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Showing papers in "Exploration Geophysics in 2010"


Journal ArticleDOI
TL;DR: In this article, a 3D inversion of entire airborne electromagnetic (AEM) surveys is proposed, based on the 3D integral equation method for computing data and sensitivities, and uses the re-weighted regularised conjugate gradient method for minimising the objective functional.
Abstract: It is often argued that 3D inversion of entire airborne electromagnetic (AEM) surveys is impractical, and that 1D methods provide the only viable option for quantitative interpretation. However, real geological formations are 3D by nature and 3D inversion is required to produce accurate images of the subsurface. To that end, we show that it is practical to invert entire AEM surveys to 3D conductivity models with hundreds of thousands if not millions of elements. The key to solving a 3D AEM inversion problem is the application of a moving footprint approach. We have exploited the fact that the area of the footprint of an AEM system is significantly smaller than the area of an AEM survey, and developed a robust 3D inversion method that uses a moving footprint. Our implementation is based on the 3D integral equation method for computing data and sensitivities, and uses the re-weighted regularised conjugate gradient method for minimising the objective functional. We demonstrate our methodology with the 3D inv...

196 citations


Journal ArticleDOI
TL;DR: In this paper, an accurate interpretation was obtained using small station spacing and accurate geophysical data processing, facilitated by combining the modelling of the data with the geological and topographic information for explored caves.
Abstract: The coastal area Marina di Capilungo located ~50km south-west of Lecce (Italy) is one of the sites at greatest geological risk in the Salento peninsula In the past few decades, Marina di Capilungo has been affected by a series of subsidenceevents,whichhaveledinsomecasestothepartialcollapse ofbuildingsandroadsurfacesTheseeventshadboth social repercussions, causing alarm and emergency situations, and economic ones in terms of the funds for restoration With the aim of mapping the subsurface karstic features, and so to assess the dimensions of the phenomena in order to preventand/orlimitthegroundsubsidenceevents,integratedgeophysicalsurveyswereundertakeninanareaof~70000m 2 at Marina di Capilungo Large volume voids such as karstic cavities are excellent targets for microgravity surveys The absent mass of the void creates a quantifiable disturbance in the earth's gravitational field, with the magnitude of the disturbance directlyproportionaltothevolumeofthevoidSmallershallowvoidscanbedetectedusingground-penetratingradar(GPR) Microgravimetric and GPR geophysical methods were therefore used An accurate interpretation was obtained using small station spacing and accurate geophysical data processing The interpretation was facilitated by combining the modelling of the data with the geological and topographic information for explored caves The GPR method can complement the microgravimetrictechniqueindeterminingcavitydepthsandinverifyingthepresenceofoff-linefeaturesandnumerousareas of small cavities, which may bedifficultto beresolved withonly microgravimetric data However, themicrogravimetric can complement GPR in delineating with accuracy the shallow cavities in a wide area where GPR measurements are difficult Furthermore, microgravity surveysin anurban environmentrequire effective and accurate consideration of the effects given by infrastructures, such as buildings, as well as those given by topography, near a gravity station The acquired negative anomaly in the residual Bouguer anomalies field suggested the presence of possible void features GPR and modelling data were used to estimate the depth and shape of the anomalous source

42 citations


Journal ArticleDOI
TL;DR: In this article, the Tilt-Depth method was used to estimate the depth to the top of magnetic basement using the best available magnetic anomaly grids covering the continental USA and Australia.
Abstract: We compute the depth to the top of magnetic basement using the Tilt-Depth method from the best available magnetic anomaly grids covering the continental USA and Australia. For the USA, the Tilt-Depth estimates were compared with sediment thicknesses based on drilling data and show a correlation of 0.86 between the datasets. If random data were used then the correlation value goes to virtually zero. There is little to no lateral offset of the depth of basinal features although there is a tendency for the Tilt-Depth results to be slightly shallower than the drill depths. We also applied the Tilt-Depth method to a local-scale, relatively high-resolution aeromagnetic survey over the Olympic Peninsula of Washington State. The Tilt-Depth method successfully identified a variety of important tectonic elements known from geological mapping. Of particular interest, the Tilt-Depth method illuminated deep (3 km) contacts within the non-magnetic sedimentary core of the Olympic Mountains, where magnetic anomalies are subdued and low in amplitude. For Australia, the Tilt-Depth estimates also give a good correlation with known areas of shallow basement and sedimentary basins. Our estimates of basement depth are not restricted to regional analysis but work equally well at the micro scale (basin scale) with depth estimates agreeing well with drill hole and seismic data. We focus on the eastern Officer Basin as an example of basin scale studies and find a good level of agreement between previously-derived basin models. However, our study potentially reveals depocentres not previously mapped due to the sparse distribution of well data. This example thus shows the potential additional advantage of the method in geological interpretation. The success of this study suggests that the Tilt-Depth method is useful in estimating the depth to crystalline basement when appropriate quality aeromagnetic anomaly data are used (i.e. line spacing on the order of or less than the expected depth to basement). The method is especially valuable as a reconnaissance tool in regions where drillhole or seismic information are either scarce, lacking, or ambiguous.

34 citations


Journal ArticleDOI
TL;DR: In this paper, two noise reduction techniques were proposed for the removal of sferics noise from airborne transient electromagnetic data using multi-resolution analysis via a stationary wavelet trawler.
Abstract: Two noise reduction techniques are proposed for the removal of sferics noise from airborne transient electromagnetic data. Both techniques use multi-resolution analysis via a stationary wavelet tra...

29 citations


Journal ArticleDOI
TL;DR: In this paper, two additional models, the differential effective medium (DEM) model and the self-consistent (SC) model, have been evaluated for several carbonates samples.
Abstract: Rock physics models play a crucial role in seismic reservoir characterisation studies. The optimal rock physics model for a sandstone reservoir might be significantly different from that of a carbonate reservoir. There are several theories that compare the elastic properties of dry and saturated rocks. These models have mainly been explained by poroelastic theoriesoreffectivemediumtheories.TheGassmann'smodelwhichiscommonlyusedinpetroleumrockphysicsissuitable for rocks with spherical and interconnected pores at low frequencies. These assumptions do not necessarily meet the conditions of carbonate rocks. In this work, two additional models, the differential effective medium (DEM) model and the self-consistent(SC)modelhavebeenexaminedforseveralcarbonatesamples.Ultrasonic30carbonateand5sandstonecore samples from an oilfield in south-west Iran were measured in the laboratory. The results show that the DEM model gives the best compatibility with the dense and low porous carbonate samples. These results are confirmed by well log data from the same area.

29 citations


Journal ArticleDOI
TL;DR: A change in processing strategy to specifically identify and subtract this additive spurious response from a valid earth response has led to the extraction of quantitative AEM data at early delays in the 10 to 20 ?
Abstract: While data in AEM systems such as VTEM is continuously sampled at 5 or 10 microsecond intervals, conventional processing steps in the past have not provided useful data close to transmitter current turnoff. In historic VTEM data for example, delivered data usually starts many tens or even a hundred microseconds or so after turnoff. Experiments in deconvolution of early time data at high altitude identified that the symptoms of "problem" early-time data were that the underlying cause had a linear phase response, resulting in a consistent "system" exponential response in time-domain. This unwanted or spurious response is experimentally determined to be of variable amplitude, and of either sign, usually reducing the data at early delays, but occasionally enhancing the combined response. A change in processing strategy to specifically identify and subtract this additive spurious response from a valid earth response has led to the extraction of quantitative AEM data at early delays in the 10 to 20 ?s range. The process can be applied to historic data. System bandwidth limitations do of course provide a limit to the accurate sampling of the earth response at extremely early delays. More recently, hardware changes have increased the system bandwidth and reduced the need for software corrections to acquired data.

27 citations


Journal ArticleDOI
Jun Ji1
TL;DR: In this article, the IRLS (iteratively reweighted least-squares) method was used as a robust inversion and showed its possible improvement in robustness when dealing with data having many outliers.
Abstract: In spite of some minor drawbacks such as nonuniqueness and higher computational cost, finding the least- absolute (l1 norm) error solution to solve an optimisation problem is mostly known to give a better answer than the classical least-squares (l2 norm) method. This is because the robust property of the median value is affected little by outlier values and the solution of the least l1 norm error corresponds to the solution of minimum median error. Several variants of the l1 norm such as the Huber norm and the Hybrid norm have the same robust properties as the l1 norm. The optimisation methods based on l1 norm obtain their robustness by reducing the influence of outliers significantly, although never ignoring it. Therefore, if the proportion of outliers increases, most of the methods based on l1 norm may begin to be affected by the outliers. In such a case, other types of robust measures such as Tukey's Biweight (Bisquare weight) norm, which excludes outliers in computing the misfit measure, could perform better. This paper describes the application of the Biweight norm using the IRLS (iteratively reweighted least-squares) method as a robust inversion and shows its possible improvement in robustness when dealing with data having many outliers.

22 citations


Journal ArticleDOI
TL;DR: In this article, the authors link the observed distribution of upper-mantle P-wave velocity to the major geological features of the upper crust, which here comprise terranes of the Delamerian and Lachlan orogens.
Abstract: Acquisition of teleseismic data in south-western New South Wales during 2007 formed the latest stage of a rolling deployment of seismometers over south-eastern Australia, and allowed a revised tomographic model to be constructed for the lithospheric mantle under Victoria and southern NSW. Our aim here is to link the observed distribution of upper-mantle P-wave velocity to the major geological features of the upper crust, which here comprise terranes of the Delamerian and Lachlan orogens. We have extended the definition of the boundaries of these terranes under cover by the use of the tilt-filter of total magnetic intensity, which provides an image with detailed resolution for sources at depths down to ~5 km. We proceed to infer the distribution of deeper sources in the middle and lower crust by two approaches to the use of potential-field images: we exploit the relationship between wavenumber and source depth, through the application of a 20-km low-pass filter to the total magnetic intensity grid; and we ...

22 citations


Journal ArticleDOI
TL;DR: In this paper, three geophysical methods were applied to produce 3D images of total root volume in the soil in an urban environment, including ground-penetrating radar, electrical resistivity tomography, and seismic refraction tomography.
Abstract: How does the tree root system develop in the subsoil? This question is important to the development and urban planning disciplines, especially when the trees occur near building foundations and underground utilities. This interest is based on the potential for buildings to suffer subsidence or structural damage from nearby trees. The inspection of both the extension of the tree roots and the degree of decay in wood are still undertaken using classical single-point and destructive methods. However, as pointed out by several authors, geophysical methods provide an alternative method of studying root architecture in a non-invasive fashion. In this paper, three geophysical methods were applied to produce 3D images of total root volume in the soil in an urban environment. The three geophysical methods used were ground-penetrating radar, electrical-resistivity tomography, and seismic refraction tomography. Each of the geophysical methods alone is able to isolate root system but cannot resolve the ring structures.

22 citations


Journal ArticleDOI
TL;DR: In this paper, the authors proposed a workflow that involves first attenuating short-period water-layer related multiples (WLRMs) and then suppressing other long-period free surface multiples using conventional SRME.
Abstract: Multiples due to shallow water are observed in seismic data acquired in various places such as the Gippsland Basin of Australia. These short period multiple reflections often pose problems to the interpretation of geological structures. They are not easily handled by conventional surface-related multiple elimination (SRME) methods because the recorded primary waterbottom reflection, which is required by SRME, is often indistinct in shallow water situations due to the near offset gap. Hence, predictive deconvolution in the x-t or ?-p domain is frequently used for attenuating shallow water multiples. However, besides multiples, deconvolution also attenuates primary events that have a periodicity which is close to that of the water-layer. In this paper, we present a workflow that involves first attenuating short-period water-layer related multiples (WLRMs) ? a process that we term shallow water demultiple (SWD); and then suppressing other longperiod free surface multiples using conventional SRME. SWD is a wavefield-consistent method that first makes use of WLRMs in the data to reconstruct the missing water-bottom primary reflection and then uses the reflection for predicting shallow WLRMs. It is data driven and takes into account the spatial varying nature of subsurface structures. Since the WLRM model predicted by SWD has similar amplitude and phase as the input data, very short matching filters, which are not possible if deconvolution is used, can be utilised in the adaptive subtraction process. We demonstrate, through real-data examples, that our workflow provides an optimal multiple attenuation solution in shallow water environment in comparison with conventional methods such as ?-p deconvolution or SRME alone.

20 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the ability of simulated annealing (SA) to optimise the global optimal parameters for the CRS stack method by the conventional three-step search method, especially for complex structures and lowfold data.
Abstract: The common reflection surface (CRS) stack is an alternative method of producing a zero-offset stacked section with a higher signal-to-noise ratio (SNR) than the conventional normal moveout (NMO)/dip moveout (DMO) stack method. Since, however, it is difficult to determine global optimal parameters for the CRS stack method by the conventional three-step search method, especially for complex structures and low-fold data, we investigate the ability of simulated annealing (SA) to optimise our estimation of these parameters. We show a detailed but practical procedure for the application of SA to the CRS stack method. We applied the CRS stack method with SA to numerically modelled seismic reflection data, and to multichannel marine seismic data over complicated geological structures around the Median Tectonic Line (MTL) in Japan. We used the results of the conventional three-step search algorithm as the initial model for the SA search and showed that with this approach SA can estimate CRS parameters accurately within a reasonable number of calculations. The CRS stack method with this approach provided a clearer seismic profile with a higher SNR than either a conventional NMO stack method or a conventional CRS stack method.

Journal ArticleDOI
TL;DR: In this paper, a method of extracting ridge and valley based on maxima and minima of a balanced plan curvature dataset is suggested and compared with other ridge detection algorithms and is applied to aeromagnetic and gravity datasets from South Africa.
Abstract: The identification of lineaments in potential field data is an important step in many interpretation projects. A method of extracting ridges and valleys based on maxima and minima of a balanced plan curvature dataset is suggested here. The method is compared with other ridge detection algorithms and is applied to aeromagnetic and gravity datasets from South Africa. Source code in Matlab format is available from the author on request.

Journal ArticleDOI
TL;DR: In this article, a versatile discrete conductor model is used to generate the maximum signal-to-noise ratio along an airborne electromagnetic (AEM) profile by varying the position of the conductor below and to the side of the airborne traverse, a sensitivity crosssection can be generated that shows the volume of material that influences the AEM response.
Abstract: A versatile discrete conductor model is used to generate the maximum signal-to-noise ratio along an airborne electromagnetic (AEM) profile. By varying the position of the conductor below and to the side of the airborne traverse, a sensitivity cross-section can be generated that shows the volume of material that influences the AEM response. This type of section accounts for both the coupling of the transmitter with the model and the coupling of the induced current flow with the receiver. Some previous definitions of ‘volumes of influence’ sometimes called ‘footprints’ do not take into account the coupling of the primary field to the target and the secondary field to the receiver. The versatile discrete conductor model can also account for target strike (variable orientation of the current flow) by considering only specific components or orientations of the primary field at the conductor. For a vertical dipole transmitter, the vertical or z-component receiver is generally better for detecting targets at greater depth and the lateral detection range is maximum for the transverse or y component. The in-line or x component is best for sensing conductors where the currents are constrained to flow in a vertical plane perpendicular to the flight direction of the AEM system. The sensitivity cross-sections can also be used for survey design: for example, in order to ensure effective exploration down to 200 m the HeliGEOTEM system must fly with a flight line spacing of 210 m, whereas the more powerful MEGATEM system can achieve equivalent depth penetration with a 300 m line spacing. The discrete conductor model could also be used to estimate the ‘volume of influence’ in ‘moving footprint’ 3D inversion schemes.

Journal ArticleDOI
TL;DR: In this paper, three canonical parameters, consisting of the arrival time, attenuation, and dispersion time, were extracted from the first and second peaks of themeasured radar signals.
Abstract: Apulseradarsystemhasbeendevelopedrecentlytodetectdormantundergroundtunnelsthataredeeplylocated at depths of hundreds of metres. To check the ability of the radar system to detect an obliquely oriented tunnel, five different boreholepairsinthetunneltestsitewerechosensothatthehorizontallines-of-sightcutthetunnelaxisobliquely,in15 � steps. Thepulseradarsignaturesweremeasuredoveradepthrangeof20maroundthecentreoftheair-filledtunnel.Threecanonical parameters, consisting of the arrival time, attenuation, and dispersion time were extracted from the first and second peaks of themeasuredradarsignatures.Usingthoseparameters,theradarsystemcandetectobliquelyorientedtunnelsatvariousangles up to 45 � from the transmitter-receiver line of sight.

Journal ArticleDOI
Yi Xie1, Kefeng Xin1, James Sun1, Carl Notfors1, Ajoy Biswal2, MK Balasubramaniam2 
TL;DR: In this article, an efficient prestack beam Q migration approach was developed to compensate for the frequency dependent dissipation effects in the migration process, which can be used for amplitude preserving processing that is important for AVO analysis when transmission anomalies are present.
Abstract: Spatial variations in the transmission properties of the overburden cause seismic amplitude attenuation, wavelet phase distortion and seismic resolution reduction on deeper horizons. This poses problems for the seismic interpretation, tying of migration images with well-log data and AVO analysis. We developed an efficient prestack beam Q migration approach to compensate for the frequency dependent dissipation effects in the migration process. A 3D tomographic amplitude inversion approach may be used for the estimation of absorption model. Examples show that the method can mitigate these frequency dependent dissipation effects caused by transmission anomalies and should be considered as one of the processes for amplitude preserving processing that is important for AVO analysis when transmission anomalies are present.

Journal ArticleDOI
TL;DR: Geophysical monitoring of producing gas reservoirs will thus play an important role in two ways; enhancing the gas recovery factor of these projects by improving the reservoir model and understanding geologic flow complexity, and monitoring any required CO2 injection to ensure it is being safely stored in the subsurface for the longterm.
Abstract: WA is poised to embark on several major new energy developments. These include $250+ Billion AUD investment in new gas reserves that will begin production in the Carnarvon, Browse and Bonaparte basins. Developing these new gas reserves will require handling many Millions of tons per year in CO2 released as a natural byproduct of the LNG process. To avoid venting this natural CO2 into the atmosphere, which may be bad for both the environment and business, the CO2 will have to be disposed of safely. One of the best available options is to accelerate nature's course by re-injecting and storing CO2 into deep rock formations, termed "geo-sequestration". Geophysical monitoring of producing gas reservoirs will thus play an important role in two ways; (1) enhancing the gas recovery factor of these projects by improving the reservoir model and understanding geologic flow complexity, and (2) monitoring any required CO2 injection to ensure it is being safely stored in the subsurface for the longterm. In addition to petroleum applications, there is a strong interest in developing clean-coal initiatives by capturing the CO2 generated at coal-fired power plants and injecting it into the subsurface. Geophysical techniques will therefore also play a key role in monitoring and verification strategies for clean-coal CO2 sequestration projects.

Journal ArticleDOI
TL;DR: In this paper, the capability of airborne electromagnetics (AEM) to give a significant contribution to the understanding of the hydrogeology within, below and at the margin of the Venice Lagoon, Italy is investigated.
Abstract: A comprehensive investigation of the exchange between surficial waters and groundwater in transitional environments, and hence the mixing of waters characterized by a different salt concentration, is an issue of paramount importance considering the ecological, cultural, and socio-economic relevance of transitional environments. Acquiring information, which can improve the process understanding, is often logistically challenging, and generally expensive and slow in these areas. This applies both for punctual, invasive borehole measurements as well as for ground-based non invasive geophysical surveys. Here we investigate the capability of airborne electromagnetics (AEM) to give a significant contribution to the understanding of the hydrogeology within, below and at the margin of the Venice Lagoon, Italy. The quasi-3D modelling of the SkyTEM data by the spatially constrained inversion (SCI) methodology allows to accurately distinguish several hydrogeological features, both underneath the lagoon and the farmland beside it. Hydrogeological features resolved are, for example, the extent of the saltwater intrusion in coastal aquifers and the transition between the upper salt saturated and the underlying fresher sediments below the lagoon bottom, and areas of probable submarine groundwater discharge. While the AEM data show a high degree of coherence with available ancillary information, both from wells and from other geophysical techniques such as high resolution off-shore seismic, they improve greatly the understanding of the hydrogeology in the Venice lagoon, both at large scale and in detail. The research highlights the great potential of AEM to improve significantly the hydrogeological characterization of subsurface processes in lagoons, wetlands, and deltas worldwide.

Journal ArticleDOI
TL;DR: In this article, the results of test surveys over the Caber VMS deposit are illustrated with the results from different VTEM systems for the same geometry target with changing conductance, as well as resistivity-depth transforms of the real data.
Abstract: The helicopter borne time-domain VTEM system has been in constant development since 2002 through complex and multicomponent technical improvements in an effort to satisfy exploration and mining industry requirements. Progress in the geoelectrical informational level of data obtained with different VTEM systems is illustrated with the results of test surveys over the Caber VMS deposit. Forward plate modeling with different systems for the Caber deposit geometry target with changing conductance, as well as resistivity-depth transforms of the real data, both serve to illustrate significant broadening of the conductance aperture and increasing depth of investigation over time. Most of all the increasing sensitivity of the VTEM system is highlighted.

Journal ArticleDOI
TL;DR: In this paper, a helicopter-borne AFMAG system that measures the magnetic field response in the frequency range 25-600 Hz of naturally occurring currents in the subsurface is analyzed by forward modeling and inverting synthetic ZTEM data using a 2D algorithm for a range of conductivity scenarios.
Abstract: ZTEM is a helicopter-borne AFMAG system that measures the magnetic-field response in the frequency range 25-600 Hz of naturally occurring currents in the subsurface. The resolution of this system is analyzed by forward modeling and inverting synthetic ZTEM data using a 2D algorithm for a range of conductivity scenarios. ZTEM data acquired at the Forrestania test site are compared with overlapping VTEM data. Conductivitydepth sections derived from both data sets show broad agreement, but indicate better spatial resolution for the VTEM data. The response due to bedrock conductor IR2 is strong for the VTEM system and subtle on the ZTEM profiles, which appear to be dominated by responses to larger, elongated structures. Products derived from the ZTEM data, including apparent conductivity, phase and Karous-Hjelt filtered grids appear to map geologic structure, complementing the information gathered from the VTEM data.

Journal ArticleDOI
TL;DR: In this article, the authors developed a reservoir modelling workflow to reproduce the heterogeneous distribution of effective permeability that impacts on the performance of SAGD (Steam Assisted Gravity Drainage), the in-situ bitumen recovery technique in the Athabasca Oil Sands.
Abstract: This study was conducted to develop a reservoir modelling workflow to reproduce the heterogeneous distribution of effective permeability that impacts on the performance of SAGD (Steam Assisted Gravity Drainage), the in-situ bitumen recovery technique in the Athabasca Oil Sands. Lithologic facies distribution is the main cause of the heterogeneity in bitumen reservoirs in the study area. The target formation consists of sand with mudstone facies in a fluvial-to-estuary channel system, where the mudstone interrupts fluid flow and reduces effective permeability. In this study, the lithologic facies is classified into three classes having different characteristics of effective permeability, depending on the shapes of mudstones. The reservoir modelling workflow of this study consists of two main modules; facies modelling and permeability modelling. The facies modelling provides an identification of the three lithologic facies, using a stochastic approach, which mainly control the effective permeability. The permeability modelling populates mudstone volume fraction first, then transforms it into effective permeability. A series of flow simulations applied to mini-models of the lithologic facies obtains the transformation functions of the mudstone volume fraction into the effective permeability. Seismic data contribute to the facies modelling via providing prior probability of facies, which is incorporated in the facies models by geostatistical techniques. In particular, this study employs a probabilistic neural network utilising multiple seismic attributes in facies prediction that improves the prior probability of facies. The result of using the improved prior probability in facies modelling is compared to the conventional method using a single seismic attribute to demonstrate the improvement in the facies discrimination. Using P-wave velocity in combination with density in the multiple seismic attributes is the essence of the improved facies discrimination. This paper also discusses sand matrix porosity that makes P-wave velocity differ between the different facies in the study area, where the sand matrix porosity is uniquely evaluated using log-derived porosity, P-wave velocity and photographically-predicted mudstone volume.

Journal ArticleDOI
TL;DR: In this paper, the Laplace-transformed wavefield is used to recover the initial velocity model of a damped wavefield over a range of damping constants, which can be used for either migration or for subsequent frequency-domain inversion.
Abstract: Wecomparedtheabilityoffullwaveforminversiontorecoverbackgroundvelocitymodelsfromdatacontaining nolow-frequencyinformationusingthefrequencyandLaplacedomains.Low-frequencyinformationiscrucialforrecovering backgroundvelocitywhenusingfrequency-domainwaveforminversions.However,thedearthoflow-frequencyinformation in fielddatamakesfrequency-domaininversionimpracticalwithoutaccuratestartingvelocitymodels.Instead,byperforming waveform inversion in theLaplace domain, onecan recovera smoothvelocity modelthat canbeused for either migration or for subsequentfrequency-domain inversionas anaccurate initial velocitymodel.The Laplace-transformed wavefieldcanbe thought of as the zero-frequency component of a damped wavefield over a range of damping constants. In this paper, we compare results obtained from both frequency- and Laplace-domain inversions and confirm that the Laplace-domain inversioncanbeusedtorecoverbackgroundvelocityfromrealdatawithoutlow-frequencyinformation.Wealsodemonstrate thattheLaplace-domaininversioncanprovidethefrequency-domaininversionwithsmoothinitialvelocitymodelsforbetter inversion results.

Journal ArticleDOI
TL;DR: In this article, the authors developed an automatic velocity analysis algorithm by using bootstrapped differential semblance (BDS) and Monte Carlo inversion, which provides a higher velocity resolution than conventional semblance, as a coherency estimator.
Abstract: The goal of automatic velocity analysis is to extract accurate velocity from voluminous seismic data with efficiency. In this study, we developed an efficient automatic velocity analysis algorithm by using bootstrapped differential semblance (BDS) and Monte Carlo inversion. To estimate more accurate results from automatic velocity analysis, the algorithm we have developed uses BDS, which provides a higher velocity resolution than conventional semblance, as a coherency estimator. In addition, our proposed automatic velocity analysis module is performed with a conditional initial velocity determination step that leads to enhanced efficiency in running time of the module. A new optional root mean square (RMS) velocity constraint, which prevents picking false peaks, is used. The developed automatic velocity analysis module was tested on a synthetic dataset and a marine field dataset from the East Sea, Korea. The stacked sections made using velocity results from our algorithm showed coherent events and improved the quality of the normal moveout- correction result. Moreover, since our algorithm finds interval velocity (vint) first with interval velocity constraints and then calculates a RMS velocity function from the interval velocity, we can estimate geologically reasonable interval velocities. Boundaries of interval velocities also match well with reflection events in the common midpoint stacked sections.

Journal ArticleDOI
TL;DR: In this paper, the effects and characteristics of seismic wave propagation under different settings involving changes in basalt properties, its thickness, lateral extent, relative position to the shot position and various forms of inhomogeneity were investigated.
Abstract: Seismic reflection surveying is one of the most widely used and effective techniques for coal seam structure delineation and risk mitigation for underground longwall mining. However, the ability of the method can be compromised by the presence of volcanic cover. This problem arises within parts of the Bowen and Sydney Basins of Australia and seismic surveying can be unsuccessful. As a consequence, such areas are less attractive for coal mining. Techniques to improve the success of seismic surveying over basalt flows are needed. In this paper, we use elastic wave-equation-based forward modelling techniques to investigate the effects and characteristics of seismic wave propagation under different settings involving changes in basalt properties, its thickness, lateral extent, relative position to the shot position and various forms of inhomogeneity. The modelling results suggests that: 1) basalts with high impedance contrasts and multiple flows generate strong multiples and weak reflectors; 2) thin basalts have less effect than thick basalts; 3) partial basalt cover has less effect than full basalt cover; 4) low frequency seismic waves (especially at large offsets) have better penetration through the basalt than high frequency waves; and 5) the deeper the coal seams are below basalts of limited extent, the less influence the basalts will have on the wave propagation. In addition to providing insights into the issues that arise when seismic surveying under basalts, these observations suggest that careful management of seismic noise and the acquisition of long-offset seismic data with low-frequency geophones have the potential to improve the seismic results.

Journal ArticleDOI
TL;DR: In this article, a semi-automated two-stage equivalent source approach was proposed to remove cultural noise and image subtle geological anomalies from high-resolution aeromagnetic data.
Abstract: High-resolution aeromagnetic surveys are commonly used to locate subtle anomalies that are important in mineral and oil exploration. However, such anomalies, especially in highly populated areas, are often masked by undesirable magnetic signals from near surface man-made objects – known as ‘cultural noise’ – making post processing and interpretation of the aeromagnetic data difficult. Magnetic data need to be cleaned of this cultural noise before applying advanced processing and interpretation methods. Conventional algorithms for cultural noise removal tend to identify and remove noise signals, either manually or using non-linear filters. These methods are often combined with Fast Fourier Transform filters to smooth the result. These algorithms usually introduce artificial anomalies, have difficulty interpolating across edited sections and rarely yield clean data. For these reasons, we have developed a semi-automated two stage equivalent source approach to remove cultural noise and image subtle geological anomalies. A theoretical example that combines a magnetic anomaly due to a dyke with three cultural noise sources is used to test the effectiveness of the proposed method. Comparison of the equivalent source and conventional results shows that the equivalent source method more closely recovers the original magnetic data. We then demonstrate the practical utility of the two stage equivalent source approach using a high-resolution aeromagnetic dataset from Harberton Bridge, Ireland.

Journal ArticleDOI
TL;DR: In this paper, the authors propose a robust method for the depth of investigation (DOI) calculation of 1D EM geophysical models, which is based on the actual model output from the inversion and includes the full system response.
Abstract: All geologists or geophysicists want to know the depth of investigation (DOI) for their final survey models. For diffusive methods, such as groundbased or airborne EM, there is no specific depth below which there is no information on the resistivity structure, but the question is to what depth the model is most reliable. We present a new robust concept for the calculation of DOI that is valid for any 1D EM geophysical model. The method is based on the actual model output from the inversion and includes the full system response, contrary to assuming e.g. planar waves over a homogeneous halfspace. Equally important, the data noise and the number of data points is integrated in our calculation. Our methodology is based on a recalculated sensitivity (Jacobian) matrix of the final model and it can thus be used on any model type for which a sensitivity matrix can be calculated. Contrary to other sensitivity matrix methods we define a global and absolute threshold value contrary to defining a relative, say 5%, sensitivity limit. The threshold limit will apply to all 1D inverted data and will thus produce comparable numbers of DOI.

Journal ArticleDOI
TL;DR: In this article, the shear wave velocity of the core and rock-fill zone of a dam is determined by using the surface wave method on the crest and sloping surface of the dam.
Abstract: In current design practice, the shear wave velocity (Vs) of the core and rock-fill zone of a dam, one of the characteristics essential for seismic response design, is seldom determined by field tests. This is because the borehole seismic method is often restricted in application, due to stabilisation activities and concern for the security of the dam structure, and surface wave methods are limited by unfavourable in-situ site conditions. Consequently, seismic response design for a dam may be performed using Vs values that are assumed, or empirically determined. To estimate Vs for the core and rock-fill zone, and to find a reliable method for measuring Vs, seismic surface wave methods have been applied on the crest and sloping surface of the existing ‘M’ dam. Numerical analysis was also performed to verify the applicability of the surface wave method to a rock-fill dam. Through this numerical analysis and comparison with other test results, the applicability of the surface wave method to rock-fill dams was verified.

Journal ArticleDOI
TL;DR: In this paper, the authors used ground-penetrating radar (GPR) data to determine subglacial topography and internal features of the Fourcade Glacier on King George Island in Antarctica.
Abstract: To determine subglacial topography and internal features of the Fourcade Glacier on King George Island in Antarctica, helicopter-borne and ground-towed ground-penetrating radar (GPR) data were recorded along four profiles in November 2006. Signature deconvolution, f-k migration velocity analysis, and finite-difference depth migration applied to the mixed-phase, single-channel, ground-towed data, were effective in increasing vertical resolution, obtaining the velocity function, and yielding clear depth images, respectively. For the helicopter-borne GPR, migration velocities were obtained as root-mean-squared velocities in a two-layer model of air and ice. The radar sections show rugged subglacial topography, englacial sliding surfaces, and localised scattering noise. The maximum depth to the basement is over 79 m in the subglacial valley adjacent to the south-eastern slope of the divide ridge between Fourcade and Moczydlowski Glaciers. In the ground-towed profile, we interpret a complicated conduit above possible basal water and other isolated cavities, which are a few metres wide. Near the terminus, the GPR profiles image sliding surfaces, fractures, and faults that will contribute to the tidewater calving mechanism forming icebergs in Potter Cove.

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TL;DR: In this article, the authors present a balanced overview of the anatomy of the Lusi mud volcanic system with particular emphasis on these critical uncertainties and their influence on the disaster, including the source of the erupted water (shales versus deep carbonates), the fluid flow pathways (purely fractures versus mixed fracture and wellbore), and disputes over the subsurface geology (nature of deep carbonate, lithology of lithological unit between shales and carbonates).
Abstract: Early in the morning of the 29th of May 2006, hot mud started erupting from the ground in the densely populated Porong District of Sidoarjo, East Java. With initial flow rates of ~5000 cubic meters per day, the mud quickly inundated neighbouring villages. After almost four years, the ?Lusi? eruption has expelled over 73 million cubic meters of mud at an average rate of approximately 64000 cubic meters per day and at maximum rates of 170000m3/day. The mud flow has now covered over 700 hectares of land to depths of over 25 meters, engulfing a dozen villages and displacing approximately 40000 people. In addition to the inundated areas, other areas are also at risk from subsidence and distant eruptions of gas. However, efforts to stem the mud flow or monitor its evolution are hampered by an overall lack of knowledge and consensus on the subsurface anatomy of the Lusi mud volcanic system. In particular, the largest and most significant uncertainties are the source of the erupted water (shales versus deep carbonates), the fluid flow pathways (purely fractures versus mixed fracture and wellbore) and disputes over the subsurface geology (nature of deep carbonates, lithology of lithological unit between shales and carbonates). This study will present the first balanced overview of the anatomy of the Lusi mud volcanic system with particular emphasis on these critical uncertainties and their influence on the disaster.

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TL;DR: In this article, the Fourier series expansion is combined with a 2D goal-oriented high-order self-adaptive hp finite-element refinement strategy, where h denotes the element size and p the polynomial order of approximation within each element.
Abstract: Resistivity logging instruments are designed to measure the electrical resistivity of a formation, and this can be directly interpreted to provide a water-saturation profile. However, resistivity logs are sensitive to borehole and shoulder- bed effects, which often result in misinterpretation of the results. These effects are emphasised more in the presence of tool eccentricity. For precise interpretation of short- and long-normal logging measurements in the presence of tool eccentricity, we simulate and analyse eccentricity effects by combining the use of a Fourier series expansion in a new system of coordinates with a 2D goal-oriented high-order self-adaptive hp finite-element refinement strategy, where h denotes the element size and p the polynomial order of approximation within each element. The algorithm automatically performs local mesh refinement to construct an optimal grid for the problem under consideration. In addition, the proper combination of h and p refinements produces highly accurate simulations even in the presence of high electrical resistivity contrasts. Numerical results demonstrate that our algorithm provides highly accurate and reliable simulation results. Eccentricity effects are more noticeable when the borehole is large or resistive, or when the formation is highly conductive.

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TL;DR: In this article, the authors used a combination of very low frequency electromagnetic (VLF-EM), Stratagem EH4 and controlled source audiofrequency magnetotelluric (CSAMT) measurements.
Abstract: Geophysical and geological studies attempting to search and explore new mineralisation resources were carried out in the Longtoushan area, Inner Mongolia, northern China. The Longtoushan area is located within a large alteration zone, which is covered by a variety of altered lava, bioclast limestone and Quaternary sediments. In this case study, ground geophysical surveys played an important role in defining the mineralised structures beneath Quaternary sediment cover. The aim of this study was to investigate the subsurface mineralisation in the area using a combination of very low frequency electromagnetic (VLF-EM), Stratagem EH4 and controlled source audiofrequency magnetotelluric (CSAMT) measurements. These successful surveys in the area revealed that the VLF, Stratagem EH4 and CSAMT methods were effective in detecting the unseen mineralised system and results from the different methods confirmed each other. VLF measurements at 2875 stations along 35 lines identified the principal mineralised shear system as two sub-parallel linear conductive belts, tracing the principal mineralised system for a further 650 m of strike length beneath cover loess, in a NW–SE-trending direction. The subsequent Stratagem EH4 soundings along four parallel traverses perpendicular to the mineralised trend indicated that the principal mineralised shear structure, extending for more than 600 m in the dip direction, was almost vertically dipping above the 200 m level but changed to the SSW direction at a steep angle at depth. A CSAMT survey on two lines confirmed that the ore-bearing mineralised zone was a conductive belt and contained a low resistivity anomaly of less than 500 Ωm.