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8th EEGS-ES Meeting
- Conference date: 08 Sep 2002 - 12 Sep 2002
- Location: Aveiro, Portugal
- ISBN: 972-789-071-7
- Published: 08 September 2002
41 - 60 of 131 results
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Comparison of 1D laterally constrained inversion and 2.5D inversion of CVES resistivity data with drilling data as apriori information
More LessData collection with a multi electrode system, such as Lund Imaging System from ABEM, is usually arranged so the dataset is suited for inversion using 2.5D inversion algorithms (Loke and Barker, 1995). This data may not always be suitable for 1D inversion. However, with a 1D inversion approach it is possible to decide the number of layers to be used in inversion and also put in apriori information, where this is available, to lock layer boundaries. This approach is likely to give a result where depths to layers are better determined than with a 2.5D approach. This paper aims to compare inversion with a 2.5D least squares algorithm, Res2Dinv, and the 1D Lateral Constrained Inversion (LCI) algorithm (Auken et al., 2000). The comparison is based on a large dataset collected with a multi electrode system in Lockarp in Southern Sweden (LTH, 2000). There are a large number of geotechnical investigations available, such as auger and core drillings, which can be used as apriori information in the inversions. The geology in the area is sedimentary and consists of Quaternary deposits underlain by Danian limestone. The Quaternary deposits consist of four individually different units. On top there is a half to two meters of high resistive sediments. The second and fourth unit consist of low resistive clay till. In between these two units a third unit consisting up to three meters of high resistive intermorainic sediments can be found in parts of the area. The resistivity data was originally acquired for 2.5D inversion. For LCI inversion the data was divided into individual soundings, one every meter, and all soundings with midpoint within a four meter distance were gathered into one sounding. This results in a dataset with soundings every four meters containing exactly the same amount of data as the original dataset. This means that a sounding might contain more than one data point with the same electrode configuration.
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Interurban - geophysical assessment of water dynamics and solute transport in urban soils
Authors M. Müller, O. Mohnke, J. Schmalholz and U. YaramanciUrban soils are subject to strong variations of environmental conditions, like water flow, solute transport or heat budget. The research project INTERURBAN has been formed to clarify the dynamics of water and materials at urban locations with special consideration of the spatial heterogeneity, the organic soil substance, and soil-biological transformation processes. In order to achieve these objectives, we work on four interdisciplinary questions: 1st The evaluation of spatial heterogeneity and its interaction with sub-processes; 2nd The influence of biotic and abiotic factors on the characteristics of the organic substance; 3rd The recording of water- and material transport from heterogeneous urban locations and application to selected locations; 4th The recording of the interaction between biotic and abiotic factors and their participation in the dynamics of the release of pollutants. These goals of the research group define the target for geophysics: The development of a noninvasive conception to map the water distribution and important structural parameters. In addition to standard methods, like geoelectrics and Ground Penetrating Radar (GPR), Spectral Induced Polarization (SIP) and Surface Nuclear Magnetic Resonance (SNMR) will be used. SNMR is the only method that allows to determine water content and pore size distribution directly from the surface and therefore a special focus in the project.
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GPR Mapping toluene infiltration in a sand model
More LessThe high resolution GPR technique of 1.5 GHz antenna is applied for mapping infiltrated dense non-aqueous phase liquid DNAPL (here toluene) in a partially saturated sand medium with time. A model box (83*9.5*61 cm3) is filled with a medium sand which is saturated with water at the lowermost 18 cm. The model contains five prismatic objects of fine (objects 1, 4) and coarse sand (objects 2, 3, 5). A toluene is infiltrated from a single driplet at the centre of the upper surface. Georadar measurements (1.5 MHz) from the external surfaces show that mapping toluene is possible in the partially saturated zones of objects 1, 2 and 4 and hardly in the other areas. The weak reflections within the low saturated region may be caused by a weak electromagnetic impedance contrast at the interface to the toluene zone.
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Preliminary result of resistivity measurements for the name project (Nitrate from aquifers and influence on carbon cycling in marine ecosystems)
Authors B. Ludovic and D. ChapellierThe NAME project is an European research project which gathers hydrogeologists, geophysicists, geochemists and marine biologists belonging to different institutions: • DHI Water & Environment, Denmark; • Netherlands Institute of Ecology, Netherlands; • Max Planck Institute, Deutschland; • Ribe County, Denmark; • Technical University of Denmark; • University of Lausanne, Switzerland. The role of nitrogen in eutrophication of coastal marine environments is well documented (Horwarth et al., 1996). It is also well known that the application of fertilizer on agricultural fields causes nitrate concentrations in groundwater to increase (Postma et al., 1991). The nitrate bearing-nutrient that enters the marine environment in this way is monitored by measuring their fluxes in outlets of rivers and estuaries. Furthermore a significant part of the nitrate is removed by denitrification in the estuarine system (Nixon, 1996). However, at extensive sections along the European coast, the groundwater is discharging directly into the sea by upward transport throw the sea bottom. The importance of this direct nutrient flux into the marine environment is virtually unknown both with respect to its quantitative importance and with respect to the processes that take place at the freshwater-seawater interface. Given the steady increase in groundwater nitrate concentrations (Howarth et al., 1996), there is a strong need to investigate and document this matter and to evaluate its significance for coastal zone management. To illustrate the potential significance of processes occurring at the freshwater-seawater interface for the impact of direct discharge of nitrate containing groundwater on the marine ecosystem, two imaginary end-member scenarios may be envisioned: • In the first scenario nitrate bearing groundwater emerges unaltered through vents in the sea bottom. In this case there is a direct input of nitrate into the marine environment which may seriously affect the state of eutrophication in the coastal marine ecosystem. • In the second scenario, the freshwater is thought to mix dispersively with seawater below the sea bottom and in this case the emerging nitrate is reduced to free nitrogen while oxidizing organic matter that is deposited from above. The two scenarios described above are purely speculative, basically because of our current ignorance concerning real world processes. A recent research interest in this subject has lead to some reports describing the seepage of groundwater through the sea bottom (Moore, 1999; Hussain et al., 1999; Bussman et al, 1999). Generally, however, the current literature reveals a disturbing lack of knowledge concerning the quantitative significance of nitrate-bearing groundwater directly discharging into the marine environment and the biogeochemical processes involved.
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Georadar estimation of ground pollution degree as a result of leakage from main pipelines
Authors A. Dolgiy, Аn. Dolgy, V. Zolotarev and V. MarkulisThis report discusses the georadar technology application to problems occurring during operation with main pipelines. Two of the authors - V. Markulis and A. Dolgiy represent an operation service of a Latvian-Russian joint venture on transportation of oil and derivate products by main pipelines LatRosTrans Ltd (figure 1 – pipeline layout). The others two authors - V. Zolotarev and Аn. Dolgy represent Radar Systems Inc., Latvia.
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Induced polarisation survey at a waste site in southern sweden
More LessTime-domain induced polarisation and apparent resistivity pseudosections were measured on a former waste site in Southern Sweden, just outside the city of Kristianstad. The landfill is still in service, except an approximately 400 m x 400 m area, which has been covered by a rather inhomogeneous layer of clay and clay till. Since the landfill is to become a green leisure area, the aim of the study was to be able to assess the thickness of that covering layer with good precision in order to guide the rehabilitation process. A grid of auger drillings, down to a maximum depth of 3 m and placed approximately every 50 m yielded punctual information on the stratigraphy of the material covering the waste, and was used as reference. Apparent resistivity and chargeability pseudo-sections were measured to get a continuous picture of the distribution of the material.
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Signal optimization for electrokinetic metal decontamination: sample and fiels scale model experiments
Authors F. Cherubini, G. Losito, A. Trova and R. AngeliniAs well known, the decontamination of polluted soils by heavy metals is an environmental problem that concerns wide areas of the planet. The electrokinetic decontamination (Acar Y.B. et al., 1993; Acar Y.B. et al., 1995; Alshawabken A.N. et al., 1999) is a recent "in situ" method, but in the current technology it requests high levels of power (some MW), with consequent security and cost problems. So the main purpose of our research has been to find an experimental procedure that requests low voltage levels to obtain the metal mobilisation.
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An application of magnetic and ecletromagnetic methods for environmental research
Authors P. Barsukov, E. Fainberg and E. KhabenskyIn 1941 when the German armies invaded Sevastopol (Crimea), the Soviet sappers blew up the arsenals of the Black Sea fleet stored in the limy massif galleries known as the Inkerman galleries. However, when undermining only small part of the arsenal has been detonated, the massif was strongly destroyed and the basic part of the arsenal has remained under blockages galleries. According archive data the area of the galleries at the moment of explosion was more than 1 hectare, there were from 10000 up to 30000 tons of ammunition, mainly large aerial bombs, sea mines, torpedoes, charges for ship artillery of the big calibers, etc. At present, there are some crevices in the destroyed limy massif through which it was possible to penetrate into the blockages of the arsenal and to find kept ammunition on the area about 40 m2. The find state of the ammunition is quite satisfactory, that allows using the mine clearing and the further recycling technologies. At the moment of explosion the vaults of the galleries were destroyed and sank on 10-15 m. The rock massif has intensively cracked and turned to a congestion of limestone blocks in the volume up to 1000-5000 m3. Cracks are filled with the clay formations and water supplied. Intensively progressing karstic processes in cracked limestone create danger of explosive objects of the arsenal corrosion, which is situated within the city boundaries. On territory of Inkerman galleries SINECO Company carried out measurements of a full vector of magnetic field on a uniform mesh 5m×5m. Several powerful sign-variable anomalies were revealed there, however their locality and huge gradients (up to 200-400 nT/m) testified to the small size and small depth of bedding (not more than 1-3 m). As the goal of the magnetic survey was the detailed mapping of the ammunition’s value, the decision to involve in researches TEM-FAST sounding technology was accepted. This technology is used successfully to solve the engineering, geological, hydro-geological and environmental problems. Primary goals of the research were the following: 1) Mapping of the ammunition on the depth of 10-30 m from a surface; 2) Three-dimensional mapping of sluggies in the limestone massif. For the solution of these tasks TEM-FAST 48 HPC (AEMR ltd.) system with 25m×25m antenna has been used. This system defined transient characteristics of the media in a range of times from 2-4 μs up to 16 ms, both in one-loop configuration, and in configuration loop-in-loop. Preliminary full-scale test has shown that well conducting layers of limestone screen metal objects even of 5 ton aerial bomb size which practically are not visible at depths more than 10 meters. However the super paramagnetic effect (SPM), caused frequency dependence of a magnetic susceptibility of metals χ(ω), is confidently registered even for much big depths. When measurements of transient characteristics of an electromagnetic field the voltage on receiving antenna U(t) in case of SPM-effect [1] is inversely to time U(t) ~1/t1+δ (|δ|<<1). In regulation E (t) = t*U (t)/I the response of the media at t > 100 μs weakly depends on time E(t) ~ t−δ. At the same time the induction signal caused by attenuation of the curl currents in rock, attenuates much faster: E(t) ~ 1/tn, where n ~ 1-3. Examples of the transients for two sites: P-38 and P-44 in the above regulation are given in the fig. 1.
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Eigenimage analysis of shallow seismic sections
More LessThe appearance of seismic events related to various subsurface objects is to a large extent defined by their coherency on seismic sections. In particular, shallow subsurface inhomogeneities can be expected to appear as the areas of poor coherency. Identifying and separating coherent and non-coherent events on the sections can be performed using the technique of eigenimage analysis. This technique is based on the singular value decomposition (SVD) of the sections represented as data matrices. A similar approach is implemented in the Karhunen-Loeve (or principal component) reconstruction method (Hemon and Mace, 1978). The technique has been successfully applied to various geophysical problems (Jones and Levy, 1987; Freire and Ulrych, 1988; Jackson et al., 1991; de Franco and Musacchio, 2001).
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The use of constraints in 2D and 3D resistivity modelling
Authors M. H. Loke and J. W. LaneElectrical imaging surveys are now widely used to map areas with complex subsurface geology. The many applications include groundwater exploration, mineral exploration, environmental and engineering studies (Dahlin, 1996; Griffiths and Barker, 1993). While 2D surveys are more commonly used due to time and cost limitations, 3D surveys are also used in very complex areas such as in mineral exploration surveys (White et al., 2001). A cell-based inversion model is normally used to model such complex structures. The subsurface is divided into a large number of rectangular cells and an inversion algorithm is used to determine the resistivity of the cells. The inversion of resistivity data gives non-unique results. Thus some assumptions are made about the subsurface so as to limit the range of plausible models. The regularized least-squares optimization method where constraints can be easily added into the inversion procedure (Ellis and Oldenburg, 1994) is frequently used. The effects of constraints on the inversion results are discussed in the following sections.
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P-S Wave separation in 3-D elastic seismic data
By R. SunIn geophysical exploration, reflected P- and S-wave have different velocities but are always coupled in solid earth. Receivers sense only specific displacement components without distinguishing P- or S-wave. An accurate velocity distribution is required to image the seismic energy at its reflecting point. A strategy is to separate reflected P- and S-waves before imaging such that the P- and S-wave can be imaged separately. This study intends to develop an algorithm to separate the reflected P- and S-wave in three-dimensional (3-D), three-component, prestack elastic seismic data. The 3-D elastic seismic data is downward extrapolated from the receiver locations into a vertically homogeneous elastic model (with P- and S-velocities) using the 3-D elastic wave equation. During downward extrapolation, divergence and curl of the displacement are computed and recorded independently at a certain depth. The P- and S-velocities in the elastic model are then split into two independent acoustic models. The divergence is upward extrapolated, using the acoustic wave equation, into the P-velocity model and extracted at the receiver locations to obtain the separated P-wave. The curl is extrapolated upwards, using the acoustic wave equation, into the S-velocity model and extracted at the receiver locations to obtain the separated S-wave. Tests are successful on synthetic seismic data computed for simple laterally heterogeneous 3-D models and if the velocities used in the extrapolations are incorrect.
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Pulled array seismic (PAS) – A new method for shallow, high-resolution reflection seismic data acquisition
Authors J. F. Jensen, J. Ringgaard, P. Skjellerup and T. Vangkilde-PedersenAlong with the increasing demands in Denmark for new and deeper groundwater resources, the application of shallow reflection seismic methods has been intensified. While the traditionally low-priced and extensively used electric and electromagnetic methods are used successfully for mapping of the more shallow geology, reflection seismic is an ideal method for mapping of deep (up to several hundreds of metres) buried quaternary valleys and deep tertiary (Miocene) aquifers as well as faults. The cost for carrying out shallow onshore reflection seismic surveys has, however, been a limiting factor towards the use of seismic data in mapping of groundwater resources. Previously, results using a towed land streamer with gimbal mounted geophones and a pipe gun or sledgehammer as seismic source, has been presented by van der Veen and Green, 1998 and van der Veen et. al. 2001. RAMBOLL has developed a new method for shallow reflection seismic data acquisition called Pulled Array Seismic or PAS using a towed trail of conventional geophones and a seismic vibrator as energy source. The new method is considerably faster and more cost-effective compared to traditional shallow reflection seismic data acquisition, and the data quality is fully comparable also to deeper conventional seismic data. Since the first commercial data acquisition with this new method took place in August 2000 there has been a substantially interest for the method. RAMBOLL has thus acquired 106 km of Pulled Array Seismic on 28 lines for 6 different clients in Denmark and southern Sweden.
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Model recognition by 1-D laterally constrained inversion of resistivity data
Authors E. Auken, N. Foged and K. I. SørensenSeveral 2-D “minimum structure” inversion algorithms (de Groot-Hedlin and Constable, 1990) are available for interpretation of profile oriented resistivity data (e.g. Oldenburg and Li, 1994; Loke, 1995). For very large data sets these algorithms are intensive to run and the models are often hard to interpret in terms of a layered geological model which is the most suitable model type in sedimentary environments. As an alternative to the 2-D “minimum structure” inversion algorithm, we have developed the 1-D Laterally Constrained Inversion (1-D LCI) algorithm. The LCI algorithm is parameterized and uses a series of laterally constrained 1-D models in the inversion. The 1-D LCI algorithm has for some years been used for some years by routine to invert multi electrode data (Wisén and Auken, this conference) as well as Pulled Array Continues Electric Sounding (PACES) data (e.g. Sørensen, 1996). The aim of this abstract and presentation is first to present a statistic study of the ability of the LCI algorithm to recognize a geological model, second to present the LCI algorithm itself. PACES data are used to map the upper 25 – 30 m of the geology for a detailed, regional mapping of protective clay caps of aquifers (Christensen and Sørensen, 1998). With the resolution capabilities and the equivalence problem of resistivity data in mind it is important to know not only the geological/hydrogeophysical model but also the probability of the model. One approach to analyze the quality of an inverted model section is to generate data over some simplified models, invert them and compare the inversion results to the original model. The decisive factor doing this is how representative the forward model is. A more thorough approach is to generate data from a large number of stochastic model sections which reflect the expected geological environment in a given area of investigation. Subsequently, the data must be submitted to the same data processing and inversion process as field data. Based on a comparison between the original model and the inverted model, it is possible to calculate a statistical measure for the quality of the inverted model.
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2-D Laterally constrained inversion (LCI) of resistivity data
Authors A. V. Christiansen, E. Auken, K. I. Sørensen and T. Smith2D laterally constrained inversion (2D LCI) is a new approach to interpretation of continuous resistivity data using an inversion scheme similar to that used for 1D LCI. The only difference is that the forward code in the inversion scheme is 2D. This enables a layered 2D inversion in sedimentary areas where the layering has been disturbed in a degree, that a 1D model is inadequate, but a layered model is still applicable. The layered 2D inversion provides a full analysis of the model, and can include a priori information just as in the 1D case. Preliminary results on synthetic data show that a layered 2D approach enables a more detailed description of 2D structures, which the 1D formulation can not resolve. The model complexity is controlled and balanced by the lateral constraints on depths and resistivities.
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Shallow seismic reflection profiling with a towed geophone system: preliminary results
Authors R. Marques Moura and M. J. Senos MatiasIn recent years some workers have been experimenting with towed receiver array systems in an effort to increase the productivity of land seismic reflection acquisition. The works of Eiken (1989) and Van der Veen (1998) are key references. To our knowledge, there is at least one firm (in Denmark) that provides this type of acquisition service on a regular basis. Based on some of these promising results and due to our need in arranging a time efficient method for profiling we decided to design and build our own towed array system.
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Modelling of the phase behaviour in the SNMR-Signal
Authors M. Braun, M. Hertrich and U. YaramanciSurface Nuclear Magnetic Resonance (SNMR) is a fairly new geophysical method for groundwater exploration and aquifer characterization (e.g. Yaramanci 2000). The NMR-Experiment is conducted by a coincident transmitter and receiver loop at the surface exciting the protons of water molecules underground with the Larmor frequency of the earth’s magnetic field. Conducting a SNMR measurement with increasing excitation intensity yields a complex sounding curve, where the amplitude of the relaxation signal is determined by the numbers of protons i.e. the water content and the phase is sensitive to the subsurface conductivity.
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Weighted F-K for surface wave analysis
By C. StrobbiaTests based on surface waves (SWM) consist of three steps: acquisition, processing and inversion. The acquisition is the observation of Rayleigh wave propagation in time and space: the vertical velocity at the free surface is usually recorded in two or more points at known distances from the source. To assess the dispersion characteristics on a wide frequency range, the source has to produce either a single broad-band signal or a series of monochromatic signals over a wide frequency band. The processing extracts from field records the information about Rayleigh waves dispersion. The inversion estimates site mechanical properties as a function of depth from the experimental dispersion characteristics. The amount of information extracted by the processing and its uncertainty, which strongly depend on the acquisition parameters, affect the reliability of the final result. Several processing techniques can be used; the basic principle is the same and some of them are formally identical. Nevertheless their different sensitivity to coherent and random noise leads to peculiar quality assessment and filtering capabilities. I compare here different processing techniques for dispersion curve extraction, and propose a new algorithm based on f-k processing and coherence between signal pairs. The procedure reduces the effects of noise by discarding low coherence signals and allows a great increase of the useful frequency band. Moreover non-uniformly spaced arrays can be processed and poor quality signals are filtered out automatically.
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Mapping a quartz vein using three electrode arrays at Santo Domingo, Nicaragua
Authors A. Mendoza and T. DahlinThe multi-electrode electrical resistivity method has become one of the most efficient geophysical methods used in a wide range of applications, which is also suitable when disturbances in the ground are to be avoided. The selection of the right electrode configuration can provide better effectiveness, depending on the target being investigated. In this work a case of the use of three electrode arrays for vertical quartz vein localization is presented. The investigations were carried out within the framework of a multi disciplinary research and training programme funded by Sida/SAREC, with the aim of mapping groundwater systems in an area contaminated by mining activities. The study area is close to the village Santo Domingo, central Nicaragua, were gold mining has been an important economic activity for over 140 years. Three Continuous Vertical Electrical Soundings (CVES) were performed crossing El Cuatro gold-bearing quartz vein. This is one of the more representative geological structures in the area because of its characteristic tectonic settings, steep topography and its hydrogeological significance for groundwater occurrence.
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Hitem – A high moment / high production TEM system
Authors J. E. Danielsen, E. Auken and K. I. SørensenDuring the nineties the transient electromagnetic method (Nabighian and Macnae, 1991) became the method preferred for mapping the extension of aquifers in the sedimentary environments in Denmark. The most commonly used system has been the Geonics PROTEM 47 system with a 40 x 40 m2 transmitter loop. More than 45,000 soundings have been carried out by use of this system. However, for many tasks its penetration depth is insufficient and a wish rose to develop a high moment TEM system (HiTEM) with the same model resolution capabilities and similar production rates as the 40 x 40 m2 system. The development of the HiTEM system to its present state took about 3 years. The system is capable of transmitting up to 75 A in a 30 x 30 m2 transmitter loop. This offers a magnetic moment of the system about 15 times larger than the conventional 40 x 40 m2 system and a penetration depth up to 300 m compared to the 120-150 m of the PROTEM 47 system. Since spring 2001 more than 2,000 HiTEM soundings have been carried out. Several array configurations were tested, as significant technical difficulties arise when high currents are transmitted in a small loop and the Earth response is measured in the centre of the loop. For this reason and because of expected IP-effects (Flis et al., 1989), the receiver coil was moved outside the loop. This configuration is more complex to handle in the data interpretation process than the central loop configuration, and it took some time to gain knowledge of the strengths and limitations of the configuration. Thorough 3-D modelling of various arrays and model scenarios (Toft, 2001) showed that the offset configuration is extremely sensible to small-scale, near-surface inhomogeneities at early times. In addition, the offset of the receiver coil has to be known with an impossible accuracy in a practical field situation. All this led to the characteristic HiTEM sounding which includes both a central and an offset loop measurement. Early times are measured in the central loop configuration, and late times are measured in the offset loop configuration at 2.4 A and 75 A, respectively. Both sensitivities and locations of the arrays differ, but they are interpretatively the same sounding. A traditional joint inversion of the two data sets often results in a poor data fit due to the model inconsistencies between the data sets. The solution to this has been the Mutually Constrained Inversion (MCI) approach. In a MCI inversion two models, one for each data set, are constrained to each other, and the MCI algorithm inverts the two data sets concurrently and produces two similar models. The hardness of the constraint serves as a handle which the interpreter can use to meet certain requirements: the harder the constraint, the more similar the out coming models, but the more consistent data sets are asked to provide a satisfying fit to data.
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First gravity surveys of the EEZ programme in Galicia Margin (Spain)
Authors P. Llanes, J. Álvarez, A. Muñoz-Martín and A. CarbóIncluded in the systematic mapping programme of the Hidrographic and Oceanographic Research Plan for the Spanish Exclusive Economic Zone (EEZ), gravity surveys have been carried up in Galicia margin. Using gravity data collected during the cruise carried out in 2001 aboard Hesperides Research Vessel, we have made the Free Air anomaly and Bouguer anomaly maps for this area. Sea bottom terrain correction has been calculated for all readings, -45.60 as maximum value and 36.86 mGal as medium value have been obtained. Typical oceanic Bouguer anomaly values, between 70 and 320 mGal, can be observed in the map, being located the highest values in the southwestern area.
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