Migration for velocity and attenuation perturbations

Migration maps seismic data to reflectors in the Earth. Reflections are not only caused by small‐scale variations of the velocity and density but also of the quality factor that describes attenuation. We investigated scattering due to velocity and attenuation perturbations by computing the resolution function or point‐spread function in a homogeneous background model. The resolution function is the migration image of seismic reflection data generated by a point scatterer. We found that the resolution function mixes velocity and attenuation parameter perturbations to the extent that they cannot be reconstructed independently. This is true for a typical seismic setting with sources and receivers at the surface and a buried scatterer. As a result, it will be impossible to simultaneously invert for velocity and attenuation perturbations in the scattering approach, also known as the Born approximation. We proceeded to investigate other acquisition geometries that may resolve the ambiguity between velocity and attenuation perturbations. With sources and receivers on a circle around the scatterer, in 2D, the ambiguity disappears. It still shows up in a cross‐well setting, although the mixing of velocity and attenuation parameters is less severe than in the surface‐to‐surface case. We also consider illumination of the target by diving waves in a background model that has velocity increasing linearly with depth. The improvement in illumination is, however, still insufficient to remove the ambiguity.     

卫星遥感结合地面观测估算非均匀地表区域能量通量

卫星遥感在研究非均匀陆面上地－气间能量和水循环过程时有其独到的作用。文中介绍了一种利用ＬＡＮＤＳＡＴＴＭ资料进行非均匀陆面上区域能量平衡研究的参数化方案。并以夏季和深秋两个景的ＴＭ资料为个例,结合“黑河实验”（ＨＥＩＦＥ）期间的地面观测资料分析了实验区非均匀陆面上地表特征参数及能量平衡各分量的区域分布及季节差异,同时将所得的结果与地面观测的“真值”作了比较,所得的这些结果是基本可信的。这种方法仍然处于发展的初始阶段,对此也进行了讨论。     

Mantle resistance against Gibraltar slab dragging as a key cause of the Messinian Salinity Crisis

The Messinian Salinity Crisis (5.97–5.33 Ma) was caused by the closure of the Atlantic‐Mediterranean gateways that cut through the Gibraltar orogenic system. The geodynamic drivers underlying gateway closure and re‐opening are still debated. Here, we interrogate the gateway successions to find the imprints of surface deformation, infer the timing and nature of associated geodynamic drivers, and test such inferences against numerical simulations of slab dynamics. We find that since the latest Miocene, a tectonic framework was established in the gateway region dominated simultaneously by (a) relative plate convergence, (b) slab tearing under the eastern Betic Cordillera and (c) mantle resistance against north‐northeastward dragging of the Gibraltar slab by the African plate's absolute motion. We propose that mantle‐resisted slab dragging and slab tearing operated in concert closing the gateways that caused the Messinian Salinity Crisis, whereas sinking of heavy oceanic lithosphere located between buoyant continental plates re‐opened the Strait of Gibraltar at 5.33 Ma.     

Artificial recharge efficiency assessment by soil water balance and modelling approaches in a multi-layered vadose zone in a dry region

To assess recharge through floodwater spreading, three wells, approx. 30 m deep, were dug in a 35-year-old basin in southern Iran. Hydraulic parameters of the layers were measured. One well was equipped with pre-calibrated time domain reflectometry (TDR) sensors. The soil moisture was measured continuously before and after events. Rainfall, ponding depth and the duration of the flooding events were also measured. Recharge was assessed by the soil water balance method, and by calibrated (inverse solution) HYDRUS-1D. The results show that the 15 wetting front was interrupted at a layer with fine soil accumulation over a coarse layer at the depth of approx. 4 m. This seemed to occur due to fingering flow. Estimation of recharge by the soil water balance and modelling approaches showed a downward water flux of 55 and 57% of impounded floodwater, respectively.     

A parameterisation for the threshold shear velocity to initiate deflation of dry and wet sediment

A new parameterisation for the threshold shear velocity to initiate deflation of dry and wet particles is presented. It is based on the balance of moments acting on particles at the instant of particle motion. The model hence includes a term for the aerodynamic forces, including the drag force, the lift force and the aerodynamic-moment force, and a term for the interparticle forces. The effect of gravitation is incorporated in both terms. Rather than using an implicit function for the effect of the aerodynamic forces as reported earlier in literature, a constant aerodynamic coefficient was introduced. From consideration of the van der Waals force between two particles, it was further shown that the effect of the interparticle cohesion force between two dry particles on the deflation threshold should be inversely proportional to the particle diameter squared. The interparticle force was further extended to include wet bonding forces. The latter were considered as the sum of capillary forces and adhesive forces. A model that expresses the capillary force as a function of particle diameter squared and the inverse of capillary potential was deduced from consideration of the well-known model of Fisher and the Young–Laplace equation. The adhesive force was assumed to be equal to tensile strength, and a function which is proportional to particle diameter squared and the inverse of the potential due to adhesive forces was derived. By combining the capillary-force model and the adhesive force model, the interparticle force due to wet bonding was simplified and written as a function of particle diameter squared and the inverse of matric potential. The latter was loglinearly related to the gravimetric moisture content, a relationship that is valid in the low-moisture content range that is important in the light of deflation of sediment by wind. By introducing a correction to force the relationship to converge to zero moisture content at oven dryness, the matric potential–moisture content relationship contained only one unknown model parameter, viz. moisture content at −1.5 MPa. Working out the model led to a rather simple parameterisation containing only three coefficients. Two parameters were incorporated in the term that applies to dry sediment and were determined by using experimental data as reported by Iversen and White [Sedimentology 29 (1982) 111]. The third parameter for the wet-sediment part of the model was determined from wind-tunnel experiments on prewetted sand and sandy loam aggregates. The model was validated using data from wind-tunnel experiments on the same but dry sediment, and on data obtained from simulations with the model of Chepil [Soil Sci. Soc. Am. Proc. 20 (1956) 288]. The experiments showed that soil aggregates should be treated as individual particles with a density equal to their bulk density. Furthermore, it was shown that the surface had to dry to a moisture content of about 75% of the moisture content at −1.5 MPa before deflation became sustained. The threshold shear velocities simulated with our model were found to be in good agreement with own observations and with simulations using Chepil's model.     

The palaeolimnology of Lake Fidler, a meromictic lake in south-west Tasmania and the significance of recent human impact

Three meromictic lakes in the World Heritage Area of south-west Tasmania possess unusual microbiological communities. Their meromixis is maintained by periodic incursions of brackish water from the nearby Gordon River which, in its lower reaches, is a salt-wedge estuary. In 1977 the construction of a dam in the middle reaches of the river restricted penetration of the salt-wedge and meromixis rapidly declined in all three lakes. A palaeolimnological study was carried out on one of the lakes, Lake Fidler, firstly to determine the history of meromixis and its associated microbiological communities, and secondly to assess whether the recent and rapid decline of meromixis is inconsistent with natural rates of development of the Gordon River meromictic lakes. One part of this study included the analysis of the stratigraphy of fossil diatoms from a 17-metre sediment core dating back 8000 yrs. Detrended Correspondence Analysis and Analog Matching were used to compare diatom species assemblages in core samples with diatom samples from a reference dataset consisting of a selection of lake and river sites in the lower Gordon River valley. Five distinct stratigraphic zones were identified in the core. These zones indicated specific stages in the development of the Gordon River lakes from river backwaters to ectogenically-maintained meromictic lakes which will, finally, become terrestrialised by encroaching rainforest. The onset of a stratified water column was identified by the emergence of a dominant freshwater algal flora which suggested that the lake had developed a mixolimnion and become meromictic ca. 2070 ± 50 ¹⁴C yrs ago. In the context of this long history of meromixis, the rapid demise in meromictic stability following construction of the dam is judged to be inconsistent with natural rates of development. The palaeolimnological studies, of which this paper is one part, prompt recommendations for a management strategy to prevent the further decay of these meromictic lakes in the World Heritage Area.