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1.
Sea‐bed diffractions are frequently observed for several of the fields in the Norwegian Sea and the Barents Sea. This is a challenge in time lapse seismic analysis, since diffracted multiples are difficult to remove by processing and therefore is a major source of poor time lapse data quality. In this work we test if the diffractions can be used for enhanced 4D interpretation. By analysing the time‐shift of the sea‐bed diffraction hyperbola between the base and monitor it is tested if changes in water velocity and tides can be estimated. Two models using time lapse diffraction analysis are tested: the first one simply adds time‐shifts for the two branches of the diffraction hyperbola and this average time‐shift is then used to estimate the water velocity change. The other method uses an inversion method based on the diffraction equation for a point diffractor to estimate the velocity change. In‐line common‐midpoint shifts are estimated by subtracting the time‐shifts of both hyperbola branches followed by direct inversion. The diffraction based time‐shifts are compared to time‐shifts estimated by standard cross‐correlation of the sea‐bed reflection. The averaging method gives slightly higher uncertainties, while the inversion using an exact traveltime equation gives similar uncertainties compared to the sea‐bed reflection method.  相似文献   

2.
Rising sea levels, owing to climate change, are a threat to fresh water coastal aquifers. This is because saline intrusions are caused by increases and intensification of medium‐large scale influences including sea level rise, wave climate, tidal cycles, and shifts in beach morphology. Methods are therefore required to understand the dynamics of these interactions. While traditional borehole and galvanic contact resistivity (GCR) techniques have been successful they are time‐consuming. Alternatively, frequency‐domain electromagnetic (FEM) induction is potentially useful as physical contact with the ground is not required. A DUALEM‐421 and EM4Soil inversion software package are used to develop a quasi two‐ (2D) and quasi three‐dimensional (3D) electromagnetic conductivity images (EMCI) across Long Reef Beach located north of Sydney Harbour, New South Wales, Australia. The quasi 2D models discern: the dry sand (<10 mS/m) associated with the incipient dune; sand with fresh water (10 to 20 mS/m); mixing of fresh and saline water (20 to 500 mS/m), and; saline sand of varying moisture (more than 500 mS/m). The quasi 3D EMCIs generated for low and high tides suggest that daily tidal cycles do not have a significant effect on local groundwater salinity. Instead, the saline intrusion is most likely influenced by medium‐large scale drivers including local wave climate and morphology along this wave‐dominated beach. Further research is required to elucidate the influence of spring‐neap tidal cycles, contrasting beach morphological states and sea level rise.  相似文献   

3.
Sea levels are expected to rise as a result of global temperature increases, one implication of which is the potential exacerbation of sea water intrusion into coastal aquifers. Given that approximately 70% of the world's population resides in coastal regions, it is imperative to understand the interaction between fresh groundwater and sea water intrusion in order to best manage available resources. For this study, controlled investigation has been carried out concerning the temporal variation in sea water intrusion as a result of rising sea levels. A series of fixed inland head two‐dimensional sea water intrusion models were developed with SEAWAT in order to assess the impact of rising sea levels on the transient migration of saline intrusion in coastal aquifers under a range of hydrogeological property conditions. A wide range of responses were observed for typical hydrogeological parameter values. Systems with a high ratio of hydraulic conductivity to recharge and high effective porosity lagged behind the equilibrium sea water toe positions during sea‐level rise, often by many hundreds of meters, and frequently taking several centuries to equilibrate following a cease in sea‐level rise. Systems with a low ratio of hydraulic conductivity to recharge and low effective porosity did not develop such a large degree of disequilibrium and generally stabilized within decades following a cease in sea‐level rise. This study provides qualitative initial estimates for the expected rate of intrusion and predicted degree of disequilibrium generated by sea‐level rise for a range of hydrogeological parameter values.  相似文献   

4.
This paper analyzes the results of a theoretical simulation of sea water intrusion and its dynamics. The assignment of hydrochemical facies identifies whether the aquifer is in the phase of sea water intrusion or freshening, indicating the status of the aquifer in terms of the advance or regression of the saline front. A new multi-rectangular diagram is proposed that aids interpretation of these important processes through the representation and evolution of hydrochemical facies (hydrochemical facies evolution diagram, HFE-D). As an example, the HEF-D is applied to an alluvial aquifer in the Vinaroz-Peñíscola Plain (Spain), where Ca-Cl facies characterize the sea water intrusion phase, while Na-MixHCO3/MixSO4 facies characterize a freshening stage.  相似文献   

5.
A variable‐density groundwater flow and dispersive solute transport model was developed for the shallow coastal aquifer system near a municipal supply well field in southeastern Florida. The model was calibrated for a 105‐year period (1900 to 2005). An analysis with the model suggests that well‐field withdrawals were the dominant cause of salt water intrusion near the well field, and that historical sea‐level rise, which is similar to lower‐bound projections of future sea‐level rise, exacerbated the extent of salt water intrusion. Average 2005 hydrologic conditions were used for 100‐year sensitivity simulations aimed at quantifying the effect of projected rises in sea level on fresh coastal groundwater resources near the well field. Use of average 2005 hydrologic conditions and a constant sea level result in total dissolved solids (TDS) concentration of the well field exceeding drinking water standards after 70 years. When sea‐level rise is included in the simulations, drinking water standards are exceeded 10 to 21 years earlier, depending on the specified rate of sea‐level rise.  相似文献   

6.
Stephen B. Shaw 《水文研究》2017,31(21):3729-3739
There remains continued use of non‐linear, logistic regression models for predicting water temperature from air temperature. A dominant feature of these non‐linear models is an upper bound on river water temperature. This upper bound is often attributed to a large increase in evaporative cooling at high air temperatures, but the exact conditions under which such an increase may occur have not been thoroughly explored. To better understand the appropriateness of the non‐linear model for predicting river water temperatures, it is essential to understand the physical basis for the upper bound and when it should and should not be included in the statistical model. This paper applies and validates an energy balance model against 8 river systems spread across different climate regions of the United States. The energy balance model is then used to develop a diagram relating vapour pressure deficit and air temperature to water temperature. With knowledge of present or future vapour pressure deficit (difference between saturation and actual vapour content in the atmosphere) conditions in a given climate, the diagram can be used to predict the likelihood of an upper bound in the air–water temperature relationship. This investigation offers a fundamental physical explanation of the most appropriate form of statistical models that should be used for predicting future water temperature from air temperature in different geographic regions with different climate conditions. In general, climatic regions that have only a slight increase in vapour pressure deficit with increasing air temperature (typically humid regions) would not be expected to have an upper bound. Conversely, climatic regions in which vapour pressure deficit sharply increases with increasing air temperature (typically arid regions) would be expected to have an upper bound.  相似文献   

7.
For small tropical islands with limited freshwater resources, understanding how island hydrology is influenced by regional climate is important, considering projected hydroclimate and sea level changes as well as growing populations dependent on limited groundwater resources. However, the relationship between climate variability and hydrologic variability for many tropical islands remains uncertain due to local hydroclimatic data scarcity. Here, we present a case study from Kiritimati, Republic of Kiribati (2°N, 157°W), utilizing the normalized difference vegetation index to investigate variability in island surface water area, an important link between climate variability and groundwater storage. Kiritimati surface water area varies seasonally, following wet and dry seasons, and interannually, due to hydroclimate variability associated with the El Niño/Southern Oscillation. The NIÑO3.4 sea surface temperature index, satellite‐derived precipitation, precipitation minus evaporation, and local sea level all had significant positive correlations with surface water area. Lagged correlations show sea level changes and precipitation influence surface water area up to 6 months later. Differences in the timing of surface water area changes and variable climate‐surface water area correlations in island subregions indicate that surface hydrology on Kiritimati is not uniform in response to climate variations. Rather, the magnitude of the ocean–atmosphere anomalies and island–ocean connectivity determine the extent to which sea level and precipitation control surface water area. The very strong 2015–2016 El Niño event led to the largest surface water area measured in the 18‐year data set. Surface water area decreased to pre‐event values in a similarly rapid manner (<6 months) after both the very strong 2015–2016 event and the 2009–2010 moderate El Niño event. Future changes in the frequency and amplitude of interannual hydroclimate variability as well as seasonal duration will thus alter surface water coverage on Kiritimati, with implications for freshwater resources, flooding, and drought.  相似文献   

8.
Aquifers are inherently susceptible to contamination and coastal aquifers in specific are highly vulnerable to sea water intrusion. For efficient planning and management of coastal aquifers in Kayalpattu and Tiruchopuram villages, which extend over 4·05 km2, it is essential to delineate and predict the extent of intrusion into the shallow aquifer. Management of ground water in coastal aquifers is composed of major elements that should be properly evaluated, and special attention is given to the sea water intrusion problem. Different data, like hydro‐geomorphological and depth‐wise iso‐apparent resistivity, are integrated spatially using a geographical information system. The stack‐unit mapping approach is used to delineate the zones with iso‐apparent resistivity of less than 10 Ω m have been found to be increasing in areal extent with reference to depth. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

9.
We compare selected marine electromagnetic methods for sensitivity to the presence of relatively thin resistive targets (e.g., hydrocarbons, gas hydrates, fresh groundwater, etc.). The study includes the conventional controlled‐source electromagnetic method, the recently introduced transient electromagnetic prospecting with vertical electric lines method, and the novel marine circular electric dipole method, which is still in the stage of theoretical development. The comparison is based on general physical considerations, analytical (mainly asymptotic) analysis, and rigorous one‐dimensional and multidimensional forward modelling. It is shown that transient electromagnetic prospecting with vertical electric lines and marine circular electric dipole methods represent an alternative to the conventional controlled‐source electromagnetic method at shallow sea, where the latter becomes less efficient due to the air‐wave phenomenon. Since both former methods are essentially short‐offset time‐domain techniques, they exhibit a much better lateral resolution than the controlled‐source electromagnetic method in both shallow sea and deep sea. The greatest shortcoming of the transient electromagnetic prospecting with vertical electric lines and marine circular electric dipole methods comes from the difficulties in accurately assembling the transmitter antenna within the marine environment. This makes these methods significantly less practical than the controlled‐source electromagnetic method. Consequently, the controlled‐source electromagnetic method remains the leading marine electromagnetic technique in the exploration of large resistive targets in deep sea. However, exploring laterally small targets in deep sea and both small and large targets in shallow sea might require the use of the less practical transient electromagnetic prospecting with vertical electric lines and/or marine circular electric dipole method as a desirable alternative to the controlled‐source electromagnetic method.  相似文献   

10.
With the increasing use of permanently installed seismic installations, many of the issues in time‐lapse seismic caused by the lack of repeatability can be reduced. However, a number of parameters still influence the degree of reliability of 4D seismic data. In this paper, the specific impact of seawater velocity variations on time‐lapse repeatability is investigated in a synthetic study. A zero‐lag time‐lapse seabed experiment with no change in the subsurface but with velocity changes in the water column is simulated. The velocity model in the water column is constant for the baseline survey while the model for the repeat survey is heterogeneous, designed from sea salinity and temperature measurements in the West of Shetlands. The difference section shows up to 80% of residual amplitude, which highlights the poor repeatability. A new dynamic correction which removes the effect of seawater velocity variations specifically for permanent installations is developed. When applied to the synthetic data, it reduces the difference residual amplitude to about 3%. This technique shows substantial improvement in repeatability beyond conventional time‐lapse cross‐equalization.  相似文献   

11.
In the development and testing of water‐surface multiple‐removal algorithms, it is valuable to have accurate synthetic seismograms which exhibit multiples, for which the multiple‐free solution is known. A method is presented for constructing 2D and 3D solutions of the acoustic wave equation in water, by combining the solution from a primary source with other scaled solutions of secondary sources, which simulate diffractors. The computation involves function evaluation rather than numerical solution of differential equations and is consequently accurate and comparatively fast. The analytic formulae on which the method is based give insights into methods for multiple removal. Generalized reflection coefficients, defined on a horizontal plane above the diffractors, are derived and used to construct the integral equations which are the basis for many multiple‐removal schemes.  相似文献   

12.
Ground water quality assessment using multi-rectangular diagrams   总被引：2，自引：0，他引：2  
Ahmad N  Sen Z  Ahmad M 《Ground water》2003,41(6):828-832
A new graphical technique is proposed here for classifying chemical analyses of ground water. In this technique, a diagram is constructed using rectangular coordinates. The new diagram, called a multi-rectangular diagram (MRD), uses adjacent multi-rectangles in which each rectangle represents a specific ground water type. This new diagram has the capability to accommodate a large number of data sets. MRDs have been used to classify chemical analyses of ground water in the Chaj Doab area of Pakistan to illustrate this new approach. Using this graphical method, the differentiated ground water types are calcium bicarbonate, magnesium bicarbonate, sodium bicarbonate, and sodium sulfate. Sodium bicarbonate emerges as the most abundant ground water type. MRDs also offer a visual display of the Chebotarev sequence of ground water quality evolution.  相似文献   

13.
水化学分析方法在地下水异常核实中的应用   总被引：1，自引：0，他引：1  
张磊  刘耀炜  任宏微  柯云龙 《地震》2019,39(1):29-38
地震前兆识别中的异常核实工作是提高地震分析预报能力的重要环节。"四图一法"是地下水异常核实中有效的水化学分析方法,其中Piper图和Schoeller图用于判定地下水化学类型和补给关系,Gibbs图用于判定地下水化学成分来源,Giggenbach三角图用于判定地下水的水岩平衡状态和是否有深部活动信息,以及混合比例法用于计算地下水的混合程度。同时,介绍了水化学分析方法在异常核实应用中的水样采集、测试方法和基本原理。使用水化学分析方法,有助于判定异常的构造或非构造活动影响因素,提高异常判定的科学水平。  相似文献   

14.
The importance of the study of fresh‐saline water incursion cannot be over‐emphasized. Borehole techniques have been widely used, but they are quite expensive, intrusive, and time consuming. The electrical resistivity method has proved very successful in groundwater assessment. This advanced technique uses the calculation of Dar‐Zarrouk (D‐Z) parameters, namely longitudinal unit conductance, transverse unit resistance, and longitudinal resistivity has been employed by using 50 vertical electrical sounding points to assess the groundwater and delineate the fresh‐saline water interface over 1045 km2 area of Khanewal in Southern Punjab of Pakistan. The x‐y plots and maps of D‐Z parameters were produced to establish a decipherable vision for the occurrence and distribution of different water‐bearing formations of fresh‐saline water aquifers through a complicated situation of intermixing of different resistivity ranges for fresh‐saline water bodies. This technique is useful to reduce the ambiguity produced by the process of equivalence and suppression which cause intermixing in differentiating fresh, brackish, and saline aquifers during interpretation. The fresh‐saline water interface is correlated very well with the previous studies of water quality analysis carried out in Khanewal area. The results suggest that the D‐Z parameters are useful for demarcating different aquifer zones. The behavior and pattern of D‐Z parameters with respect to occurrence and distribution of different water‐bearing formations were effectively identified and delineated in the study area.  相似文献   

15.
Although it has been increasingly acknowledged that groundwater flow pattern is complicated in the three‐dimensional (3‐D) domain, two‐dimensional (2‐D) water table‐induced flow models are still widely used to delineate basin‐scale groundwater circulation. However, the validity of 2‐D cross‐sectional flow field induced by water table has been seldom examined. Here, we derive the analytical solution of 3‐D water table‐induced hydraulic head in a Tóthian basin and then examine the validity of 2‐D cross‐sectional models by comparing the flow fields of selected cross sections calculated by the 2‐D cross‐sectional model with those by the 3‐D model, which represents the “true” cases. For cross sections in the recharge or discharge area of the 3‐D basin, even if head difference is not significant, the 2‐D cross‐sectional models result in flow patterns absolutely different from the true ones. For the cross section following the principal direction of groundwater flow, although 2‐D cross‐sectional models would overestimate the penetrating depth of local flow systems and underestimate the recharge/discharge flux, the flow pattern from the cross‐sectional model is similar to the true one and could be close enough to the true one by adjusting the decay exponent and anisotropy ratio of permeability. Consequently, to determine whether a 2‐D cross‐sectional model is applicable, a comparison of hydraulic head difference between 2‐D and 3‐D solutions is not enough. Instead, the similarity of flow pattern should be considered to determine whether a cross‐sectional model is applicable. This study improves understanding of groundwater flow induced by more natural water table undulations in the 3‐D domain and the limitations of 2‐D models accounting for cross‐sectional water table undulation only.  相似文献   

16.
A new method for time‐lapse signal separation and enhancement using singular‐value decomposition is presented. Singular‐value decomposition is used to separate a 4D signal into its constituent parts: common geology, time‐lapse response and noise. Synthetic tests which demonstrate the advantages of the singular‐value decomposition technique over traditional differencing methods are also presented. This signal separation and enhancement technique is used to map out both the original and moved oil–water contacts across the Nelson Field. The singular‐value decomposition technique allows the oil–water contact to be mapped across regions which would have been missed using traditional differencing methods. In particular, areas toward the edges of the field are highlighted by the technique. The oil–water contact is observed to move upwards across the field, with the largest movements being associated, as anticipated, with natural production. The results obtained are broadly consistent with those predicted by the reservoir simulator model. Singular‐value decomposition is demonstrated to be a useful tool for enhancing the time‐lapse signal and for gaining confidence in areas where traditional differencing fails.  相似文献   

17.
Surface‐related multiples are attenuated for one sail line and one streamer of a 3D data set (courtesy of Compagnie Générale de Géophysique). The survey was carried out in the Gulf of Mexico in the Green Canyon area where salt intrusions close to the water‐bottom are present. Because of the complexity of the subsurface, a wavefield method incorporating the full 3D volume of the data for multiple removal is necessary. This method comprises modelling of the multiples, where the data are used as a prediction operator, and a subtraction step, where the model of the multiples is adaptively removed from the data with matching filters. The accuracy of the multiple model depends on the source/receiver coverage at the surface. When this coverage is not dense enough, the multiple model contains errors that make successful subtraction more difficult. In these circumstances, one can either (1) improve the modelling step by interpolating the missing traces, (2) improve the subtraction step by designing methods that are less sensitive to modelling errors, or (3) both. For this data set, the second option is investigated by predicting the multiples in a 2D sense (as opposed to 3D) and performing the subtraction with a pattern‐based approach. Because some traces and shots are missing for the 2D prediction, the data are interpolated in the in‐line direction using a hyperbolic Radon transform with and without sparseness constraints. The interpolation with a sparseness constraint yields the best multiple model. For the subtraction, the pattern‐based technique is compared with a more standard, adaptive‐subtraction scheme. The pattern‐based approach is based on the estimation of 3D prediction‐error filters for the primaries and the multiples, followed by a least‐squares estimation of the primaries. Both methods are compared before and after prestack depth migration. These results suggest that, when the multiple model is not accurate, the pattern‐based method is more effective than adaptive subtraction at removing surface‐related multiples while preserving the primaries.  相似文献   

18.
A modified discrete element method is briefly introduced and used for modelling reservoir geomechanical response during fluid injection and depletion. The modified approach works as a continuum method until some local failure is initiated, after which it behaves like a discrete element method on a polygonal lattice. The method is advantageous for modelling fracture developments in rocks. It is applied here to synthetic models of two reservoirs taken from the North Sea (Gullfaks and Elgin‐Franklin). For Gullfaks, two cases of water injection were modelled, one with low horizontal effective stress and the other with low vertical effective stress. Vertical fractures are developed in the first case, whereas horizontal fractures are developed in the second case. This would not have been seen using traditional methods. Based on 4D seismics data for the Gullfaks field, one may envision that horizontal fractures could have been formed. The Elgin‐Franklin synthetic model is used to study various scenarios of changing stress field around the depleting reservoir. Based on 4D seismics data from this field, one may see changes that could be interpreted in terms of possible fault reactivation.  相似文献   

19.
The rough‐sea reflection‐response varies (1) along the streamer (2) from shot to shot and (3) with time along the seismic trace. The resulting error in seismic data can be important for time‐lapse imaging. One potential way of reducing the rough‐sea receiver error is to use conventional statistical deconvolution, but special care is needed in the choice of the design and application windows. The well‐known deconvolution problem associated with the non‐whiteness of the reflection series is exacerbated by the requirement of an unusually short design window – a requirement that is imposed by the non‐stationary nature of the rough‐sea receiver wavelet. For a synthetic rough‐sea data set, with a white 1D reflection series, the design window needs to be about 1000 ms long, with an application window about 400 ms long, centred within the design window. Although such a short design window allows the deconvolution operator to follow the time‐variation of the rough‐sea wavelet, it is likely to be too short to prevent the non‐whiteness of the geology from corrupting the operator when it is used on real data. If finely spatial‐sampled traces are available from the streamer, the design window can be extended to neighbouring traces, making use of the spatial correlations of the rough‐sea wavelet. For this ‘wave‐following’ approach to be fruitful, the wind (and hence the dominant wave direction) needs to be roughly along the line of the streamer.  相似文献   

20.
The problem of quantifying the effects of flexible plants on flow resistance and eddy viscosity by vegetated floodplains is first addressed with a one‐dimensional (1D) approximation based upon the so‐called lateral distribution method. The estimates so obtained are then tested with two‐dimensional (2D) numerical simulations based on the full shallow water equations through the use of the computational code Telemac‐2D. Data obtained on a physical model of the Besòs River (Spain), whose floodplains were covered with plastic ornamental plants to mimic the effect of flexible vegetation, is used for the validation of the numerical results. Additionally, the values of flow resistance estimated numerically with the 1D and 2D simulations are compared with values obtained in a rectangular flume under flow conditions (slope, water depth and artificial lining) similar to those used on the reduced model. It is then established that as more physical mechanisms are included in the mathematical model used to study the problem, the ratio between the floodplain and the main channel flow resistance coefficient increases. The approach demonstrates that whenever enough flow data is available, the lateral distribution method delivers values of flow resistance and eddy viscosity which are highly consistent with 2D numerical modelling. This finding could mean considerable savings in the burdensome task of specifying flow resistance and turbulence dissipation values for 2D modelling of large compound channel systems. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

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