrane.The Kapuskasing Structure Zone is a sequence of lower crustal rocks that have been thrust upward along the Ivanhoe Lake cataclastic zone and are now exposed at the surface. As part of the Kapuskasing Transect, 340 km of regional deep reflection data and 20 km of “ high resolution” data (20 m station spacing, 20 m source interval and a higher sweep range) over the cataclastic zone were recorded in 1987–1988. The Abitibi sub-province of Ontario and Quebec contains mineralized belts for which it is famous. As a preliminary program for the Abitibi-Grenville Transect, 130 km of regional and 65 km of high-resolution Seismic profiles also were recorded in 1987–1988. These were centred over major fault zones with which mineralization is related. Data quality is excellent and the data are in the process of being interpreted.

Deep crustal structure and genesis from contrasting reflection patterns: an integrated approach

An integrated approach including technique development into data acquisition, processing and modeling is used to aid the Final interpretation of crustal reflection data by minimizing the many unknowns. Thus, a project may start out with a series of experiments involving data acquisition designed to attack a particular problem and conclude with modeling as a test to the interpretation of a problem.

Interpretation of migrated seismic reflection profiles across the northern Appalachians in Maine

A simple line-migration technique has been developed and applied to deep crustal reflection data collected in the northern Appalachians of Quebec and central Maine. Preliminary interpretation of these data, combined with the results of gravity and refraction studies, shows thinning of the crust in two distinct steps beneath the Silurian-Devonian Merrimack synclinorium and shows evidence for Mesozoic crustal stretching and dike intrusion.

Towards an understanding of the southern Appalachian Piedmont crustal transition—A multidisciplinary approach

The Piedmont of the southern Appalachians is characterized by significant geophysical and geological anomalies which indicate there is a major crustal transition. Multiple hypotheses, including a suture zone and a subducted continental margin, have been presented to explain the variations. Although crustal seismic reflection data have provided significant constraints, there are ambiguities inherent in the interpretation of such data. The ambiguities can be reduced by careful consideration of related geophysical and geological observations. Although the importance of correlating crustal reflection data with known geologic features by tracing reflections to drill holes or to the surface cannot be overestimated, only rarely are such correlations possible. In almost all interpretations of crustal reflection structure it is necessary to constrain the model with methods such as seismic refraction, gravity, magnetics, conductivity, and surface geology (including palinspastic reconstructions). When information from these techniques is incorporated into interpretations of the Piedmont crustal structure, the model which appears to be most consistent with the observations is one in which the upper crust of the Piedmont is decoupled from the lower crust, and in which the lower crust thins eastward. The lower crust may be a subducted Precambrian continental margin and its associated transition toward thinner, basinal crust.

Deep reflection experiments in northeastern Australia, 1976–1978

Between 1976 and 1978 the Australian Bureau of Mineral Resources (BMR) recorded deep crustal reflection data at seven sites in northeastern Australia over continuous profiles up to 15 km long by simply extending the recording time to 16 sec during normal sedimentary basin surveys. The record sections show many events with variable strength, continuity, dip, and spatial distribution. By comparing the sections from the longer and the shorter perpendicular traverses, it is possible to discriminate between primary reflections and diffractions, multiples, and other noise events. Based on their character the reflections can be grouped into zones which are interpreted in terms of the nature and structure of the crust. Most of the reflection sections show, below the sedimentary reflections, a thin (2–3 sec) reflection‐free zone underlain by a thick (9 sec or more) zone of numerous reflection segments which varies in thickness and the distribution of reflection segments. The data thus suggest that the upper crust under the sediments is similar in seismic character throughout northeastern Australia. On the other hand, the deeper crust under the Georgina and Drummond basins is significantly different in seismic signature and thickness from that under the Bowen basin and the northern margin of the Galilee basin. It is concluded that good quality deep reflections can be recorded with little extra effort during sedimentary basin surveys using modern multiple‐fold common‐depth‐point (CDP) techniques, and that the data recorded on long traverses, cross‐spreads, and expanded spreads provide information on the structure and composition of the crust and upper mantle with a resolution greater than has been possible before. Such information is valuable in studying the evolution of mineral and petroleum provinces and the lithosphere in general.

The Laramide orogeny: Evidence from cocorp deep crustal seismic profiles in the wind river mountains, wyoming

Deep crustal reflection data that are critical for the interpretation of Laramide structure have been obtained by the Consortium for Continental Reflection Profiling (COCORP). The Laramide orogeny, which occurred from the late Cretaceous to early Eocene, is characterized in Wyoming by large uplifts of Precambrian basement, commonly flanked by reverse faults. The attitude of these faults at depth has been a major tectonic problem and is very important for deciding whether horizontal or vertical crustal movements were primarily responsible for the basement uplifts. COCORP has run 158 km of deep seismic reflection profiles (recording to 20-sec two-way travel time) across the southeastern end of the Wind River Mountains, the largest of these Laramide uplifts. Reflections from the thrust fault flanking the Wind River uplift can be clearly traced on the profiles to at least 24-km depth and possibly as deep as about 36 km with a fairly uniform apparent dip of 30°–35°. Other reflection events subparallel to the main Wind River thrust are present in the seismic profiles and may represent other faults. There is at least 21 km of crustal shortening along the thrust. There is no evidence in the reflection profiles for large-scale folding of the basement; the Wind River Mountains were formed predominantly by thrust movements. Gravity anomalies in the Wind River Mountains can be modeled by a thrust that displaces dense material in the lower crust. If the thrust ever cut the Moho, the effect is not observed in the gravity today. A proposed model for the presence of uplifted basement in Wyoming invokes a shallowly dipping, subducted Farallon plate beneath the North American continent; drag between the two plates localized compressional stresses in an area over 800 km into the North American plate causing large thrusts to develop. The earth's crust seems to have fractured as a fairly rigid plate