Search

A VLSI design methodology is proposed for the efficient generation of multiple pseudorandom number sequences based on a simplification of Cauwenberghs' counterpropagation technique. We demonstrate that the counterpropagation of two sequences can be replaced by one propagating and one nonpropagating sequence, requiring as few as half the number of flip-flops, while still allowing new circuits to be added to the system without additional calculations-there is no need to keep track of random starting values, tap combinations, or time shifts. Moreover we extend our method from multiple bit sequences to multiple random number sequences that are uniformly distributed over any range of integers. In particular we address the more general problem of generating sequences over the range [0,K] where K+1 is any desired integer, including a power of two or a prime number. To this end we demonstrate that the simple concatenation of random bits to form random bytes is a special case of a more general concept whereby random integers distributed over prime number ranges are concatenated to form random integers distributed over any range. We find that the proposed design compares favorably with design strategies based on cellular automata, both in terms of statistical properties and implementation efficiencies.

We report seismic evidence for a unique 300 km thick layer at the base of the mantle beneath the south Atlantic ocean, with a steeply dipping edge, anomalously low shear wave velocities linearly decreasing from 2% (top) to 10–12% (bottom), and a maximum P velocity decrease of 3% relative to the preliminary reference Earth model (PREM). These characteristics can be best explained by partial melt driven by a compositional change produced early in the Earth’s history and a vertical thermal gradient within the layer. This boundary layer may provide an explanation for the distinctive isotope geochemical DUPAL anomaly observed at some surface ocean islands.

The M(w) = 8.3 deep (636 kilometers) Bolivian earthquake of 9 June 1994 was the largest deep-focus earthquake ever recorded. Seismic data from permanent stations plus portable instruments in South America show that rupture occurred on a horizontal plane and extended at least 30 by 50 kilometers. Rupture proceeded at 1 to 3 kilometers per second along the down-dip azimuth of the slab and penetrated through more than a third of the slab thickness. This extent is more than three times that expected for a metastable wedge of olivine at the core of the slab, and thus appears to be incompatible with an origin by transformational faulting. These large events may instead represent slip on preserved zones of weakness established in oceanic lithosphere at the Earth's surface.

A new methodology has been recently proposed for the efficient generation of multiple pseudo-random bit sequences that are statistically uncorrelated [R. Kuehnel, March 2003]. Random sequences that are uniformly distributed over a range [0,K], where K+ 1 is a power of 2, can be constructed by forming a vector of M independent bit sequences, where M = log/sub 2/(K + 1). In this paper we demonstrate that this method of construction represents a special case of a more generalized approach in which K can be any positive integer. The procedures described here can be used to efficiently generate multiple independent random sequences that are uniformly distributed over any range.

The Brazilian Lithosphere Seismic Project is part of a multinational geological and geophysical program to investigate the structure, tectonics, and geologic evolution of the continental lithosphere beneath SE Brazil. The project, which will run for a period of at least 18 months, is a collaborative effort by the Carnegie Institution of Washington and the Institute of Astronomy and Geophysics of the University of Sao Paulo. The broad band seismic studies are aimed at mapping the fine structure of the continental crust and lithospheric mantle beneath Brazil and interpreting it in terms of the origin, evolution, and composition of the Brazilian shield. The first nine stations of a planned 14 station portable array of digital REFTEK dataloggers and the broad band threecomponent Streckeisen STS-2 seismometers have been installed, or are in the process of being installed, in southeast Brazil (see Figure 1). The seismic array traverses the four major tectonic provinces of the region --- the Archean Sao Francisco craton, the surrounding late Proterozoic BrasilianolPan-African mobile belts, the intracratonic Parana Basin, and the coastal Ribeira belt, a complex transcurrent shear system. Stations are configured about 50-100 km apart in an approximately east-west saw-tooth array along 200 S latitude, with an E-W aperture of approximately 800 km. The station configuration is designed to capitalize on the abundance of high-quality seismic sources, particularly Andean, that surround and illuminate the Brazilian shield. Digital data are recorded continuously at 10 slsec and in triggered mode at 50 slsec. Timing is via GPS clocks and is accurate to better than 1 msec absolute. The matched STS-2 three component recording covers the frequency band 0.0083 to 20 Hz. Preliminary results from receiver function analysis indicate that crustal thicknesses beneath the Sao Francisco craton and surrounding Archean crust are about 40 to 42 km. Crustal velocities in these regions increase approximately linearly with depth and there is no evidence of major midcrustal discontinuities. Results from receiver functions and from two station surface wave phase velocity inversion indicate that mantle shear wave velocities are high, about 4.8 to 4.9 krn/sec as in other cratonic areas, in the uppermost mantle beneath the Sao Francisco craton and surrounding area and to at least 130 km depth beneath the Parana Basin.