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Wide-angle reflections generated by five controlled blasts and over 110 timed quarry blasts in the Southern Appalachians were used to test models for isostatic compensation of topography. The profiles cross the Appalachian gravity gradient and gravity low and sample the highest elevations within the orogen. Migration of P, SV, and SH reflections suggests that crustal thickness varies from 35 to 39 km within the coastal plain and 37-39 km within the Carolina terrane. It increases northwestward from 40 to 45 km across the Inner Piedmont, and then thickens to 50-52 km along the southeastern flank of the Blue Ridge Mountains. Crustal thickness within the Blue Ridge Mountains ranges from 47 to 56 km. Receiver functions for broadband stations GOGA and MYNC show a similar pattern in crustal thickness. Assuming Airy compensation, the correlation between elevation and Moho depth suggests a range of 50-150 kg/m3 for the density contrast between the crustal root and mantle. The greatest Moho depths are associated not with the tallest peaks, but rather with the broadest portions of the mountain chain. This observation is consistent with regional bending of the lithosphere. However, the planar basement surface suggests that the root either predates Alleghanian thrusting, and therefore is unrelated to the present topography, or formed in response to some other mechanism. Bounds on curvature of the basement surface suggest a lower bound of 30-40 km for the effective elastic thickness of the lithosphere. This is consistent with previous estimates for the southern Appalachians based on analysis of gravity data. [ABSTRACT FROM AUTHOR]

Data for two broadband stations and three-component recordings of wide-angle reflections are used to constrain the velocity structure and depth to Moho beneath a portion of the southern Appalachians. Broadband data from the U. S. National Seismic Network (USNSN) station GOGA (Carolina terrane/Inner Piedmont boundary), constrained by average crustal Vp derived from wide-angle data, suggest that crustal thickness for individual back azimuths ranges from 41 to 43 km. This is consistent with wide-angle estimates of 38-40 km for the Carolina terrane and 40-45 km for the Inner Piedmont. Average crustal Vp/Vs is 1.72-1.76, compared with 1.74-1.76 derived from SmSIPmP travel-time ratios for the Carolina terrane. The results for station MYNC (Blue Ridge Mountains), again constrained by wide-angle data, suggest a crustal thickness of 50-52 km for individual back azimuths. Average Vp/Vs is 1.75-1.77. The range in average Vp/Vs derived from wide-angle data is broader (1.74-1.80), indicating variations in average composition across the Blue Ridge Mountains, as suggested by gravity data. Large-amplitude direct P and Ps arrivals on transverse-component receiver functions also indicate significant lateral variations in structure beneath MYNC. Comparison of migrated images underscores the differences in resolution for single-station broadband data and wide-angle data recorded with short-aperture arrays. Ps waveforms for MYNC show a double peak. The earlier peak migrates to depths of 48.5-51 km. in agreement with wide-angle events interpreted as Moho. For the later peak, migrated depths range from 60 to 65 km, compared with 59-68 km for peaks in wide-angle reflectivity observed for the Blue Ridge Mountains. The two methods yield consistent estimates for crustal thickness that lend support to models that predict a significant crustal root beneath the high elevations of the Blue Ridge. [ABSTRACT FROM AUTHOR]

Migration of wide-angle reflections generated by quarry blasts suggests that crustal thickness increases from 38 km beneath the Carolina Terrane to 47-51 km along the southeastern flank of the Blue Ridge. The migration algorithm, developed for generating single-fold images from explosions and earthquakes recorded with isolated, short-aperture arrays, uses the localized slant stack as an intermediate data set. In contrast with other methods, it includes an interpretive step that is based on the assumption that all coherent P-wave energy consists of reflections from planar interfaces. Each sample in the slant stack is mapped into a planar, dipping segment with a length that is determined by the recording aperture. Migrated sections from within the Blue Ridge show increases in reflectivity at depths of 20 and 40 km. High apparent reflectivity from 40 to 50-55 km suggests a layered zone in the lower crust that is similar to models proposed for the Cumberland Plateau in Tennessee and the Adirondacks. The migration results are consistent with regional gravity data and with the occurrence of crustal roots beneath the Urals, another Paleozoic orogen. [ABSTRACT FROM AUTHOR]

Quarry blasts can be effective, low-cost energy sources for seismic imaging, provided that one can deconvolve the extended source signatures produced by tipple firing. This study uses real and synthetic quarry-blast data to compare the performance of several methods for deconvolving mixed-delay signals. The problem considered here is the recovery of the effective source function along a given azimuth from traces in a shot gather. The tests focus particularly on design criteria for minimum-entropy filters and optimization of source-wavelet estimates derived by inversion of minimum-entropy filter coefficients. Predictive deconvolution reduces tinging but can generate coherent artifacts when the source wavelet is not minimum delay. Wiener filtering using source-wavelet estimates derived by least-squares inversion of minimum-entropy filter coefficients preserves relative amplitudes, allows the user to specify the degree of spiking, and avoids delays in the output. Of 47 blasts recorded in Georgia and Tennessee with 15- to 19-channel arrays, 37 yielded a localized wavelet estimate in which the duration of the most energetic portion of the wavelet estimate trace was close to the reported duration of the blast. In general, extraction of the source wavelet directly from quarry-site recordings is complicated by nonlinear effects, interference from S and Rayleigh waves, and the variation of the source wavelet with azimuth. In spite of those complications, waveforms observed at quarry sites were similar to wavelets derived from field traces for about 10 of the blasts studied. Wavelet estimates derived from minimum-entropy filter coefficients are not affected by static shifts between traces. Where statics have been removed and where recording arrays are long enough to resolve differences in ray parameter for overlapping events, localized slant stacks can be a useful alternative for estimating the source wavelet. Methods, such as Wiener filtering, that... [ABSTRACT FROM AUTHOR]