Estimation of groundwater storage from seismic data using deep learning

Convolutional neural networks can provide a potential framework to characterize groundwater storage from seismic data. Estimation of key components, such as the amount of groundwater stored in an aquifer and delineate water table level, from active-source seismic data are performed in this study. The data to train, validate and test the neural networks are obtained by solving wave propagation in a coupled poroviscoelastic–elastic media. A discontinuous Galerkin method is applied to model wave propagation, whereas a deep convolutional neural network is used for the parameter estimation problem. In the numerical experiment, the primary unknowns estimated are the amount of stored groundwater and water table level, while the remaining parameters, assumed to be of less of interest, are marginalized in the convolutional neural network-based solution. Results, obtained through synthetic data, illustrate the potential of deep learning methods to extract additional aquifer information from seismic data, which otherwise would be impossible based on a set of reflection seismic sections or velocity tomograms.     

Influence of the South Atlantic convergence zone and SouthAtlantic Sea surface temperature on interannual summerrainfall variability in Southeastern South America

Summary  In subtropical Argentina, Paraguay and southern Brazil, precipitation is most abundant during summer but its interannual variability is large. At this time a zone of low-level convergence, upper-level divergence, and intense convection is developed to the north of this area. This feature is known as the South Atlantic convergence zone (SACZ) and seems to be related to the interannual variability of summer rainfall to its south. The aim of this work is to document this relationship. Reduced (increased) precipitation in southern Brazil, most of Uruguay and northeastern Argentina is associated with a strong (weak) SACZ and a northward (southward) displacement of it, while increased (reduced) rainfall occurs further south in subtropical Argentina. Also, warm (cold) SST in the region 20° S–40° S and west of 30° W is likely accompanied by a southward (northward) shift of the SACZ. Aside of this relation with the SACZ that affect on the precipitation field of Southeastern South America, the proximate Atlantic Ocean SST seems to force the precipitation over this region by other mechanisms as well. The result of this additional SST forcing is to enhance the signal of the SACZ in northeastern Argentina, Uruguay and southern Brazil and to oppose the SACZ effect in southern subtropical Argentina. Received July 24, 1999 Revised July 5, 2000     

Upper-air wave trains over the Pacific Ocean and wintertime cold surges in tropical-subtropical South America leading to Freezes in Southern and Southeastern Brazil

Summary ?An examination is made of the relationships between circulation and convection over South America as related to strong cooling in southern and southeastern Brazil during austral wintertime. Correlations between sub monthly (2–30 days) near-surface air temperature in southeastern Brazil reveal the preferred path of a quasi-stationary Rossby wave impinging on to the region from mid-latitudes and emanating from the tropical western Pacific. The large amplitude upper-level trough in middle latitudes, which extends into the tropics, has been pointed out as one of the major features of the cold situation. These waves embedded in westerly flow are an example of wintertime tropical-extratropical interactions leading to cooling in southeastern South America. This is suggested by the results from the observational and baroclinic-modeling studies presented here. However, in addition to the right synoptic situation associated with this intermediate modulation, local controls are important on determining the degree of cooling and the occurrence of freezes. Received August 25, 2000; revised September 1, 2001; accepted December 1, 2001     

Pb-Pb dating constraints on the accretion and cooling history of chondrites

We have analyzed the Pb isotopic compositions of whole-rocks and various components (CAIs, chondrules, and/or mineral separates) of two carbonaceous chondrites, Allende (CV3) and Murchison (CM2), and nine ordinary chondrites, Sainte Marguerite (H4), Nadiabondi and Forest City (H5), Kernouvé (H6), Bjurböle (L/LL4), Elenovka and Ausson (L5), Tuxtuac (LL5), and Saint-Séverin (LL6) by MC-ICP-MS. Three CAI fractions from Allende define an isochron with an age of 4568.1 ± 9.4 Ma (MSWD = 0.08) and plot on the same isochron as fragments of the Efremovka inclusion E60 analyzed by Amelin et al. [Amelin, Y., Krot, A. N., Hutcheon, I. D., and Ulyanov, A. A. (2002a). Lead isotopic ages of chondrules and calcium-aluminum-rich inclusions. Science297, 1679-1683]. When these two groups of samples are combined, the isochron yields an age of 4568.5 ± 0.5 (MSWD = 0.90), which is our best estimate of the age of the Solar System. Chondrules and pyroxene-olivine fractions from the ordinary chondrites yield ages that reflect the blocking of Pb isotope equilibration with the nebular gas. The combination of these ages with the corresponding metamorphic phosphate ages provides constraints on the thermal history of the different chondrite parent bodies. Among the H chondrites, Sainte Marguerite cooled to below ∼1100 K within a few My at 4565 Ma and to ∼800 K at 4563 Ma. Nadiabondi appears to have experienced a slightly more protracted cooling history with the corresponding interval lasting from 4559 to 4556 Ma. The data from Forest City and Kernouvé show evidence of late-stage perturbation with resulting U/Pb fractionation. Likewise, Pb isotopes in Tuxtuac (LL5) record a cooling history lasting from ∼4555 to 4544 Ma, which may indicate that the cooling history for the LL parent body was more prolonged than for the H parent body. We suggest a thermal evolution model for the growth of the planetary bodies based on the release of radiogenic heat from ²⁶Al and ⁶⁰Fe. This model incorporates the accretion rate, which determines the time at which the radiogenic heat becomes efficiently trapped, and the terminal size of the parent body, which controls its overall thermal inertia. The parent bodies of carbonaceous chondrites, which show little indication of metamorphic transformation, collect cooler nebular material at a relatively late stage. Small asteroids of ∼10-50 km radius accreting within 1-3 My could be the parent bodies of H and LL chondrites. The parent body of the L chondrites is likely to be a larger asteroid (r > 100 km) or possibly the product of collisions of smaller planetary bodies.     

Hafnium isotopes in Jack Hills zircons and the formation of the Hadean crust

New bulk Hf and Pb isotope data were obtained for 63 leached single zircons from Jack Hills (JH), Western Australia, using solution chemistry and, respectively, MC-ICP MS and ICP-MS. With the exception of one “young” zircon at 3.32 Ga, the remainder of the selected grains were previously dated at > 3.9 Ga by ion-microprobe. This work extends and complements the solution chemistry data of Harrison et al. [Harrison, T.M., Blichert-Toft, J., Müller, W., Albarède, F., Holden, P., Mojzsis, S.J., 2005. Heterogeneous Hadean hafnium: evidence of continental crust at 4.4 to 4.5 Ga. Science 310, 1947–1950.] but uses bulk rather than in situ Pb–Pb ages to interpret the Hf isotope data. This larger data set is used to explore whether the host rocks of the JH zircons formed as a succession of pulses or rather as a single event, and to calculate the age and assess the nature of their crustal protolith. We find that the parent granites of the JH zircons analyzed here formed during a single pulse 4.1 ± 0.1 Ga ago by the remelting of a 4.30–4.36 Ga old protolith. Monte Carlo modeling indicates that the ¹⁷⁶Lu/¹⁷⁷Hf ratios of this material (< 0.01) are unlike the ratios of modern-type oceanic crust and island arc rocks but rather fit a tonalite–trondhjemite–granodiorite (TTG) source. TTGs themselves derived their inordinately enriched character from a basaltic progenitor which corresponds to the missing enriched reservoir identified by the ¹⁴³Nd–¹⁴⁴Nd, ¹⁴²Nd–¹⁴⁴Nd, and ¹⁷⁶Hf/¹⁷⁷Hf systematics of Archean rocks. We speculate that crystallization of the magma ocean in the presence of garnet left the upper mantle and an early basaltic crust enriched in incompatible elements. Reaction of this early crust with the overlying hydrosphere and subsequent foundering into the mantle gave rise at ～ 4.3 Ga to the TTG protolith of the JH granites. Dating the onset of plate tectonics therefore depends on whether TTGs can be considered as subduction zone magmas or not.     

Changes over time in near-saturated hydraulic conductivity of peat soil following reclamation for agriculture

Reclamation of peat bogs for agriculture changes the physical and chemical characteristics of the peat matrix, for example, drainage and tillage accelerate decomposition, altering peat porosity, pore size distribution, and hydraulic properties. This study investigated changes in near-saturated hydraulic conductivity over time after drainage of peat soil for agricultural use by conducting tension infiltrometer measurements in a mire that has been gradually drained and reclaimed for agriculture during the past 80 years (with fields drained 2, 12, 40, and 80 years before the measurements). At pore water pressure closest to saturation (pressure head −1 cm), hydraulic conductivity in the newest field was approximately nine times larger than that in the oldest field, and a decreasing trend with field age was observed. A similar (but weaker) trend was observed with −3 cm pressure head (approximately four times larger in the newest field in comparison to the oldest), but at −6 cm head, there were no significant differences. These results indicate that peat degradation reduces the amount of millimetre-sized pores in particular. They also indicate that changes in peat macroporosity continue for several decades before a new steady state is reached.