A time-series analysis framework for the flood-wave method to estimate groundwater model parameters     

Christophe Obergfell  Mark Bakker  Kees Maas 《Hydrogeology Journal》2016,24(7):1807-1819

The flood-wave method is implemented within the framework of time-series analysis to estimate aquifer parameters for use in a groundwater model. The resulting extended flood-wave method is applicable to situations where groundwater fluctuations are affected significantly by time-varying precipitation and evaporation. Response functions for time-series analysis are generated with an analytic groundwater model describing stream–aquifer interaction. Analytical response functions play the same role as the well function in a pumping test, which is to translate observed head variations into groundwater model parameters by means of a parsimonious model equation. An important difference as compared to the traditional flood-wave method and pumping tests is that aquifer parameters are inferred from the combined effects of precipitation, evaporation, and stream stage fluctuations. Naturally occurring fluctuations are separated in contributions from different stresses. The proposed method is illustrated with data collected near a lowland river in the Netherlands. Special emphasis is put on the interpretation of the streambed resistance. The resistance of the streambed is the result of stream-line contraction instead of a semi-pervious streambed, which is concluded through comparison with the head loss calculated with an analytical two-dimensional cross-section model.  相似文献   

A Comparison of Strategies for Seismic Interferometry   总被引：2，自引：0，他引：2  

Roel Snieder  Masatoshi Miyazawa  Evert Slob  Ivan Vasconcelos  Kees Wapenaar 《Surveys in Geophysics》2009,30(4-5):503-523

The extraction of the response from field fluctuations excited by random sources has received considerable attention in a variety of different fields. We present three methods for the extraction of the systems response that are based on cross-correlation, deconvolution, and the solution of an integral equation, respectively. For systems that are invariant for time-reversal the correlation method requires random sources on a bounding surface only, but when time-reversal invariance is broken, for example by attenuation, a volume distribution of sources is needed. For this reason the correlation method is not useful for diffusive or strongly attenuating systems. We provide examples of the three methods and compare their merits and drawbacks. We show that the extracted field may satisfy different boundary conditions than does the physical field. This can be used, for example, to suppress surface-related multiples in exploration seismology, to study the coupling of buildings to the subsurface, and to remove the airwave in controlled source electromagnetics (CSEM).  相似文献   

Modeling time series of ground water head fluctuations subjected to multiple stresses     

von Asmuth JR  Maas K  Bakker M  Petersen J 《Ground water》2008,46(1):30-40

The methods behind the predefined impulse response function in continuous time (PIRFICT) time series model are extended to cover more complex situations where multiple stresses influence ground water head fluctuations simultaneously. In comparison to autoregressive moving average (ARMA) time series models, the PIRFICT model is optimized for use on hydrologic problems. The objective of the paper is twofold. First, an approach is presented for handling multiple stresses in the model. Each stress has a specific parametric impulse response function. Appropriate impulse response functions for other stresses than precipitation are derived from analytical solutions of elementary hydrogeological problems. Furthermore, different stresses do not need to be connected in parallel in the model, as is the standard procedure in ARMA models. Second, general procedures are presented for modeling and interpretation of the results. The multiple-input PIRFICT model is applied to two real cases. In the first one, it is shown that this model can effectively decompose series of ground water head fluctuations into partial series, each representing the influence of an individual stress. The second application handles multiple observation wells. It is shown that elementary physical knowledge and the spatial coherence in the results of multiple wells in an area may be used to interpret and check the plausibility of the results. The methods presented can be used regardless of the hydrogeological setting. They are implemented in a computer package named Menyanthes (www.menyanthes.nl).  相似文献   

Seismic reflection coefficients of faults at low frequencies: a model study     

Joost van der Neut  Mrinal K. Sen  Kees Wapenaar 《Geophysical Prospecting》2008,56(3):287-292

We use linear slip theory to evaluate seismic reflections at non‐welded interfaces, such as faults or fractures, sandwiched between general anisotropic media and show that at low frequencies the real parts of the reflection coefficients can be approximated by the responses of equivalent welded interfaces, whereas the imaginary parts can be related directly to the interface compliances. The imaginary parts of low frequency seismic reflection coefficients at fault zones can be used to estimate the interface compliances, which can be related to fault properties upon using a fault model. At normal incidence the expressions uncouple and the complex‐valued P‐wave reflection coefficient can be related linearly to the normal compliance. As the normal compliance is highly sensitive to the infill of the interface, it can be used for gas/fluid identification in the fault plane. Alternatively, the tangential compliance of a fault can be estimated from the complex‐valued S‐wave reflection coefficient. The tangential compliance can provide information on the crack density in a fault zone. Coupling compliances can be identified and quantified by the observation of PS conversion at normal incidence, with a comparable linear relationship.  相似文献   

The Influence of Code Multipath on the Estimated Parameters of the Geometry-Free GPS Model     

Kees De Jong 《GPS Solutions》1999,3(2):11-18

In this article, the influence of biases in GPS code observations on the estimated parameters of the geometry-free model is investigated. This is done for undifferenced as well as double-differenced data from short baselines, that is, baselines for which ionospheric effects may be assumed absent. It is shown how introducing a linear model for code multipath affects the original model parameters. The performance of the original and extended model is illustrated by analyzing data from a single receiver and a short baseline. ? 1999 John Wiley & Sons, Inc.  相似文献   

Review paper: Virtual sources and their responses,Part II: data‐driven single‐sided focusing          下载免费PDF全文

Kees Wapenaar  Jan Thorbecke  Joost van der Neut  Evert Slob  Roel Snieder 《Geophysical Prospecting》2017,65(6):1430-1451

In Part I of this paper, we defined a focusing wave field as the time reversal of an observed point‐source response. We showed that emitting a time‐reversed field from a closed boundary yields a focal spot that acts as an isotropic virtual source. However, when emitting the field from an open boundary, the virtual source is highly directional and significant artefacts occur related to multiple scattering. The aim of this paper is to discuss a focusing wave field, which, when emitted into the medium from an open boundary, yields an isotropic virtual source and does not give rise to artefacts. We start the discussion from a horizontally layered medium and introduce the single‐sided focusing wave field in an intuitive way as an inverse filter. Next, we discuss single‐sided focusing in two‐dimensional and three‐dimensional inhomogeneous media and support the discussion with mathematical derivations. The focusing functions needed for single‐sided focusing can be retrieved from the single‐sided reflection response and an estimate of the direct arrivals between the focal point and the accessible boundary. The focal spot, obtained with this single‐sided data‐driven focusing method, acts as an isotropic virtual source, similar to that obtained by emitting a time‐reversed point‐source response from a closed boundary.  相似文献   

Review paper: Virtual sources and their responses,Part I: time‐reversal acoustics and seismic interferometry          下载免费PDF全文

Kees Wapenaar  Jan Thorbecke 《Geophysical Prospecting》2017,65(6):1411-1429

A focusing acoustic wave field, emitted into a medium from its boundary, converges to a focal spot around the designated focal point. Subsequently, the focused field acts as a virtual source that emits a field propagating away from the focal point, mimicking the response to a real source at the position of the focal point. In this first part of a two‐part review paper on virtual sources and their responses, we define the focusing wave field as the time reversal of an observed point‐source response. This approach underlies time‐reversal acoustics and seismic interferometry. We analyse the propagation of a time‐reversed point‐source response through an inhomogeneous medium, paying particular attention to the effect of internal multiples. We investigate the differences between emitting the focusing field from a closed boundary and from an open boundary, and we analyse in detail the properties of the virtual source. Whereas emitting the time‐reversed field from a closed boundary yields an accurate isotropic virtual source, emitting the field from an open boundary leads to a highly directional virtual source and significant artefacts related to multiple scattering. The latter problems are addressed in Part II, where we define the focusing wave field as an inverse filter that accounts for primaries and multiples.  相似文献   

Acoustic directional snapshot wavefield decomposition     

Max Holicki  Guy Drijkoningen  Kees Wapenaar 《Geophysical Prospecting》2019,67(1):32-51

Up–down wavefield decomposition is effectuated by a scaled addition or subtraction of the pressure and vertical particle velocity, generally on horizontal or vertical surfaces, and works well for data given on such surfaces. The method, however, is not applicable to decomposing a wavefield when it is given at one instance in time, i.e. on snapshots. Such situations occur when a wavefield is modelled with methods like finite-difference techniques, for the purpose of, for example, reverse time migration, where the entire wavefield is determined per time instance. We present an alternative decomposition method that is exact when working on snapshots of an acoustic wavefield in a homogeneous medium, but can easily be approximated to heterogeneous media, and allows the wavefield to be decomposed in arbitrary directions. Such a directional snapshot wavefield decomposition is achieved by recasting the acoustic system in terms of the time derivative of the pressure and the vertical particle velocity, as opposed to the vertical derivative in up–down decomposition for data given on a horizontal surface. As in up–down decomposition of data given at a horizontal surface, the system can be eigenvalue decomposed and the inverse of the eigenvector matrix decomposes the wavefield snapshot into fields of opposite directions, including up–down decomposition. As the vertical particle velocity can be rotated at will, this allows for decomposition of the wavefield into any spatial direction; even spatially varying directions are possible. We show the power and effectiveness of the method by synthetic examples and models of increasing complexity.  相似文献   

The L3–6 chondritic regolith breccia Northwest Africa (NWA) 869: (II) Noble gases and cosmogenic radionuclides     

Kees WELTEN  Marc W. CAFFEE  Luitgard FRANKE  A. J. Timothy JULL  Marlene D. LeCLERC  Knut METZLER  Ulrich OTT 《Meteoritics & planetary science》2011,46(7):970-988

Abstract– We measured cosmogenic radionuclides and noble gases in the L3–6 chondrite breccia Northwest Africa (NWA) 869, one of the largest meteorite finds from the Sahara. Concentrations of ¹⁰Be, ²⁶Al, and ³⁶Cl in stone and metal fractions of six fragments of NWA 869 indicate a preatmospheric radius of 2.0–2.5 m. The ¹⁴C and ¹⁰Be concentrations in three fragments yield a terrestrial age of 4.4 ± 0.7 kyr, whereas two fragments show evidence for a recent change in shielding, most likely due to a recent impact on the NWA meteoroid, approximately 10⁵ yr ago, that excavated material up to approximately 80 cm deep and exposed previously shielded material to higher cosmic‐ray fluxes. This scenario is supported by the low cosmogenic ³He/²¹Ne ratios in these two samples, indicating recent loss of cosmogenic ³He. Most NWA samples, except for clasts of petrologic type 4–6, contain significant amounts of solar Ne and Ar, but are virtually free of solar helium, judging from the trapped ⁴He/²⁰Ne ratio of approximately 7. Trapped planetary‐type Kr and Xe are most clearly present in the bulk and matrix samples, where abundances of ¹²⁹Xe from decay of now extinct ¹²⁹I are highest. Cosmogenic ²¹Ne varies between 0.55 and 1.92 × 10^?8 cm³ STP g^?1, with no apparent relationship between cosmogenic and solar Ne contents. Low cosmogenic (²²Ne/²¹Ne)_c ratios in solar gas free specimens are consistent with irradiation in a large body. Combined ¹⁰Be and ²¹Ne concentrations indicate that NWA 869 had a 4π cosmic‐ray exposure (CRE) age of 5 ± 1 Myr, whereas elevated ²¹Ne concentrations in several clasts and bulk samples indicate a previous CRE of 10–30 Myr on the parent body, most probably as individual components in a regolith. Unlike many other large chondrites, NWA 869 does not show clear evidence of CRE as a large boulder near the surface of its parent body. Radiogenic ⁴He concentrations in most NWA 869 samples indicate a major outgassing event approximately 2.8 Gyr ago that may have also resulted in loss of solar helium.  相似文献   

From thick to thin regional identities?     

Kees Terlouw 《GeoJournal》2012,77(5):707-721

Regions and regional identity have become more important over the last decades. At the same time regions have become less discernable as distinct historically rooted spatial entities. Globalisation and the decline of collective identities through individualisation transform both this regional reality and how regions are conceptualised. This article analyses the shifts in types of regional identities used by regional administrations in an increasingly competitive environment. It uses the contrast between ‘thick’ traditional and historical rooted well-established regional identities, and ‘thin’ regional identities which are more transitory and focus more on economic competitiveness. These concepts are used to analyse the regional identity of regional administrations in Northwest Germany and the Netherlands. Hybrid regional identities combining a locally specific mix of thick and thin elements of regional identity, and which link up with regional identities at other relevant scales, appear to be the most effective regional identities for regional administrations facing the challenges of both globalisation and the decline in collective identities.  相似文献