The lacustrine Itapema Formation in the Santos Basin locally comprises 102 m thick clinoforms identified seismically and corroborated by several well penetrations. Individual clinoforms, as proven by well penetrations, are composed of 102 m thick successions of basinward-dipping molluscan grainstones and rudstones. Manual dip picking of borehole images show upward increasing dips consistent with seismic geometries and a predominance of longshore sediment transport. Clinoforms are bound at their top and base by strata with significantly lower dips recognizable both on seismic and borehole images. Elevated gamma-ray log responses together with sidewall core samples indicate that these intervals correspond to more argillaceous facies which are interpreted as lake flooding events. While the existence of bona fide clinoforms is demonstrated by a range of subsurface data their precise origin remains enigmatic. The majority of the bivalve genera that make up the grain-supported carbonates appear to be infaunal or semi-infaunal. As such the clinoforms represent large bars produced through the re-working of bivalves from lower energy depositional environments by shore-parallel currents.
The Mw 5.8 earthquake that occurred in Louisa County, Virginia, on 23 August 2011 provided an opportunity to record with several “high density” seismic arrays, in addition to traditional, sparse temporary seismic networks. Traditional aftershock networks consist of a few dozen stations spread over tens of kilometers. As a result, the recorded seismic waveforms suffer from spatial aliasing that is so severe that many types of waveform processing are not applicable. Here we report the results of recording with a large number of oil industry‐type instruments deployed at a spacing that is an order of magnitude closer than in traditional deployments. The objective was to image subsurface structure with array methods, using the aftershocks as sources. The dense array recorded continuously for 12 days and consisted of 172 vertical component seismometers that were placed at 200–400 m and a 60 km long three‐component regional profile with stations every 2 km. We demonstrate how processing techniques from Vertical Seismic Profiling can produce high‐resolution 3‐D reflection images of structure beneath the array. These images display reflectivity that correlates with that observed on a nearby deep reflection survey collected by the U.S. Geological Survey. Of particular interest is a strong reflector imaged across multiple profiles. Our analysis demonstrates how a surface array of seismometers can provide 3‐D images of structure using microearthquake sources when wavefields are sampled sufficiently densely.
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