Multiscale seismic imaging of active fault zones for hazard assessment: A case study of the Santa Monica fault zone, Los Angeles, California

Geophysics ◽  
1998 ◽  
Vol 63 (2) ◽  
pp. 479-489 ◽  
Author(s):  
Thomas L. Pratt ◽  
James F. Dolan ◽  
Jackson K. Odum ◽  
William J. Stephenson ◽  
Robert A. Williams ◽  
...  
Keyword(s):  
High Resolution ◽  
Los Angeles ◽  
Hazard Assessment ◽  
Seismic Reflection ◽  
Seismic Profile ◽  
Strike Slip ◽  
Alluvial Fan ◽  
Depth Range ◽  
Seismic Reflection Profile ◽  
Santa Monica

High‐resolution seismic reflection profiles at two different scales were acquired across the transpressional Santa Monica Fault of north Los Angeles as part of an integrated hazard assessment of the fault. The seismic data confirm the location of the fault and related shallow faulting seen in a trench to deeper structures known from regional studies. The trench shows a series of near‐vertical strike‐slip faults beneath a topographic scarp inferred to be caused by thrusting on the Santa Monica fault. Analysis of the disruption of soil horizons in the trench indicates multiple earthquakes have occurred on these strike‐slip faults within the past 50 000 years, with the latest being 1000 to 3000 years ago. A 3.8-km-long, high‐resolution seismic reflection profile shows reflector truncations that constrain the shallow portion of the Santa Monica Fault (upper 300 m) to dip northward between 30° and 55°, most likely 30° to 35°, in contrast to the 60° to 70° dip interpreted for the deeper portion of the fault. Prominent, nearly continuous reflectors on the profile are interpreted to be the erosional unconformity between the 1.2 Ma and older Pico Formation and the base of alluvial fan deposits. The unconformity lies at depths of 30–60 m north of the fault and 110–130 m south of the fault, with about 100 m of vertical displacement (180 m of dip‐slip motion on a 30°–35° dipping fault) across the fault since deposition of the upper Pico Formation. The continuity of the uncomformity on the seismic profile constrains the fault to lie in a relatively narrow (50 m) zone, and to project to the surface beneath Ohio Avenue immediately south of the trench. A very high‐resolution seismic profile adjacent to the trench images reflectors in the 15 to 60 m depth range that are arched slightly by folding just north of the fault. A disrupted zone on the profile beneath the south end of the trench is interpreted as being caused by the deeper portions of the trenched strike‐slip faults where they merge with the thrust fault.

A high resolution seismic reflection profile at the Prince Ranch, South Dakota

1989 ◽  
Author(s):  
Robert A. Williams ◽  
K.W. King ◽  
D.L. Carver ◽  
D.M. Worley
Keyword(s):  
High Resolution ◽  
South Dakota ◽  
Seismic Reflection ◽  
Seismic Reflection Profile ◽  
Reflection Profile

PROCESSING OF SEISMIC DIFFRACTIONS FROM CARBONATE NODULES IN CLAY FORMATIONS

Geophysics ◽  
2021 ◽  
pp. 1-64
Author(s):  
Cinzia Bellezza ◽  
Flavio Poletto ◽  
Biancamaria Farina ◽  
Giorgia Pinna ◽  
Laurent Wouters ◽  
...  
Keyword(s):  
High Resolution ◽  
Seismic Reflection ◽  
Synthetic Data ◽  
Seismic Profile ◽  
Radioactive Waste Disposal ◽  
Small Scale ◽  
Boom Clay ◽  
Seismic Image ◽  
Imaging Results ◽  
Clay Formation

The problem of localizing small (relative to wavelength) scatterers by diffractions to enhance their use in identifying small-scale details in a seismic image is extremely important in shallow exploration, to identify interesting features such as fractures, caves and faults. The conventional approach based on seismic reflection is limited in resolution by the Rayleigh criterion. In certain acquisition geometries, such as crosswell surveys aimed at obtaining high resolution signals, the availability of suitable datasets for effective migration depends on the spatial extent of the available source and receiver data intervals. With the aim of overcoming the resolution limits of seismic reflection, we studied the detectability, response, and location of meter- and possibly sub-meter-dimension carbonate concretions (septaria) in the Boom Clay Formation (potential host rocks for radioactive waste disposal) by diffraction analysis of high-frequency signals. We investigated diffraction wavefields by signal separation, focusing, and high-resolution coherency analysis using the MUltiple Signal Classification (MUSIC) method and semblance. The investigation was performed for two different surveys in Belgium, a shallow and high resolution Reverse Vertical Seismic Profile (RVSP) and a near-offset crosswell application at Kruibeke and ON-MOL-2 sites, respectively. The data analysis is supported by synthetic wavefield modeling. The multi-offset RVSP provides the appropriate geometry to observe and investigate the septaria diffractions both from depth and the surface. The crosswell approach, calibrated using synthetic data in the analysis of wavefield patterns in 2D, shows promising imaging results with field data of a selected diffraction zone in the interwell area.

Shallow, high‐resolution seismic imaging at the Ansil mining camp in the Abitibi greenstone belt

Geophysics ◽  
1998 ◽  
Vol 63 (2) ◽  
pp. 379-391 ◽  
Author(s):  
Gervais Perron ◽  
Andrew J. Calvert
Keyword(s):  
High Resolution ◽  
Seismic Reflection ◽  
Fresnel Zone ◽  
Massive Sulfide ◽  
Sulfide Mineral ◽  
Reflection Coefficients ◽  
Seismic Reflection Profile ◽  
Volcanic Stratigraphy ◽  
Seismic Reflection Data ◽  
Deep Mine

Volcanic rocks in, and around, the Ansil mining camp host a large number of massive sulfide mineral deposits. A high‐resolution seismic reflection profile was shot across the camp with the objective of mapping the contacts between the different volcanic units at which most of the ore bodies have been found. Numerous exploration boreholes define the geology to a depth of 1600 m and allow a precise comparison with the recorded reflections. Geophysical logs obtained in one deep borehole suggest that reflection coefficients between the andesite‐rhyolite units of the volcanic stratigraphy are around 0.05, but few corresponding reflections can be identified in the seismic data. Those reflections in the surface seismic profile that can be correlated with the subsurface geology originate from diorite sills, at which reflection coefficients are between 0.05 and 0.11. We suggest that reflections are observed from the diorite sills because the sills were intruded as sheets, some along fault planes, resulting in interfaces that extend over an area much greater than the first Fresnel zone. The contacts between the rhyolite‐andesite volcanic units may be highly variable spatially, preventing any strong reflection response, in contrast to the results of 1-D synthetic seismograms calculated from the borehole logs. Thus, the strength of a reflection from a lithological contact in igneous rock is likely to be related as much to the way the contact was created as to the magnitude of any local change in seismic impedance across it. Although it did not prove possible to map the volcanic stratigraphy of the Ansil mining camp, reflections interpreted to be from the disused, 1300-m-deep mine galleries were recorded. The seismic reflection data also indicate that the tonalitic Flavrian pluton, which underlies the volcanics, is an imbricated, tabular body, crosscut by diorite sills. Seismic reflections in the pluton arise from the sills and, possibly, primary magmatic layering.

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