ALBERT

Description: We investigate charge generation as a function of stress in fine-grained gabbro for both nominally dry samples and samples fully saturated with electrically conductive brine fluids similar to those observed in active earthquake fault zones. These experiments address a number of proposed and reported electrical precursory and coseismic phenomena associated with earthquakes. Compressive load was applied to one end of the sample in repetitive cycles using a pair of precision steel platens driven by a large hydraulic press. The samples were tested by cycling between constant low stress and constant high stress values with a 200-s periodicity. Net charge transport between the stressed and unstressed sample ends was monitored with a picoammeter. For the nominally dry samples, stress-stimulated current (SSC) transients on the order of 50–400 pA peak-to-peak were observed with a decay time constant ~10 s during stress loading and unloading. Under constant compressive loads of ~22 MPa, small negative polarity SSC of ~15 pA magnitude was observed as an offset from the baseline current at low load (5 MPa) conditions. For the fluid-saturated samples, neither transients nor SSCs were observed as a function of stress when the load was cycled, an observation that is consistent with more rapid internal self-discharge due to higher electrical conductivity of the sample. Because the Earth’s crust is fluid saturated, observation of significant electrical charge buildup is not expected during the observed slow stress accumulation prior to earthquakes or during any slow precursory stress release that may occur in the region of earthquake nucleation. However, observation of coseismic charge generation due to electrokinetic, triboelectric, and other processes may occur during earthquake stress drops, surface rupture, and seismic-wave arrivals from dynamic rupture.

Description: We investigate charge generation as a function of stress in fine-grained gabbro for both nominally dry samples and samples fully saturated with electrically conductive brine fluids similar to those observed in active earthquake fault zones. These experiments address a number of proposed and reported electrical precursory and coseismic phenomena associated with earthquakes. Compressive load was applied to one end of the sample in repetitive cycles using a pair of precision steel platens driven by a large hydraulic press. The samples were tested by cycling between constant low stress and constant high stress values with a 200-s periodicity. Net charge transport between the stressed and unstressed sample ends was monitored with a picoammeter. For the nominally dry samples, stress-stimulated current (SSC) transients on the order of 50-400 pA peak-to-peak were observed with a decay time constant approximately 10 s during stress loading and unloading. Under constant compressive loads of approximately 22 MPa, small negative polarity SSC of approximately 15 pA magnitude was observed as an offset from the baseline current at low load (5 MPa) conditions. For the fluid-saturated samples, neither transients nor SSCs were observed as a function of stress when the load was cycled, an observation that is consistent with more rapid internal self-discharge due to higher electrical conductivity of the sample. Because the Earth"s crust is fluid saturated, observation of significant electrical charge buildup is not expected during the observed slow stress accumulation prior to earthquakes or during any slow precursory stress release that may occur in the region of earthquake nucleation. However, observation of coseismic charge generation due to electrokinetic, triboelectric, and other processes may occur during earthquake stress drops, surface rupture, and seismic-wave arrivals from dynamic rupture.

Description: Papers focused on land surface, atmospheric, and ocean properties are reported. Specific comments pertaining to polarization, models and inversion, and measurements, are given. Recommendations are: continued research into the application potential of the BRDF (Bidirectional Reflectance Distribution Function) and polarization properties of ground surface and atmospheric targets; three dimensional models, which account for the statistical behavior of remotely sensed data, should be extended and inverted in order to support analysis of data potentially covering rolling terrain such that pixels represent heterogeneous mixtures of surface cover types and project ground footprints with sizes between 10 to 6 km, the ground pixel sizes of planned future sensors; available reflectance models should be further validated by means of multi dimensional (directional, spectral, temporal) field data and existing models should be intercompared in more depth to evaluate their performance and limitations; existing methods for model inversion should be validated in more depth in order to quantify the practical limitations and the expected accuracy of the parameters retrieved and new approaches should be developed based upon apriori knowledge of plant canopy development and spectral BRDF properties; there is a need to establish a protocol of validation and intercomparison of the indices and compositing techniques which have been proposed during these last years.

Description: The authors examine the hypothesis that some part of the ecosystem-dependent variability of the vegetation indices is attributable to the effects of specular reflection of sunlight by leaves. A new class of vegetation indices, or 'minus specular' vegetation indices, is defined to account for the effects of specularly reflected light. Results show that the 'minus specular' indices, when compared to the traditional vegetation indices, potentially provide better estimates of the photosynthetic activity of a canopy than the traditional vegetation indices, particularly as a function of sun and view angles.

Description: The hypothesis tested was that some part of the ecosystem-dependent variability of vegetation indices was attributable to the effects of light specularly reflected by leaves. 'Minus specular' indices were defined excluding effects of specular light which contains no cellular pigment information. Results, both empirical and theoretical, show that the 'minus specular' indices, when compared to the traditional vegetation indices, potentially provide better estimates of the photosynthetic activity within a canopy - and therefore canopy primary production - specifically as a function of sun and view angles.

Description: Among the most important short-term dynamic biological processes are diurnal changes in canopy water relations. Plant regulation of water transport through stomatal openings affects other gaseous transport processes, often dramatically decreasing photosynthetic fixation of carbon dioxide during periods of water stress. Water stress reduces stomatal conductance of water vapor through the leaf surface and alters the diurnal timing of stomatal opening. Under non-water stressed conditions, stomates typically open soon after dawn and transpire water vapor throughout the daylight period. During stress periods, stomates may close for part of the day, generally near mid-day. Under prolonged stress conditions, stomatal closure shifts to earlier times during the day; stomates may close by mid-morning and remain closed until the following morning - or remain closed entirely. Under these conditions the relationship between canopy greenness (e.g., measured with a vegetation index or by spectral mixture analysis) and photosynthetic fixation of carbon is lost and the remotely sensed vegetation metric is a poor predictor of gas exchange. Prediction of stomatal regulation and exchange of water and trace gases is critical for ecosystem and climate models to correctly estimate budgets of these gases and understand or predict other processes like gross and net ecosystem primary production. Plant gas exchange has been extensively studied by physiologists at the leaf and whole plant level and by biometeorologists at somewhat larger scales. While these energy driven processes follow a predictable if somewhat asymmetric diurnal cycle dependent on soil water availability and the constraints imposed by the solar energy budget, they are nonetheless difficult to measure at the tree and stand levels using conventional methods. Ecologists have long been interested in the potential of remote sensing for monitoring physiological changes using multi-temporal images. Much of this research has focused on day-to-day changes in water use, especially for agricultural applications. Ustin et al. showed seasonal changes in canopy water content in chaparral shrub could be estimated using optical methods. Vanderbilt et al. followed asymmetric diurnal changes in the reflectance of a walnut orchard, but could not attribute specific reflectance changes to specific changes in canopy architecture or physiology. Forests and shrub lands in California experience prolonged periods of drought, sometimes extending six months without precipitation. The conifer and evergreen chaparral communities common to the foothill region around the central valley of California retain their foliage throughout the summer and have low transpiration rates despite high net radiation and temperature conditions. In contrast, grasslands and drought resistant deciduous species in the same habitat are seasonally dormant in summer. Because of differences in the mechanisms of drought tolerance, rooting depth and physiology between different plant communities in the region, it is likely that they display differences in diurnal water relations. The presence of diverse plant communities provides an opportunity to investigate possible diurnal landscape patterns in water relations that could be observed by an airborne hyperspectral scanner. This investigation of AVIRIS data collected over forest and shrub land represents the continuation of a prior investigation involving spectral mixture analysis of diurnal effects in the same AVIRIS data set.

Description: A formula is derived that describes the size of the solid angle within which must be the normal to a flat specular reflector, if such a reflector is to redirect light from a small but finite extended source to a small but finite extended detector. The formula assumes the rules of geometrical optics and does not account for the effects of diffraction.

Description: The specular reflection process is shown to be a key aspect of radiation transfer by plant canopies. Polarization measurements are demonstrated as the tool for determining the specular and diffuse portions of the canopy radiance. The magnitude of the specular fraction of the reflectance is significant compared to the magnitude of the diffuse fraction. Therefore, it is necessary to consider specularly reflected light in developing and evaluating light-canopy interaction models for wheat canopies. Models which assume leaves are diffuse reflectors correctly predict only the diffuse fraction of the canopy reflectance factor. The specular reflectance model, when coupled with a diffuse leaf model, would predict both the specular and diffuse portions of the reflectance factor. The specular model predicts and the data analysis confirms that the single variable, angle of incidence of specularly reflected sunlight on the leaf, explains much of variation in the polarization data as a function of view-illumination directions.