Maximum-entropy Surrogation in Network Signal Detection

Multiple-channel detection is considered in the context of a sensor network where raw data are shared only by nodes that have a common edge in the network graph. Established multiple-channel detectors, such as those based on generalized coherence or multiple coherence, use pairwise measurements from every pair of sensors in the network and are thus directly applicable only to networks whose graphs are completely connected. An approach introduced here uses a maximum-entropy technique to formulate surrogate values for missing measurements corresponding to pairs of nodes that do not share an edge in the network graph. The broader potential merit of maximum-entropy baselines in quantifying the value of information in sensor network applications is also noted.Comment: 4 pages, submitted to IEEE Statistical Signal Processing Workshop, August 201

Temperatures of shock-induced shear instabilities and their relationship to fusion curves

New emission spectra for MgO and CaAl_2Si_2O_8 (glass) are observed from 430 to 820 nm. Taken with previous data, we suggest that transparent solids display three regimes of light emission upon shock compression to successively higher pressures: (1) characteristic radiation such as observed in MgO and previously in other minerals, (2) heterogeneous hot spot (greybody) radiation observed in CaAl_2Si_2O_8 and previously in all transparent solids undergoing shock-induced phase transformations, and (3) blackbody emission observed in the high pressure phase regime in NaCl, SiO_2, CaO, CaAl_2Si_2O_8, and Mg_2SiO_4. The onset of regime (2) may delineate the onset of shock-induced polymorphism whereas the onset of regime (3) delineates the Hugoniot pressure required to achieve local thermal equilibrium in the shocked solid. We also propose that the hot spot temperatures and corresponding shock pressures determined in regime (2) delineate points on the fusion curves of the high pressure phase

Shock temperatures in silica glass: Implications for modes of shock-induced deformation, phase transformation, and melting with pressure

Gray body temperatures and emittances of silica glass under shock compression between 10 and 30 GPa are determined. Observed radiative temperatures are higher than computed continuum temperatures for shock-compressed silica glass; however, below ∼26 GPa observed emittances are <0.02. This suggests that fused quartz deforms heterogeneously in this shock pressure range as has been observed in other minerals. Between 10 and 16 GPa, radiative temperatures decrease from 4400 K to 3200 K, whereas above 16–30 GPa, gray body temperatures of ∼3000 K with low emittances are observed. The emittances increase with pressure from 0.02 to 0.9. The pressure range from 10 to 16 GPa coincides with the permanent densification region, while the 16–30 GPa range coincides with the inferred mixed phase region along the silica glass Hugoniot. The differing radiative behaviors may relate to these modes of deformation. Based upon earlier shock recovery experiments and a proposed model of heterogeneous deformation under shock compression, the temperatures associated with low emittances in the mixed phase region probably represent the melting temperature of the high-pressure phase, stishovite, which can be expected to crystallize from a melt in hot zones. Above 20 GPa the melting temperature of stishovite would therefore be 3000 K±200 K and almost independent of pressure to 30 GPa. The effects of pressure on melting relations for the system SiO_2–Mg_2SiO_4 are considered together with the proposed stishovite melting curve and suggested maximum solidus temperatures within the mantle of ∼2370 K at 12.5 GPa and ∼2530 K at 20.0 GPa. Using the proposed stishovite melting temperatures Tm and estimates of upper mantle temperatures T, the effective viscosity, which can be considered a function of the homologous temperature T/T_m, appears to remain nearly constant from 200 to 600 km depth in the Earth

Brushless DC motor control system responsive to control signals generated by a computer or the like

A control system for a brushless DC motor responsive to digital control signals is disclosed. The motor includes a multiphase wound stator and a permanent magnet rotor. The rotor is arranged so that each phase winding, when energized from a DC source, will drive the rotor through a predetermined angular position or step. A commutation signal generator responsive to the shaft position provides a commutation signal for each winding. A programmable control signal generator such as a computer or microprocessor produces individual digital control signals for each phase winding. The control signals and commutation signals associated with each winding are applied to an AND gate for that phase winding. Each gate controls a switch connected in series with the associated phase winding and the DC source so that each phase winding is energized only when the commutation signal and the control signal associated with that phase winding are present. The motor shaft may be advanced one step at a time to a desired position by applying a predetermined number of control signals in the proper sequence to the AND gates and the torque generated by the motor may be regulated by applying a separate control signal to each AND gate which is pulse width modulated to control the total time that each switch connects its associated winding to the DC source during each commutation period

Shear Modulus Dispersion in Cracked and Fluid-Saturated Quartzites: Experimental Observations and Modeling

The effect of pore fluids on acoustic wave dispersion in rocks with low aspect ratio crack porosity is important for the interpretation of laboratory and field observations in hard rock mineral exploration environments. Here we make laboratory measurements of shear modulus dispersion at frequencies 0.01–1 Hz and at 1 MHz with different saturating fluids (dry, argon, and water saturated) in two thermally cracked quartzite samples with ~2% total porosity. Measurements are made across a range of effective pressures (10–150 MPa), with the resulting very low permeabilities of the samples varying from 1–300 × 10−21 m2. Moduli across the 0.01–1 Hz band were typically independent of frequency. The shear moduli measured at sub‐Hz frequencies are unaffected by fluid saturation, as expected for the saturated isobaric (Gassmann) regime. In marked contrast, water saturation of the cracked rocks results in very large increases in the shear moduli measured at 1 MHz and low effective pressures, indicative of saturated isolated conditions. Thus, at an effective pressure of 20 MPa, the shear moduli for the two water‐saturated quartzites increase by 74% and 98% from 1 Hz to 1 MHz. The contrast in elastic moduli between dry and water‐saturated conditions is well represented by the theoretical model developed by Walsh and others. The observed dispersion highlights the need for care in seismological application of results obtained at MHz frequencies from laboratory ultrasonic measurements.The work was funded through NSERC Discovery grants to D. R. S. and grant DP110101830 from the Australian Research Council to I. J. and D. R. S. H. S.’s visit to Canberra was funded by the NSERC Michael Smith Foreign Study Supplement

Holographic in situ stress measurements

A new instrument for measuring the in situ level of stress in boreholes has been developed. The instrument operates on the principle of locally relieving the stresses acting on a rock mass by drilling a small hole into the borehole surface and recording the resultant displacement field by holographic interferometry. Because the recording technique is optical, the entire displacement field due to stress relief is obtained. A description of the stressmeter, theory of the interferometric technique, data reduction methodology, and results of laboratory stress relief calibration tests are presented. In addition, we present results from a field deployment of the instrument in an underground shale mine in Garfield County, Colorado using a test borehole within a support pillar. Sufficient data were obtained to constrain five of six stress components at a shallow level of the test borehole, thereby demonstrating the viability of the holographic technique. The holographic stress-relief data yield an approximate EW maximum horizontal stress direction. By comparison with previous hydrofracture measurements of Bredehoeft et al., our results indicate substantial stress-relief near the pillar face, thus masking any relicts of the far-field tectonic stress

In-situ holographic elastic moduli measurements from boreholes

We have developed a unique technique employing optical holography to measure the static Young's modulus (E) from a borehole. In the experiment, a known point force induces micron scale displacements on the borehole wall which are recorded by a double-exposure hologram. Raw data consist of dark fringes superimposed on the three-dimensional image whose pattern is modeled to find E directly. In the laboratory, the holographic technique determined E on rock and metal samples to an uncertainty better than 10 percent. For example, double exposure holograms of a saw-cut sample of dolomitic marlstone gave an E of 16.8 ± 2.8 GPa in agreement with 17.2 ± 2.0 GPa predicted by published density-modulus relationships. Field tests of a holographic tool in a horizontal mine pillar borehole gave in-situ Es which range from 26.9 to 36.0 GPa. Although these data could be interpreted as localized elastic heterogeneity within the rock mass, elastic anisotropy of the rock is a possible explanation for this variation