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Exploration geophysics
Blackwell Scientific Publications
Exploration geophysics

Blackwell Scientific Publications

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0812-3985

Exploration geophysics/Journal Exploration geophysicsEISCIISTP
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    Magnetotelluric imaging of an iron-oxide copper gold (IOCG) deposit under thick cover

    Ben KayGraham HeinsonGoran BorenYing Liu...
    645-656页
    查看更多>>摘要：The presence of thick (>100 m) and electrically conductive (< 10Ω.m) ground cover is a majorimpediment in mineral exploration of deep resources. Iron-oxide copper gold (IOCG) systems inAustralia are often associated with a pronounced potential field anomaly, depending on the oxidationstate of iron, but the gravity and magnetic field signatures have little vertical resolutionunless constrained by drill hole petrophysics. Additionally, IOCG systems have deep magmaticsources, with their conductivity anomalies extending at least to the lithospheric mantle. The VulcanIOCG prospect lies about 30 km northeast of the Olympic Dam IOCG mine and is defined by asignificant gravity anomaly associated with brecciated haematite beneath 850mof sedimentarycover sequences. To image the physical properties and structural geometry of the Vulcan IOCGprospect, a 100-site broadband MT and passive seismic array was deployed in a 1 kmgrid over a9 by 9 km area. Three-dimensional inversion of MT responses resolve structure in three distinctdomains. Firstly, broad limestone-quartzite-shale stratigraphy (1–30 Ω.m) in the 850 m cover isdelineated in resistivity and corresponds well with changes in shear-wave velocity. Secondly, theregion of brecciated haematite below the cover sequences is shown to have lower resistivity (<60 Ω.m) than surrounding country rock (> 100 Ω.m). Thirdly, a more electrically conductive (<30 Ω.m) vertical zone that extends > 5 km is imaged a few kilometers to the northeast of theVulcan haematite breccia, and appears to be linked by a region of low shear-wave velocity in thedepth range 1–2 km. Two-dimensional inversion of more regional MT responses along a 200-km line passing through the Vulcan prospect suggest this vertical region of low resistivity linksto the lower-crust with anomalous resistivity of < 60 Ω.m in a similar way as imaged beneaththe Olympic Dam mine. It is suggested that this conductive region is associated with graphiteprecipitated from magmatically derived CO_2-rich fluids cooling in a reducing environment.
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    Natural source-field induced polarisation exploration of an iron-oxide copper-gold (IOCG) deposit under thick cover

    Ying LiuGraham HeinsonBen KayGoran Boren...
    657-666页
    查看更多>>摘要：Induced polarisation (IP) is a common geophysical method of exploration for disseminated sulphides.However, in areas with deep (>200 m) and conductive (10 Ωm or less) cover, thetechnique is less successful as it requires a significant transmitter source and large-offset dipoles.An alternative approach is to use natural-variations in Earth’s external magnetic field as the polarisingsource of the signal, known as natural-field IP. This paper presents a study of extracting IPinformation from electrical dipole observations during a broadband magnetotelluric (MT) programwith a 9 km by 9 km grid of 100 sites above the Vulcan IOCG deposit beneath 750 m ofcover, ∼40 km north of the Olympic Dam IOCG mine in South Australia. Inter-site transfer functionsbetween 95 sites and five reference sites at the southeast and southwest corners of the gridwere computed to determine a phase shift between horizontal electric fields in the bandwidth of1–100 s period. Phase shifts of up to – 5 degrees were centred on a region of brecciated hematitewhere drill holes intersected pyrite, and an inferred fault-zone from passive seismics that marksthe boundary between upper crust that is resistive (>100 .m) with high magnetic susceptibilityto the south of the fault, and a region of conductive crust (<10 .m) which is low magneticsusceptibility. Our study suggests that the natural-field IP method can identify regions of polarizableminerals beneath deep cover where artificial power sources cannot be feasibly deployed.Such surveys are cheaper, safer, and easier to deploy as only receivers are required, and 3D coveragecan be obtained as the source-field is of much larger dimension than the survey array. Inaddition, the natural-field IP signals are observed as part of theMTprogram so that both electricalresistivity and polarisation parameters can be determined.
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    Ambient noise tomography of an iron-oxide copper-gold (IOCG) deposit under thick cover

    Gerrit OlivierGraham HeinsonBen KayGoran Boren...
    667-677页
    查看更多>>摘要：As most outcropping and shallow mineral deposits have been found, new technology is imperativeto finding the hidden critical mineral deposits required for the renewable energy transition.One such seismic technique, called ambient noise tomography (ANT), has shown promise inrecent years as a low cost and low environmental impact passive method of three-dimensional(3D) imaging of shear-wave velocity structure of the Earth. Over the last twenty years the methodhas been well-established in academia to image crustal and regional scale geological featuresbut has seldom been used at the mineral exploration deposit-scale. In this paper we show theapplication of seismic ANT at an IOCG deposit in South Australia under more than 750mof sedimentarycover. A 100-site survey in a 10 by 10 grid with site spacing of 1 km, using 3-componentnodal seismometers with a natural-frequency of 5 Hz, was conducted over a two-week period.Data were inverted to generate a 3D velocity model to a depth of 2 km. When compared to drillholesin the survey area, the tomographic model delineates cover sequence lithologies and thedepth of crystalline basement. A velocity anomaly in the basement has characteristics of a potentialIOCG deposit and is aligned with a gravity anomaly due to brecciated haematite. The results ofthe paper indicate that ANT is a useful tool for deep cover mineral exploration that can potentiallyexpedite the discovery of new deposits.
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    Fuzzy c-mean (FCM) integration of geophysical data from an iron-oxide copper gold (IOCG) deposit under thick cover

    Simon CarterGraham HeinsonBen KayGoran Boren...
    678-689页
    查看更多>>摘要：Geophysical methods depend on a range of physical and chemical mechanisms, and eachmethod has different sensitivities and resolution of Earth parameters, and over different scalelengths. Additionally, such geophysical methods will also have their own statistical characteristics.Thus, it is a significant challenge to combine different methods through a single inversionframework. Rather than attempting to find an optimal and single model for different geophysicalresponses, an alternative approach is to use FCM clustering to identify clusters of parametersfrom two or more different geophysical data sets or models that have similar statistical properties.In this paper, we apply the FCM approach to integrate data and model sets for an array of100 broadband magnetotelluric (MT) and 100 passive seismic receivers spaced 1 kmapart on a 10by 10 grid above the Vulcan IOCG prospect in the Olympic Cu–Au Province, southern Australia.The challenge for exploration of the Vulcan prospect is that it lies beneath 750mof regolith sedimentarycover, no single geophysical method provides a unique characterization of the depositgeometry, and drilling is very expensive. Fuzzy c-mean cluster analyses are undertaken in 2D forgravity data and shear-wave velocity model data for basement depths beneath cover and in 3Dfor shear-wave velocity and electrical resistivity model data. Fuzzy c-mean clustering is shownto provide a simple and efficient method of integrating different geophysical measurements toproduce a geological framework that can be verified with drilling information.
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    3-D spatial distribution of concealed ore-forming granitoid intrusion and structures determined by the CSAMT survey of Wuxu Sb-Zn-polymetallic ore district, South China

    Yi ZhaoZhan LiangYaotian YinYanping Tang...
    690-701页
    查看更多>>摘要：Wuxu ore district is an important lead-zinc-antimony polymetallic ore field within the Danchimetallogenic belt of South China. Although previous geochemical studies have indicated thatthe deposit type is medium to low-temperature magmatic-hydrothermal, the existing metallogenicmodels remain controversial due to the thick covering layer and lack of exposure of graniticbodies. To address this issue, this study utilised four CSAMT parallel profiles across the Wuxuanticline to perform two-dimensional inversion, yielding high-resolution subsurface resistivitymodels. By comparing the resistivity anomalies along the regional strike shown by the profilesand integrating geochemical and ore deposit structural analyses, a detailed geological interpretationwas conducted. A significant high-resistivity body beneath the core region of the Wuxuanticline is interpreted to be the Yanshanian period granitic intrusion. There are westward- andeastward-dipping high-conductivity belts existing within the western and eastern wings andappear to sandwich the inferred high-resistivity code of the Wuxu anticline, respectively. Theymay represent the deep faults extending underground and are the result of decoupling of lithologicalinterface within the anticline and thereby rock fracturing. The high-conductivity featuremay be the result of filling with high-conductivity saline fluids, and possible hydrothermal alteration.The regional compression stress during the Caledonian period and the early Yanshanianperiod promoted the formation of the anticline and rock fracturing of both wings of the anticline.After that, the regional extension background of the late Yanshanian period made the faultzones reactivated, becoming the site and channels for magmatic intrusion, upward transportingof hydrothermal fluid and alteration, causing lead-zinc-antimony polymetallic ore formation.The exposed ore deposits above may also corroborate this view point. In summary, our resistivitymodel visually depicts the spatial distribution of the granitic intrusion controlling ore formationand major structures such as anticlines, fault zones, and hydrothermal alteration zones in the 3-Dspace, providing new geophysical constraints for solving the controversy over the metallogenicmodel for Wuxu ore district.
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    A rapid imaging approach for transient electromagnetic data based on gradient features

    Tong XiaXinghai ChenGuanqun ZhouMingxin Yue...
    702-715页
    查看更多>>摘要：Data processing techniques in transient electromagnetic (TEM) methods face numerouschallenges, such as the problem of 3-D imaging and the demand for new data processing techniquesarising from the development of small-loop TEM devices. We have developed an imagingalgorithm based on gradient features derived from TEM data to address these challenges. Wefound that normalising the gradient field of TEM responses under appropriate backgroundconditions enables imaging of the target. Numerical experiments conducted under theoreticalmodels have validated the feasibility of this approach and highlighted the significance of backgroundfields.Wefurther designed more complex models and achieved 3-D imaging of syntheticdata within seconds, demonstrating the method’s high efficiency and applicability to highdimensionaldata. Subsequently, we applied the algorithm to challenging small-loop TEM dataand demonstrated its superior performance compared to the inversion method, which presentsa novel approach for processing data from small-loop TEM devices. This study demonstrates thatthe algorithm possesses characteristics of high efficiency and wide applicability, suitable for TEMdata from various dimensions and types of devices. Furthermore, it holds promising potential inassisting the enhancement of the efficiency of the three-dimensional inversion of TEM data.
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    Anisotropy in shale and its impact on AVO modelling and prospect de-risk: a case study of a Central North Sea field

    Mathew Osaretin OgievaOlawale Babatunde Olatinsu
    716-731页
    查看更多>>摘要：The inherent anisotropy in shale is an attribute associated with its clay minerals platelets alignmentand layering and very common with transversely isotropic (TI) medium. This implies thatvelocity is fastest along the horizontal axis and slowest along the vertical axis, which could affectvelocity signature from sonic tools measurements in deviated wells. This may lead to wrongamplitude variation with offset (AVO) signature, inaccurate depth conversions, seismic-to-welltie difficulty etc. Nineteen (19) wells in Forties field andthirteen (13) in Huntington field wereinvestigated and interpreted for the presence of anisotropy effect within the caprock over thereservoir Forties sands of the Sele (shales) Formation. In order to analyse the Thomsen weakanisotropy parameters (ε, γ and δ) per field, wells with closer Sele depth range were considered.The well inclinations (θ), average compressional (α) and shear (β) velocities per well were utilisedfor forward and inverse modelling of Thomsen anisotropy equations. The estimated anisotropy(VTI) parameters and the P-wave (αo) and S-wave velocities (βo) at zero inclinations in Sele Formationfor the studied wells are: αo = 2448±15.9 m/s, β_o = 1003±34.8 m/s, ε = 0.15±0.02,γ = 0.33±0.10, δ = 0.030±0.015 in the Forties field Sele, while αo = 2775±23.6 m/s, βo =1306±47.9 m/s, ε = 0.22±0.02, γ = 0.43±0.14, δ = 0.022±0.018 in the Huntington field. AVOresponses were compared, assuming both isotropic and anisotropic P–P reflectivity models.Result shows an increase in AVO gradient for all anisotropic case in both fields. AVO signatureswere either completely or averagely masked in both fields using the isotropic assumptions. Thereis also an increase in AVO gradient for all anisotropic cases in both fields. Velocity decreases ofabout 1.2% and 3.5% were observed in the Sele Formation for the Forties and Huntington fieldsrespectively for anisotropy. The corrected two-way traveltime (TWT) should in principle allowfor a better seismic-to-well tie estimation. Similarly, using the faster deviated velocity (i.e. uncorrectedvelocity) in any depth conversion would result in under estimation of the actual depthacross the field.
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    The use of high productivity vibroseis techniques for 2D onshore seismic acquisition

    Tim Dean
    732-739页
    查看更多>>摘要：Vibroseis is the most popular source for land seismic acquisition, being used whenever accessis possible due to its repeatability, controllable bandwidth, and low-cost. Despite the increasingpopularity of 3D surveys, 2D surveys continue to be acquired. Over the last 30 years several highproductivityVibroseis techniques have been introduced for 3D surveys, many enabled by thewidespread adoption of nodal acquisition systems, but there is little evidence of their adoptionfor 2D surveys. In this paper I show how the use of a combination of the employment of a nodalacquisition system and a distanced separated acquisition technique increased productivity by61% over flip-flop and 118% over single-vibrator acquisition, with no detrimental effects on dataquality. In the future I anticipate that the employment of such techniques will be routine, particularlygiven that there is potential to increase productivity even further through the employmentof additional vibrators.
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    Self-adaptive numerical dispersion suppressed method for shallow seismic simulation

    Yanli LiuYongbo DiaoGuangpeng LiZhongyun Chen...
    740-751页
    查看更多>>摘要：Near-surface in seismic exploration generally refers to the low deceleration zone and a sectionof stratum below it, which can be described in geophysical language as “low velocity”, “low Q”,and “Free surface”, etc. Due to the presence of low-velocity layers in near-surface, shallow seismicsimulation is plagued by numerical dispersion. Numerical dispersion affects the accuracyof seismic wave simulation, especially severe in shallow areas containing low-velocity layers. Toimprove the simulated quality, denser grids or higher-order difference schemes are used, butboth of which would seriously reduce computational efficiency, especially for frequency-domainnumerical modeling. Differentiation is replaced by difference in wave field simulation, whichwould inevitably result in numerical errors. These numerical errors are manifested as the differencebetween the phase velocity of the discretized wave field and the medium velocity. Thewaves with different wavenumber components have different phase velocities, and the largerthe wavenumber, the more that its phase velocity lags the group velocity. Based on this theoreticalanalysis, a regularization factor is added to the frequency-domain acoustic wave equationto correct the phase velocity of high wavenumber components. According to the Von Neumannstability requirements, a regularization factor that adaptively changes with simulation parametersis derived. Compared to the fixed regularization factor, adaptive regularization factor canbetter match the velocity field and protect the effective wave field to the maximum extentwhile suppressing numerical dispersion. The improved acoustic wave equation can effectivelysuppress numerical dispersion without increasing the number of grids. Different numerical modelsdemonstrate the effectiveness and efficiency of the improved acoustic equation with theadaptive regularization factor for suppressing numerical dispersion.
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    Seismic coloured inversion to explore the hydrocarbon prospectivity of a field in the Upper Assam basin

    Siddharth GariaArnab Kumar PalShreya KatreSatyabrata Nayak...
    752-774页
    查看更多>>摘要：This study demonstrates an integrated approach using a seismic-coloured inversion to derivethe hydrocarbon potential of a field in the Upper Assam basin. The well-log analysis indicatedthe presence of hydrocarbon in the form of gas, oil, and water in sandstones with thin streaks oflimestone and shale layers. The Acoustic Impedance (AI) model was generated from post-stackseismic data using a coloured inversion. Shale volume (Vshale), density, porosity and water saturationare the petrophysical parameters spatially populated using a multilayered feed-forwardneural network on the inversion-based AI model. The estimated range of model-derived parametersvaries as Velocity: 2136–5034 m/s, Vshale: 0–48%, Density: 2.14–2.72 gm/cc, Porosity: 6–29%,Water saturation: 9–28%, and demonstrates a reasonable correlation to well-log data. Seismicattribute for thin-fault likelihood was used to interpret the major faults. From the combinedanalysis based on interpreted faults and modelled parameters, the northwestern part of thestudy area displays a thick hydrocarbon-bearing zone (due to increasing thickness and reservoirquality). This interpretation is based on the consideration that faults are open. Thus, assumingcontinuity in the sequence, this northwestern region can be considered as one of the promisingcandidates for further exploration.
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