AN EVALUATION OF FRACTAL DIMENSIONS TO QUANTIFY CHANGES IN THE MORPHOLOGY OF FLUVIAL SUSPENDED SEDIMENT PARTICLES DURING BASEFLOW CONDITIONS

The morphology of suspended sediment particles reflects the origin of the suspended load and any modifying processes that may have occurred during transport and storage in the aquatic system. The objective of this study was to evaluate the use of four fractal dimensions to quantify visually observed changes in the morphology of fluvial suspended sediment particles during baseflow conditions. Samples were collected during summer low flow in a small stream on the Canadian Prairies. Particle morphology data were obtained with a transmitted light microscope, a CCD camera and an image analysis system. The morphology of the particle population was characterized using four fractal dimensions (D, D_K, D₁ and D₂). D was derived from the area–perimeter relationship and showed an increase from 1·26±0·02 on 30 June, to 1·34±0·02 on 4 July, to 1·42±0·01 on 7 July. Visually, the increase in D represented the formation of large particles with intricate shapes and increased perimeters. D_K was determined from the area–rank relationship and varied from 1·86±0·01 on 30 June, to 1·90±0·00 on 4 July, to 1·74±0·00 on 7 July. The decrease in D_K between 4 July and 7 July would indicate a greater concentration of the particle area over a small number of large particles. Although the decrease in D_K is consistent with observed changes in the particle size distributions, D_K should be used with the considerable caution because D_K varied more than one standard error between replicates. D₁ and D₂ were determined from the length–perimeter and length–area relationships, respectively. D₁ proved to be of little value for quantifying changes in particle morphology as it showed little change with time, despite considerable visual changes. D₂ however, was useful, despite some variation between replicates. Over the sampling period, D₂ for the composite data sets showed a steady decrease from 1·74±0·02 on 30 June, to 1·68±0·02 on 4 July, to 1·60±0·01 on 7 July. This change in D₂ indicates that, through time, the larger particles became longer and thinner relative to the smaller ones. This study shows that temporal changes in D, D_K and D₂ were consistent with, and allow quantification of, observed changes in particle morphology. D₁ did not reflect observed morphological changes, and is therefore of little value for this type of study. The changes in particle morphology coincided with an increase in primary production in the form of algae. © 1997 John Wiley & Sons, Ltd.     

Sand transport and deposition within arrays of non-erodible cylindrical elements

The study concerns sand deposition within a regular array of vertical cylinders placed in the path of a sand-laden wind. Twelve wind tunnel experiments using three preselected shear velocities (28·78, 32·86 and 45·1 cm s⁻¹), with associated rates of sand feed (0·3, 2·0 and 3·8 g cm⁻¹ s⁻¹), and four roughness element concentrations (λ = 0·046, 0·092, 0·184 and 0·369) were carried out to evaluate the factors that affect sand deposition and sand flux in the presence of immobile rough elements. The measurements showed that as the concentration of non-erodible elements increased, the percentage reduction in the initial sand flux increased and a particularly sharp reduction occurred when λ ≥ 0·18. The pattern of reduction was found to be q_red = q_eq (d/H) [Δy/(Δy−d)](0·68 −3·5λ) when λ ≤ 0·18, and q_red = q_eq(d/H)[Δy/(Δy−d)](0·025) when λ > 0·18, where q_eq is the equilibrium rate of sand transport arriving at the best bed, d is the diameter of the cylinder, H is the height of the cylinder, and Δy is the width of unit area associated with a cylinder. The experimenal results also showed that the sand flux downstream of the array started to increase immediately upon the commencement of burial of the array's cylinders. Thus the sand deposition and sand flux along an array consisting of regularly distributed, non-erodible elements were shown to be neither uniform nor steady. Copyright © 1999 John Wiley & Sons, Ltd.     

Paleomagnetic results of the Cretaceous marine sediments in Tongyouluke, southwest Tarim

Southwest Tarim (hereafter SW Tarim) is one of afew areas that well developed Cretaceous marinesedimentary rocks in China [1]. The Cretaceous marinesediments are stretched in front area along the Tian-shan and Kunlun Mountains. Toward the center ofTarim Basin, the Cretaceous sediments are buried bygreat thickness of Tertiary and Quaternary sedimentswith little exposure. Compared with the Cretaceousterrestrial strata of north Tarim, the Cretaceous marinestrata of SW Tarim continue and d…     

North Qinling Paleozoic granite associations and their variation in space and time: Implications for orogenic processes in the orogens of central China

Integrated zircon U-Pb dating and whole rock geochemical analyses have been carried out for two typical S-and I-type granitoids in the north Qinling. Zircon dating by SIMS of the Piaochi S-type grani- toids yields an emplacement age of 495±6 Ma. The granitoids show whole-rock εNd(t)=-8.2--8.8, zircon εHf(t)=-6―-39. The Huichizi I-type granitoids have emplacement ages of 421±27 Ma and 434±7 Ma es- tablished by LA-ICP-MS and SIMS methods, respectively. Their whole-rock εNd(t)=-0.9-0.9 and zircon εHf(t)=-11-8....     

A theoretical study of correlation between scaled energy and earthquake magnitude based on two source displacement models

The correlation of the scaled energy, ê = E_s/M₀, versus earthquake magnitude, M_s, is studied based on two models: (1) Model 1 based on the use of the time function of the average displacements, with a ω^-2 source spectrum, across a fault plane; and (2) Model 2 based on the use of the time function of the average displacements, with a ω^-3 source spectrum, across a fault plane. For the second model, there are two cases: (a) As τ ≒ T, where τ is the rise time and T the rupture time, lg(ê) ~ -M_s; and (b) As τ ≪ T, lg(ê) ~ -(1/2)M_s. The second model leads to a negative value of ê. This means that Model 2 cannot work for studying the present problem. The results obtained from Model 1 suggest that the source model is a factor, yet not a unique one, in controlling the correlation of ê versus M_s.     

Influence of pH on the Toxicity of Phenol and its Two Derivatives Pentachlorophenol and Dinitrophenol to Some Fresh Water Teleosts

— The fish Colisa fasciatus, Saccobranchus fossilis and Notopterus notopterus were exposed to phenol, pentachlorophenol and dinitrophenol at four pH levels, i. e. 4.6 + 0.12, 6.0±0.14, 7.3 ±0.10 and 8.8 ±0.13, to evaluate the effect of pH on the toxicity of these chemicals. — A change in pH from 7.3 ± 0.10 to 6.0 ± 0.14 increased toxicity to a certain degree while a change from 7.3 ± 0.10 to 4.6 ± 0.12 potentiated the toxicity of phenol, pentachlorophenol and dinitrophenol 5.05, 19.30 and 22.33 times, respectively for Notopterus notopterus, 3.59, 4.39 and 4.73 times, tespectively for Colisa fasciatus and 3.04, 4.68 and 5.85 times, respectively for Saccobranchus fossilis. — A change in pH from 7.3 ± 0.10 to 8.8 ± 0.13 has a protective effect on the toxicities.     

Hydrogen isotopic compositions, distributions and source signals of individual n-alkanes for some typical crude oils in Lunnan Oilfield, Tarim Basin, NW China

Petroleum mainly comprises carbon and hydrogen elements. The stable carbon isotopic analysis for whole oil was undertaken as early as the 1930s. After decades, the stable carbon isotopic analytical methods have been developed from analysis for whole oil and oil fractions (e.g., saturated, aromatic and polar frac-tions) into compound-specific isotopic analysis with the emergence of the newly developed GC-C-IRMS analytical technique. Especially, by using com-pound-specific isotopic analytical…     

Paleomagnetic result of the Lower-Cretaceous from Luanping basin,Hebei Province and its tectonic implications*

A paleomagnetic study of about 95 samples from 16 sites sampled in the Early Cretaceous in Luanping basin in Hebei Province was reported. Stepwise thermal demagnetization was used to isolate magnetic components. Most samples have a characteristic direction with a high temperature component above 500°C. The tectonic-corrected data areD = 347.8°,I = 50.4°, α₉₅ = 7.l°, and the corresponding pole position is at 76.1°N, 346.3°E,with dp =6.4°,dm = 3.8°, paleolatitude λ = 31.1°N. This result indicates a counterclockwise post-Cretaceous rotation of 30.7° ±9.8° with respect to the stable Ordos basin in the west of North China Block, and a non-significant northward motion. This rotation could be related to local fault action or structural detachment, or regional NNW-NWWward motion and collision of Kula-Pacific plate with eastern China since the Early Cretaceous.

     

The effect of riparian land use on transport hydraulics in agricultural headwater streams located in northeast Ohio,USA

This study examined if riparian land use (forested vs agricultural) affects hydraulic transport in headwater streams located in an agriculturally fragmented watershed. We identified paired 50‐m reaches (one reach in agricultural land use and the other in forested land use) along three headwater streams in the Upper Sugar Creek Watershed in northeast Ohio, USA (40° 51′42″N, 81° 50′29″W). Using breakthrough curves obtained by Rhodamine WT slug injections and the one‐dimensional transport with inflow and storage model (OTIS), hydraulic transport parameters were obtained for each reach on six different occasions (n = 36). Relative transient storage (A_S:A) was similar between both reach types (As: A = 0·3 ± 0·1 for both agricultural and forested reaches). Comparing values of F_med²⁰⁰ to those in the literature indicates that the effect of transient storage was moderately high in the study streams in the Upper Sugar Creek Watershed. Examining travel times revealed that overall residence time (HRT) and residence time in transient storage (T_STO) were both longer in forested reaches (forested HRT = 19·1 ± 11·5 min and T_STO = 4·0 ± 3·8 min; agricultural HRT = 9·3 ± 5·3 min and T_STO = 1·7 ± 1·4 min). We concluded that the effect of transient storage on solute transport was similar between the forested and agricultural reaches but the forested reaches had a greater potential to retain solutes as a result of longer travel times. Copyright © 2009 John Wiley & Sons, Ltd.     

A new chronology for the age of Appalachian erosional surfaces determined by cosmogenic nuclides in cave sediments

The relative chronology of landscape evolution across the unglaciated Appalachian plateaus of Kentucky and Tennessee is well documented. For more than a century, geomorphologists have carefully mapped and correlated upland erosional surfaces inset by wide‐valley straths and smaller terraces. Constraining the timing of river incision into the Appalachian uplands was difficult in the past due to unsuitable dating methods and poorly preserved surface materials. Today, burial dating using the differential decay of cosmogenic ²⁶Al and ¹⁰Be in clastic cave sediments reveals more than five million years of landscape evolution preserved underground. Multilevel caves linked hydrologically to the incision history of the Cumberland River contain in situ sediments equivalent to fluvial deposits found scattered across the Eastern Highland Rim erosional surface. Cave sediments correlate with: (1) thick Lafayette‐type gravels on the Eastern Highland Rim deposited between c. 5·7 and c. 3·5 Ma; (2) initial incision of the Cumberland River into the Eastern Highland Rim after c. 3·5 Ma; (3) formation of the Parker strath between c. 3·5 Ma and c. 2·0 Ma; (4) incision into the Parker strath at c. 2 Ma; (5) formation of a major terrace between c. 2·0 Ma and c. 1·5 Ma; (6) shorter cycles of accelerated incision and base level stability beginning at c. 1·5 Ma; and (7) regional aggradation at c. 0·85 Ma. Initial incision into the Appalachian uplands is interpreted as a response to eustasy at 3·2–3·1 Ma. Incision of the Parker strath is interpreted as a response to eustasy at 2·5–2·4 Ma. A third incision event at c. 1·5 Ma corresponds with glacial reorganization of the Ohio River basin. Widespread aggradation of cave passages at c. 0·85 Ma is interpreted as the beginning of intense glacial–interglacial cycling associated with global climate change. Copyright © 2006 John Wiley & Sons, Ltd.     

Large roughness element effects on sand transport,Oceano Dunes,California

The effect of large roughness elements on sand transport efficiency was evaluated on a coastal sand sheet by measuring sand flux with two types of sand traps [Big Spring Number Eight (BSNE) and the Cox Sand Catcher (CSC)] at 30 positions through a 100 m‐long × 50 m‐wide roughness array comprised of 210 elements each with the dimensions 1·17 m long × 0·4 m high × 0·6 m wide. The 210 elements were used to create a roughness density (λ) of 0·022 (λ = n bh/S, where n is the number of elements, b the element breadth, h the element height, and S is the area of the surface that contains all the elements) in an area of 5000 m². The mean normalized saltation flux (NSF) values (NSF = outgoing sand flux/incoming sand flux) at the furthest downwind distance for the two trap types were 0·44 and 0·41, respectively. This is in excellent agreement with an empirical model prediction of 0·5. The reduction in saltation flux is similar to an earlier separate study for an equivalent λ composed of elements of similar height (0·36 m), even though the roughness element forms were different (rectangular in this study as opposed to circular) as were the horizontal porosity of the arrays (49% versus 16%). This corroborates earlier results that roughness element height is a critical parameter that enhances reduction in sand transport by wind for similar λ configurations. The available data suggest the form of the relationship between transport reduction efficiency and height is likely a power relationship with two limiting conditions: (1) for elements ≤ 0·1 m high the effect is minimized, and (2) as element height matches and then exceeds the maximum height of the saltation layer (≥ 1 m), the effect will stabilize near a maximum of NSF ≈ 0·32. Copyright © 2012 John Wiley & Sons, Ltd.     

New paleomagnetic results from Ceno-Mesozoic volcanic rocks along southern rim of the Tarim Basin,China*

Paleomagnetic samples were collected from four localities located in the southern rim of the Tarim basin. The samples were taken from volcanic rocks erupted between Jurassic and Quaternary. Detailed analysis of all samples has been carried out with progressive thermal demagnetization. A characteristic remanence (ChRM) with higher unblocking temperature has been isolated from all samples. The pole position from the middle Jurassic is at 52.5°N, 187.9°E(dp = 3.7°,dm =6.5°); the directions of the ChRM of Cretaceous correspond to a paleopole at 69.7°N, 211.6°E (dp = 9.8°,dm = 15.9°); the Quaternary pole from the Pulu site is at 79. 9°N, 183.1°E(dp = 1.6°.dm =2.4°). On the basis of these new paleomagnetic data, tectonic evolution of Tarim block is presented.

     

Snow accumulation in forests from ground and remote‐sensing data

Winter‐forest processes affect global and local climates. The interception‐sublimation fraction (F) of snowfall in forests is a substantial part of the winter water budget (up to 40%). Climate, weather‐forecast and hydrological modellers incorporate increasingly realistic surface schemes into their models, and algorithms describing snow accumulation and snow‐interception sublimation are now finding their way into these schemes. Spatially variable data for calibration and verification of wintertime dynamics therefore are needed for such modelling schemes. The value of F was determined from snow courses in open and forested areas in Hokkaido, Japan. The value of F was related to species and canopy‐structure measures such as closure, sky‐view fraction (SVF) and leaf‐area index (LAI). Forest structure was deduced from fish‐eye photographs. The value of F showed a strong linear correlation to structure: F = 0·44 ? 0·6 × SVF for SVF < 0·72 and F = 0 for SVF > 0·72, and F = 0·11 LAI. These relationships seemed valid for evergreen conifers, larch trees, alder, birch and mixed deciduous stands. Forest snow accumulation (S_F) could be estimated from snowfall in open fields (S_o) and to LAI according to S_F = S_o (1 ? 0·11 LAI) as well as from SVF according to S_F = S_o (0·56 + 0·6 SVF) for SVF < 0·72. The value of S_F was equal to S_o for SVF values above 0·72. The value of sky‐view fraction was correlated to the normalized difference snow index (NDSI) using a Landsat‐TM image for observation plots exceeding 1 ha. Variables F and S_F were related to NDSI for these plots according to: F = ?0·37NDSI + 0·29 and S_F = S_o (0·81 + 0·37NDSI). These relationships are somewhat hypothetical because plot‐size limitation only allowed one sparse‐forest observation of NDSI to be used. There is, therefore, a need to confirm these relationships with further studies. Copyright © 2004 John Wiley & Sons, Ltd.     

The effects of rock uplift and rock resistance on river morphology in a subduction zone forearc,Oregon, USA

Relationships between riverbed morphology, concavity, rock type and rock uplift rate are examined to independently unravel the contribution of along-strike variations in lithology and rates of vertical deformation to the topographic relief of the Oregon coastal mountains. Lithologic control on river profile form is reflected by convexities and knickpoints in a number of longitudinal profiles and by general trends of concavity as a function of lithology. Volcanic and sedimentary rocks are the principal rock types underlying the northern Oregon Coast Ranges (between 46°30′ and 45°N) where mixed bedrock–alluvial channels dominate. Average concavity, θ, is 0·57 in this region. In the alluviated central Oregon Coast Ranges (between 45° and 44°N) values of concavity are, on average, the highest (θ = 0·82). South of 44°N, however, bedrock channels are common and θ = 0·73. Mixed bedrock–alluvial channels characterize rivers in the Klamath Mountains (from 43°N south; θ = 0·64). Rock uplift rates of ≥0·5 mm a⁻¹, mixed bedrock–alluvial channels, and concavities of 0·53–0·70 occur within the northernmost Coast Ranges and Klamath Mountains. For rivers flowing over volcanic rocks θ = 0·53, and θ = 0·72 for reaches crossing sedimentary rocks. Whereas channel type and concavity generally co-vary with lithology along much of the range, rivers between 44·5° and 43°N do not follow these trends. Concavities are generally greater than 0·70, alluvial channels are common, and river profiles lack knickpoints between 44·5° and 44°N, despite the fact that lithology is arguably invariant. Moreover, rock uplift rates in this region vary from low, ≤0·5 mm a⁻¹, to subsidence (<0 mm a⁻¹). These observations are consistent with models of transient river response to a decrease in uplift rate. Conversely, the rivers between 44° and 43°N have similar concavities and flow on the same mapped bedrock unit as the central region, but have bedrock channels and irregular longitudinal profiles, suggesting the river profiles reflect a transient response to an increase in uplift rate. If changes in rock uplift rate explain the differences in river profile form and morphology, it is unlikely that rock uplift and erosion are in steady state in the Oregon coastal mountains. Copyright © 2006 John Wiley & Sons, Ltd.     

H_∞ drift control of time-delayed seismic structures

In this paper,an optimal H∞ control algorithm was applied to the design of an active tendon system installed at the first story of a multi-story building to reduce its interstory drift due to earthquake excitations.To achieve optimal control performance and to guarantee the stability of the control system,an optimum strategy to select control parameters γ and α was developed.Analytical expressions of the upper and the lower bounds of γ and α were obtained for a single degree-of-freedom system with state fee...     

Hydraulic characteristics of a small Coastal Plain stream of the southeastern United States: effects of hydrology and season

Various physical and biological properties affect solute transport patterns in streams. We measured hydraulic characteristics of Payne Creek, a low‐gradient upper Coastal Plain stream, using tracer experiments and parameter estimation with OTIS‐P (one‐dimensional transport with inflow and storage with parameter optimization). The primary objective of this study was to estimate the effects of varying discharge, season, and litter accumulation on hydraulic parameters. Channel area A ranged from 0·081 to 0·371 m² and transient storage area A_s ranged from 0·027 to 0·111 m². Dispersion D ranged from 1·5 to 11·1 m² min⁻¹ and exchange coefficient α ranged from 0·009 to 0·038 min⁻¹. Channel area and dispersion were positively correlated to discharge Q, whereas storage area and exchange coefficient were not. Relative storage size A_s/A ranged from 0·17 to 0·59, and was higher during fall than other seasons under a similar Q. The fraction of median travel time due to transient storage ranged from 8·8 to 34·5% and was significantly correlated with Q through a negative power function. Both metrics indicated that transient storage was a significant component affecting solute transport in Payne Creek, especially during the fall. Comparison between the measured channel area A_c and A suggested that surface storage was dominant in Payne Creek. During fall, accumulation of leaf litter resulted in larger A and A_s and lower velocity and D than during other seasons with similar discharge. Seasonal changes in discharge and organic matter accumulation, and dynamic channel morphology affected the magnitude of transient storage and overall hydraulic characteristics of Payne Creek. Copyright © 2005 John Wiley & Sons, Ltd.     

Superhelicity,helicity and potential vorticity

Abstract

‘‘Helicity'’ density H ≡ u · ω and other pseudo-scalar fields such as P ≡ ω · Vlnρ (which is related to Ertel potential vorticity) are useful quantities in theoretical fluid dynamics and magneto-fluid dynamics. Here u denotes the Eulerian flow velocity relative to the chosen frame of reference, ω ≡ V × u is the corresponding relative vorticity and ρ the mass density of the fluid. A general expression is readily obtained for ?H/?t (where t denotes time) in terms of P and the ‘‘superhelicity'’ density S ≡ ω · V × ω which, in fluids of low viscosity, has its highest values in boundary layers. One need for such a relationship became evident during an attempt to interpret the findings of laboratory experiments on thermal convection in rotating fluids in containers of various geometrical shapes and topological characteristics.

In electrodynamics an analogous expression can be found relating the time rate of change of ‘‘magnetic helicity'’ A · B to ‘‘magnetic superhelicity'’ B · ? × B (where B · ? × A is the magnetic field) and a scalar quantity analogous to P which involves non-Ohmic contributions to the relationship between the electric current density and the electric field.     

Influences of grass and moss on runoff and sediment yield on sloped loess surfaces under simulated rainfall

It is important to evaluate the impacts of grasses on soil erosion process so as to use them effectively to control soil and water losses on the Loess Plateau. Laboratory-simulated rainfall experiments were conducted to investigate the runoff and sediment processes on sloped loess surfaces with and without the aboveground parts of grasses and moss (GAM: grass and moss; NGAM: no grass and moss) under slope gradients of 5°, 10°, 15°, 20°, 25° and 30°. The results show that runoff from GAM and NGAM plots increased up to a slope gradient of 10° and decreased thereafter, whereas the runoff coefficients increased with gradient. The average runoff rates and runoff coefficients of NGAM plots were less than those of GAM plots except for the 5° slope. This behaviour may be due to the reduction in water infiltration under moss. The difference between GAM and NGAM plots in average runoff rates varied from 1·4 to 8%. At the same gradients, NGAM plots yielded significantly (α = 0·05) more sediment than GAM plots. Average sediment deliveries for different slopes varied from 0·119 to 3·794 g m⁻² min⁻¹ from GAM plots, and from 0·765 to 16·128 g m⁻² min⁻¹ from NGAM plots. Sediment yields from GAM plots were reduced by 45 to 85%, compared with those from the NGAM plots. Plots at 30° yielded significantly higher sediments than at the other gradients. Total sediments S increased with slope gradients G in a linear form, i.e. S = 9·25G − 39·6 with R² = 0·77*, for the GAM plots, and in an exponential model, i.e. S = 40·4 exp(0·1042G) with R² = 0·93**, for the NGAM plots. In all cases, sediment deliveries decreased with time, and reached a relative steady state at a rainfall duration of 14 min. Compared with NGAM plots, the final percentage reductions in sediment delivery from GAM plots were higher than those at the initial time of rainfall at all slopes. Copyright © 2006 John Wiley & Sons, Ltd.     

Seasonal rainfall–runoff relationships in a lowland forested watershed in the southeastern USA

Hydrological processes of lowland watersheds of the southern USA are not well understood compared to a hilly landscape due to their unique topography, soil compositions, and climate. This study describes the seasonal relationships between rainfall patterns and runoff (sum of storm flow and base flow) using 13 years (1964–1976) of rainfall and stream flow data for a low‐gradient, third‐order forested watershed. It was hypothesized that runoff–rainfall ratios (R/P) are smaller during the dry periods (summer and fall) and greater during the wet periods (winter and spring). We found a large seasonal variability in event R/P potentially due to differences in forest evapotranspiration that affected seasonal soil moisture conditions. Linear regression analysis results revealed a significant relationship between rainfall and runoff for wet (r² = 0·68; p < 0·01) and dry (r² = 0·19; p = 0·02) periods. Rainfall‐runoff relationships based on a 5‐day antecedent precipitation index (API) showed significant (r² = 0·39; p < 0·01) correspondence for wet but not (r² = 0·02; p = 0·56) for dry conditions. The same was true for rainfall‐runoff relationships based on 30‐day API (r² = 0·39; p < 0·01 for wet and r² = 0·00; p = 0·79 for dry). Stepwise regression analyses suggested that runoff was controlled mainly by rainfall amount and initial soil moisture conditions as represented by the initial flow rate of a storm event. Mean event R/P were higher for the wet period (R/P = 0·33), and the wet antecedent soil moisture condition based on 5‐day (R/P = 0·25) and 30‐day (R/P = 0·26) prior API than those for the dry period conditions. This study suggests that soil water status, i.e. antecedent soil moisture and groundwater table level, is important besides the rainfall to seasonal runoff generation in the coastal plain region with shallow soil argillic horizons. Copyright © 2011 John Wiley & Sons, Ltd.     

消除海底起伏影响的海洋地震波场正反向延拓

为了解决海底起伏变化对地震波场的影响问题,本文提出将（x-z）域中的曲网格映射成（ξ,η）域中的矩形网格,推导出（ξ,η）域中的二维标量声波方程,根据推导出来的波动方程采用逆时有限差分法将海面上采集到的地震波场在（ξ,η）域中向下延拓至海底面,延拓时采用海水的速度,然后采用顺时有限差分法将延拓后的地震波场再反延拓到海面上,延拓时采用海底面以下地层的速度,从而消除了海底起伏带来的负面影响。模型及实际地震资料的计算分析表明该方法不但能够校正由于海底起伏所引起的海底面下地层反射波场的不连续性还能够校正由于海底起伏所引起的地震波的动力学特征的变化。对延拓前后的地震波场进行速度反演,延拓后反演的地层速度比延拓前反演的地层速度的精度提高很多,延拓前后地震波场的叠加剖面对比表明该延拓方法能够明显提高地震波场的成像质量。