High‐resolution and super stacking of time‐reversal mirrors in locating seismic sources

Time reversal mirrors can be used to backpropagate and refocus incident wavefields to their actual source location, with the subsequent benefits of imaging with high‐resolution and super‐stacking properties. These benefits of time reversal mirrors have been previously verified with computer simulations and laboratory experiments but not with exploration‐scale seismic data. We now demonstrate the high‐resolution and the super‐stacking properties in locating seismic sources with field seismic data that include multiple scattering. Tests on both synthetic data and field data show that a time reversal mirror has the potential to exceed the Rayleigh resolution limit by factors of 4 or more. Results also show that a time reversal mirror has a significant resilience to strong Gaussian noise and that accurate imaging of source locations from passive seismic data can be accomplished with traces having signal‐to‐noise ratios as low as 0.001. Synthetic tests also demonstrate that time reversal mirrors can sometimes enhance the signal by a factor proportional to the square root of the product of the number of traces, denoted as N and the number of events in the traces. This enhancement property is denoted as super‐stacking and greatly exceeds the classical signal‐to‐noise enhancement factor of . High‐resolution and super‐stacking are properties also enjoyed by seismic interferometry and reverse‐time migration with the exact velocity model.     

Characterization of surface and ground water δ18O seasonal variation and its use for estimating groundwater residence times

¹⁸O is an ideal tracer for characterizing hydrological processes because it can be reliably measured in several watershed hydrological compartments. Here, we present multiyear isotopic data, i.e. ¹⁸O variations (δ¹⁸O), for precipitation inputs, surface water and groundwater in the Shingobee River Headwaters Area (SRHA), a well‐instrumented research catchment in north‐central Minnesota. SRHA surface waters exhibit δ¹⁸O seasonal variations similar to those of groundwaters, and seasonal δ¹⁸O variations plotted versus time fit seasonal sine functions. These seasonal δ¹⁸O variations were interpreted to estimate surface water and groundwater mean residence times (MRTs) at sampling locations near topographically closed‐basin lakes. MRT variations of about 1 to 16 years have been estimated over an area covering about 9 km² from the basin boundary to the most downgradient well. Estimated MRT error (±0·3 to ±0·7 years) is small for short MRTs and is much larger (±10 years) for a well with an MRT (16 years) near the limit of the method. Groundwater transit time estimates based on Darcy's law, tritium content, and the seasonal δ¹⁸O amplitude approach appear to be consistent within the limits of each method. The results from this study suggest that use of the δ¹⁸O seasonal variation method to determine MRTs can help assess groundwater recharge areas in small headwaters catchments. Copyright © 2005 John Wiley & Sons, Ltd.     

Raman study of apatite amorphised with swift heavy ions under various irradiation conditions

Crystallographically oriented Durango fluorapatites were exposed to swift heavy ions (Xe, Ta, Au, U) at different irradiation conditions. Beam-induced sample modifications were investigated with respect to the effect of fluence (10⁹–10¹³ ions/cm²), electronic energy loss (18–27 keV/nm), and pressure (3.6–11.5 GPa) applied during irradiation. In situ high-pressure irradiation was performed in diamond anvil cells. Confocal Raman spectroscopy was used to trace the occurring changes in the crystal lattice. Fragmentation of the crystal specimen depends on the orientation and sample thickness and was found to scale with energy loss and fluence. The radiation damage for irradiation along the c-axis was found to be larger than for the 〈hk0〉 direction, independent of the confining pressure. Observations on samples irradiated at high pressures indicate a stabilising effect, leading to reduced amorphisation in comparison to the samples irradiated without pressure.     

Chad Basin: Paleoenvironments of the Sahara since the Late Miocene

Since the mid 1990s, the Mission paléoanthropologique francotchadienne (MPFT) conducts yearly paleontological field investigations of the Miocene-Pliocene of the Chad Basin. This article synthesizes some of the results of the MPFT, with focus on the Chad Basin development during the Neogene. We propose an overview of the depositional paleoenvironments of this part of Africa at different scales of time and space, based on a multidisciplinary approach (sedimentary geology, geomorphology, geophysic, numerical simulations and geochronology). The Miocene-Pliocene paleoenvironments are examined through the sedimentary archives of the early hominids levels and the Holocene Lake Mega-Chad episode illustrates the last major paleoenvironmental change in this area. The sedimentary record of the Chad Basin since the Late Miocene can be schematized as the result of recurrent interactions from lake to desert environments.     

Detection of tropospheric and stratospheric aerosol layers by optical radar (Lidar)

Summary The National Center for Atmospheric Research Lidar Sustem is briefly described. Data will be presented showing stratification and temporal variations of optical backscattering due to atmospheric aerosol up to a height of 40 kilometers. Short lived (less than 4 seconds) layers with thicknesses and spacing of 30 meters are routinely observed over the entire range of observation. A well-defined layer of temporal stability of several hours is very often found at the tropopause. The so-called 20 kilometer sulphate layer is found to be subject to variations in altitude, thickness, and assuming a non-varying size distribution and composition, in concentration. Significant and quasistable layers exist between 25 and 40 kilometers including one at the 36 kilometer leve which has been postulated by preliminary calculations of Mateer to be responsible for the blue and white bands observed and photographed by astronauts.Paper published by Journal of Geophysical Research75 (1970), 3123–3132.     

Separation of multi-mode surface waves by supervised machine learning methods

Logistic regression, neural networks and support vector machines are tested for their effectiveness in isolating surface waves in seismic shot records. To distinguish surface waves from other arrivals, we train the algorithms on three distinguishing features of surface-wave dispersion curves in the domain: spectrum coherency of the trace's magnitude spectrum, local dip and the frequency range for a fixed wavenumber k in the spectrum. Numerical tests on synthetic data show that the kernel-based support vector machines algorithm gives the highest accuracy in predicting the surface-wave window in the domain compared to neural networks and logistic regression. This window is also used to automatically pick the fundamental dispersion curve. The other two methods correctly pick the low-frequency part of the dispersion curve but fail at higher frequencies where there is interference with higher-order modes.     

Atmospheric trace gas analysis using matrix isolation-fourier transform infrared spectroscopy

A novel cryogenic sampling method combining the matrix isolation technique with FTIR spectroscopy has been developed for atmospheric trace gas analysis. It is applicable to a wide range of molecules with detection limits typically in the 10–50 ppt range. The method is described along with some measurements of N₂O, CFCl₃, CF₂Cl₂, OCS, CS₂, SO₂ and PAN from samples collected at ground level and from an aircraft between 9 and 14 km.