- Home
- Conferences
- Conference Proceedings
- Conferences
Near Surface 2007 - 13th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 03 Sep 2007 - 05 Sep 2007
- Location: Istanbul, Turkey
- ISBN: 978-90-73781-81-8
- Published: 03 September 2007
1 - 20 of 144 results
-
-
Shear Wave Reflection Landstreamer Technology Applied to Soil Response Evaluation of Earthquake Shaking in an Urban Area
Authors A. J-M. Pugin, J.A. Hunter, D. Motazedian and G.R. BrooksShear wave seismic test locations and boreholes have outlined a buried bedrock valley in the Ottawa, Ontario, suburb of Orleans. This region is in a significant high seismic hazard zone, and the surficial materials are primarily high water-content, poorly compacted Holocene-age Champlain Sea sediments. Since the in-filled sediments exhibit an extremely low average shear wave velocity (~200 m/s) and the bedrock beneath exhibits shear wave velocities on the order of 2500 m/s, it is suspected that the buried valley may give rise to three-dimensional ground motion amplification phenomena in the event of significant earthquakes. In order to better define the buried valley and to prepare for future three-dimensional shake modeling, shear-wave reflection landstreamer lines were acquired, to confirm the depth to bedrock, and to detail its shape. Despite the large broad-frequency band of ambient traffic noise, it was possible to observe reflections from the bedrock as well as additional infra-overburden reflectors. The valley shape and its internal structure, as determined by these surveys, will form a vital contribution to the three-dimensional interpretation for soil response to earthquake shaking in the Ottawa area.
-
-
-
Non-contact Surface Wave Testing While Moving
More LessWe present experiments with a rolling multichannel microphone array where measurements can be taken continuously while moving. Leaky air-coupled surface waves are measured with ordinary non-directional audio microphones. Results show that microphones can be successfully used to produce a realistic phase velocity spectrum even while moving along the surface. The paper presents some theoretical background along with initial experimental results using the moving microphone array.
-
-
-
Combining Multiple Seismic and Ground Penetrating Radar Techniques to Analyze Geologic Controls of Riparian Meadow Complexes in the Central Great Basin, Nevada USA
Authors G.S. Baker and K. SturtevantRiparian meadow systems in the Central Great Basin are of interest because they support the majority of ecosystem diversity in the region. The riparian meadows are highly dependent upon groundwater levels, thereby making them vulnerable to fluctuations. These systems are actively degrading due to incision of the streams, resulting in a lowered water table and modification of the associated ecosystems. Geologic controls, such as bedrock geometry and sediment variability, are important in the meadows because of their control on the overall system. The current hypothesis is that the sediments associated with side-valley alluvial fans and fault-related bedrock steps interact to constrict ground water flow. Seismic reflection data and seismic refraction tomography data were collected to analyze bedrock structure and topography from 10 to 80 meters depth, while ground penetrating radar (GPR) data were collected to determine the stratigraphic variability in the upper 10 meters. These data were integrated to provide a comprehensive interpretation of the upper 80 meters of the subsurface. Seismic reflection data were processed to identify the bedrock surface. This surface was then correlated with the seismic refraction tomography to extend the bedrock surface across the meadow complexes. The large volume of GPR data were interpreted by classifying radar facies based on the characteristics of the radar reflectors. These facies (in conjunction with borehole information) confirm and extend areas of alluvial fan related sediment distribution. Integration of these three geophysical techniques is advantageous because they provide more information than could be obtained with the individual techniques.
-
-
-
Mapping Salt-loads of the Murray River, Australia, Using Airborne and In-river Electromagnetic Methods
Authors A.D. Fitzpatrick, T.J. Munday, V. Berens, M.A. Hatch and A.L. TelferThis paper examines the potential of using the FDHEM RESOLVE system as basis for mapping reaches of the river that contribute to elevated salt loads in the Murray River to the south east of Mildura in Victoria. The advantages of the airborne systems become more apparent when data coverage and acquisition costs are considered, particularly in a situation where a parallel swath approached is employed. This entails the acquisition of adjacent lines of EM data along the centre and along the margins of the river. We suggest this approach provides for a better understanding of recharge and discharge processes and links between the floodplain and the main-river channel. Compared with data acquired along the river alone, this study demonstrated our ability to use Helicopter EM data to map losing and gaining (from a salt load perspective) stretches of the river and to provide insight into which parts of the groundwater-floodplain system were significant contributors to river salt loads. The rapid acquisition of airborne EM data makes these systems more suited to providing temporal snapshots of a river-floodplain environment during dramatic climatic events, such as flooding.
-
-
-
Efficient Datasets – An Alternative Approach Analysing the Data Space
Authors M. Mueller-Petke and U. YaramanciThe quest and need for optimisation of geophysical data sets either to improve the resolution or the field work (or both) is of growing interest for geophysical applications. So far, the approach to solve the task is to analyse measures derived from the model resolution matrix and adopt the dataset iteratively. Here we present a direct method to estimate an efficient dataset. The approach is based on analysing the data space, i.e. calculating the data resolution matrix. Essentially variable quantities of data errors are taken into account to define the necessary amount of independent unique measurements. Applied to the technique of Magnetic Resonance Sounding (MRS) we show that not only the singular value distribution of the forward operator is improved but also the resolution, i.e. the inversion result of the efficient dataset compared to a logarithmic and linear spaced.
-
-
-
Preserving Horizontality Using Topography Corrected Laterally Constrained Inversion
Authors A.V. Christiansen, E. Auken, H.Ø. Andersen and K. SørensenGeophysical dataset from areas with substantial topography cause problems to many inversion routines. Commonly, the cells or layers of the model follow the topography which then favors layers boundaries of the inverted model that are topography-parallel. In many cases the geological layering is not connected to the overall topography, e.g. an erosional environment over a horizontal sub-stratum. Inverting data from such areas are penalized if the topography is not taken into account when dealing with the constraints We present a simple way of correcting the constraints entering the roughness matrix of the inversion problem so that topography is neutralized and horizontal layering is favored. Synthetic modeling confirms the superiority of the topography neutralization over traditional depth referenced inversion. Field examples will be shown in the talk.
-
-
-
Geophysical Investigation of a Diatreme Structure Near Ebersbrunn/W-Saxony in Germany
Authors H. Schütze, C. Kroner and T. JahrFrom 2003 to 2006 an extensive geophysical survey, including gravimetry, magnetics, and electromagnetics, of the structure characterized as a tuff chimney was carried out. This fossile fluid migration zone which is located near Ebersbrunn/West-Saxony in Germany is afresh of geoscientific interest as it is near an area in which unexpectedly swarm earthquakes occurred in the last years. From the gravimetric survey an elliptical Bouguer-anomaly emerges with a gravity minimum of -3 mGal and a diameter of about 1000 m. In agreement with the meas-urements of the vertical intensity in the 1960s a dissected anomaly of the magnetic total field intensity is found. The anomalies are characterized by variations of several 100 nT with al-most no recognizable minima. The degree of weathering appearently affects the presence of maxima in the total field since visibly reduced magnetic susceptibility values were found in areas in which strong weathering occurred. From the conductivity map the diatreme border could be derived which beforehand was deduced from near-surface wells. By a 3d gravimetric and magnetic modeling including the results from the other geophysi-cal surveys as well as information from geological and petrographic studies, core analyses, and well logging, details on the diatreme structure could be determined.
-
-
-
Cooperative Inversion Based on Fuzzy c-Means Cluster Analysis – Application to Field Data
Authors H. Paasche, J. Tronicke and P. DietrichIn many near surface geophysical applications it is now common praxis to collect co-located disparate geophysical data sets. The advantage of such multi-method based exploration strategies lies in its potential to reduce ambiguities and uncertainties in data analysis and interpretation. To reconstruct the physical parameter distributions underlying geophysical data requires in many cases the application of tomographic reconstruction techniques. To improve the reliability of tomographically reconstructed physical parameter models, the information content of all co-located data sets should be considered during the model-generation process. Here, we apply a novel approach based on fuzzy c-means cluster analysis and conventional single-input data set inversion algorithms for the cooperative inversion of crosshole seismic P-wave and S-wave traveltimes. The approach results in a single zoned two-parameter model outlining the major subsurface units and provides additionally P-wave and S-wave velocity fields.
-
-
-
Auxiliary Results of Collection and Classification of Surface Geoelectric Arrays
More LessRecently, we have made a classification of more than one hundred various surface geoelectric arrays ever published in geophysical literature (Szalai and Szarka 2007a, 2007b). The classification is based on three divalent parameters (as “superposition” of measurements, “focusing” of currents and “colinearity” of the array), thus we set up eight groups of geoelectric arrays. One further group was separated for about 10 so-called “composite” arrays, which cannot be classified in the aforementioned way. Here we present some application examples of the classification results. Namely, we call the attention to some hidden relationships among geoelectric arrays: (1) we give an illustration how various arrays can be derived from their root array (besides the Schlumberger-related arrays several other examples will also be given in the presentation); (2) we provide a summary of arrays, capable to measure various partial derivatives of the electric potential. Among the 21 arrays 14 are already published arrays, but there are seven possible, but not-yet-applied arrays. In this way, such missing links in the genealogic trees may lead to creation of reasonable and purposeful new arrays.
-
-
-
Modelling and Measuring Shallow Inductive Electric Field Response with Capacitive Sensors
Authors C.H. Adams and J.C. MacnaeModelling software developed shows that a new Capacitive Array Resistivity with Inductive Source (CARIS) method has potential for detecting both conductive and resistive buried objects. Detecting buried resistive objects has possible application in UXO, archaeology, civil or engineering applications, void detection, pipe delineation, or fracture detection and near surface geology. A prototype CARIS instrument has been designed and built. The prototype CARIS instrument has been tested with highly repetitive success under laboratory conditions, showing good comparison with expected results from modelling. The CARIS system has also been tested in the field. Initial results from field testing show high repeatability but also high lateral variability. This appears to be due to sensitivity to near surface moisture and consolidation contrasts.
-
-
-
Incorporating Topography into 2D Resistivity Modeling by Using Finite-element and Finite Difference Approaches
Authors E. Erdogan, I. Demirci and M.E. CanadansayarIn this study, we examine the topographic effect in two-dimensional dc resistivity forward solution. A new modeling algorithm is developed which is capable of solving Poisson’s equation by using finite-difference (FD) and finite-element (FE) approaches. In this algorithm, topography can be incorporated into the model by using three different methods with FE approach and by using two different methods with FD approach. We compared these various methods on different models of surface geometry and resistivity distribution regarding their accuracy and CPU time.
-
-
-
Incorporation of Topography into 2D Resistivity Inversion Models by Using FD Modeling with Triangular Discretization
Authors I. Demirci and M.E. CandansayarWhen interpreting direct current resistivity data collected on rough terrain topographic effects should be incorporated. We developed a new 2D inversion algorithm for dc resistivity data in which topography is incorporated in to the model using a finite difference technique.
-
-
-
Efficient 2D Inversion of Long ERT Sections
By P. TsourlosIn this work a new algorithm for the efficient 2D inversion of long ERT lines was introduced. The algorithm incorporates an experimental procedure to avoid calculation and storage of the entire Jacobian matrix. This approach speeds up the Jacobian matrix calculations and also reduces the required memory resources to store it. The efficient storage of the sparse Jacobian and Smoothness matrices and the efficient inversion using the LSQR method increase significantly the inversion speed. The application of the new algorithm to synthetic and real data sets resulted in reconstructed models of comparable accuracy to the standard approach.
-
-
-
Roll-along Inversion – A New Approach for Very Long Dc Resistivity Profiles
By T. GuentherIn some cases elongated dc resistivity profiles are measured either continuous or by a roll-along technique. The interpretation in one step is usually a time consuming numerical task with large memory requirements. We present a new approach for a subsequent solution using segmentation and sophisticated constraints. It proves to be very efficient for large segment numbers and can be used on standard pc's. We show two field data examples: First, data from a flood-protection measured with the roll-along technique are treated. Thus it is possible to distinguish cohesive and sandy zones in the dike as well as in the substratum. The second examples uses data collected by a sea-bottom towed multi-electrode cable with continuous data acquisition. The result provides valuable insight into the sedimentation structures.
-
-
-
3D Effects on 2D Resistivity Imaging – Modelling and Field Surveying Results
More LessToday resistivity surveying plays an important role in many large-scale area investigations. The existence of 2D effects on 1D resistivity modelling is a well known problem; however, former studies show that 3D effects in 2D surveying are less evident. The results presented here show that, nevertheless, there is an advantage in performing 3D inversion. A comparison between 2D inversion and 3D inversion has been made with analysis of data from two different synthetic models and three field datasets. From the synthetic study it is clear that 3D inversion give higher contrast, less inversion artefacts and better model recognition than 2D inversion. From the field studies it is also evident that 3D inversion gives models with higher contrast. With only limited ground truth data it is not always possible to determine which model is closest to the true one; however, where ground truth data is available it is clear that the 3D inversion gives a better result. In addition we show that the choice of array configuration have a significant influence on the result, with gradient array generally giving better results than the other options.
-
-
-
Application of 3-D ERT for Enhanced Subsurface Imaging
More LessIn the geotechnical and environmental engineering, high resolution imaging technologies are required and 3-D ERT method is commonly used to obtain 3-D image of subsurface with quite high resolution, especially when the vertical structures such as fault or fractures are main target. In this paper, we show several examples of 3-D ERT imaging. The survey areas in the examples are (1) Granite quarry mine, (2) Abandoned lead-zinc mine, and (3) Test boreholes for fracture system evaluation. In the first example, by combining 3-D ERT with borehole radar reflection imaging, we could obtain 3-D attitude of fractures in a granite quarry to help the future mining plan. When the subsurface shows strong electrical anisotropy, we could obtain reasonable 3-D image of subsurface, which was used in safety analysis of subsurface. In the last example, by including the 3-D borehole deviations, we could obtain more accurate image of subsurface in the 2-D ERT case with the help of 3-D ERT program. These successful 3-D ERT results demonstrate that 3-D ERT is a very powerful method that can be used for wide application areas to obtain reliable 3-D earth images.
-
-
-
Time Domain Electromagnetic (TEM) Measurements on a Buried Subglacial Valley in Northern Germany
Authors B. Tezkan, H. Mufor, S. Helwig and R. BergersBuried subglacial valleys are important groundwater reservoirs in Northern Germany. However, relative little is known about their exact locations, their sizes and the internal structure of their sediment fills. Due to the presence of a conductive clay layer at about 40m depth within the Bremerhaven-Cuxhaven valley, no information about the structure below this layer could be obtained by conventional ground TEM, SkyTEM and helicopter electromagnetic measurements. To overcome this problem, the TEM method in the central loop configuration was used with a large transmitter size of 400m by 400m and a high current of 12.7A for early times and 20A for late time measurements. Five soundings could be realized and the data were interpreted by means of OCCAM and Marquardt type 1D inversions. The inversion results were compared with SkyTEM, HEM and seismic sections of the buried valley. A conductive clay layer was found between 30 and 83m within the valley and the bottom of the valley was determined to be at a depth of 285 to 309m
-
-
-
Frequency or Time Domain HEM Systems for Defining Spatial Processes of Salinisation?
Authors T.J. Munday, D. Sattel, A. Fitzpatrick and J. ReidAirborne EM methods have considerable potential for providing detailed spatial information on the distribution of salinity in soils and groundwater, across the floodplains of the Lower Murray River in southern Australia. This potential is examined along with the relative merits of high resolution airborne electromagnetic technologies, specifically the RESOLVE frequency domain helicopter EM (FDHEM) and the SkyTEM time domain helicopter EM (TDHEM) systems. For a coincident area, these two data sets were inverted for conductivity and depth using a smooth model Occams inversion, a conductivity depth transform (CDI) obtained using EMFlow and a Laterally Constrained Inversion (LCI) technique for defining variations in near surface conductivity and sediment salt load . Results from the two systems are comparable, both indicating the presence of an extensive flushed zone adjacent to the Murray River, and identifying finer scale variations between losing and gaining groundwater on the floodplains adjacent to the Murray River. Both systems effectively map the near surface conductive Blanchetown Clay in the adjacent highlands and the high saline groundwater system at depth
-
-
-
3D Inversion of Time Domain Data with Integrated Application to Mineral Exploration
Authors D.W. Oldenburg, E. Haber, S. Napier and R. ShekhtmanWe present a 3D time domain inversion algorithm. The forward problem is solved using finite volume methods in the spatial domain and an implicit method (Backward Euler) in the time domain. A modified Gauss-Newton strategy is employed to solve the inverse problem. The practical modifications include the use of a Quasi-Newton method to generate a preconditioner for the perturbed system, and implementing an iterative Tikhonov approach in the solution to the inverse problem. In addition, we show how the size of the inverse problem can be reduced through a corrective source procedure. We invert UTEM data at San Nicolas and compare the results to previously obtained images from gravity, magnetics, and IP and with geologic information from drill holes. A consistent interpretation for the location of a massive sulfide is achieved.
-
-
-
An Approach to Calibrate EMI Multifrequency Sensor
Authors E.V. Balkov and Y.A. MansteinNowadays electromagnetic induction (EMI) methods are common approach to solving a wide range of ecological, geotechnical, archaeological and agricultural shallow depth problems. For the determination of apparent and true values of ground resistivity the device calibration should be performed. Usually calibration is a single experiment above a known conductive half-space or its simulation by metal coil. But such a way is insufficient for a multifrequency sensor calibration which action can not be explained in the terms of low frequency approximation. The paper explains the new calibration approach. It uses a minimization procedure for oversaturated system of experimental data to make possible finding and correcting quite a large amount of device parameters. Its values are needed to be precisely known for the quantitative data processing. The practical application for the existing portable multifrequency sensor NEMFIS is shown as well.
-