Depth Conversion Research Articles

3D seismic analysis of normal fault growth and interaction within a gravitational detachment delta system in the Ceduna Sub-Basin, Great Australian Bight

We use three-dimensional (3D) seismic reflection data to determine the structural evolution of thin-skinned listric fault growth, at the extensional top of a gravitationally driven delta system, in the central Ceduna Sub-Basin. We present analysis of a strike and dip-linked extensional fault system, which is decoupled at the base of a marine mud interval of late Albian age. The fault system is oriented NW-SE with strike-linkage of fault segments and dip-linkage through the Santonian interval which connects a Cenomanian-Santonian period of kilometre scale fault growth and post-Santonian normal faulting. Understanding the growth of listric faulting requires quantifying heave and throw, which involves simplistic depth conversion of fault plane time measurements to establish a fault plane model to translate throw into fault plane displacement. Our analysis constrains fault growth into six evolutionary stages: [1] early Cenomanian nucleation and isolated radial propagation of fault segments; [2] substantial segment linkage established by the latest Cenomanian; [3] late Santonian cessation of a majority of fault growth; [4] heavy erosion at continental breakup of Australia and Antarctica (c. 83 Ma); [5] early Campanian independent nucleation of the post-Santonian fault system; and [6] fault assemblages fully linked by the Cenozoic, with continued accumulation of displacement. The structural evolution of this fault system is compatible with the 'isolated fault model'. In particular, we emphasise the importance of dip-linkage in this fault system, which controls the present day geometry of the fault array.

Fringe projection profilometry for panoramic 3D reconstruction

In this paper, we introduce a panoramic profilometric system to reconstruct inner cylindrical environments. The system projects circular fringes and uses a temporal phase unwrapping technique. The recovered phase map is used to reconstruct objects placed on the inner cylindrical surface. We derived a phase to depth conversion formula for this system. The use of fringe projection allows dense reconstructions. The panoramic system is composed by a digital projector, two parabolic mirrors and a CCD camera. All these components share a common axis with a reference cylinder. This paper presents results for distinct objects.

집적영상에서 요소영상의 좌표변환을 이용한 정치실영상 구현

In this paper, we propose a depth conversion method for orthoscopic real image reconstruction in integral imaging. Pseudoscopic image has been regarded a problem in conventional integral imaging. the depth of reconstructed image is depending on a coordinate of an elemental image. The conversion from pseudoscopic to orthoscopic may be possible by analysing the geometrical relation between pickup and reconstruction system of elemental image. The feasibility of the proposed method has been confirmed through preliminary experiments as well as ray optical analysis.

BSRs, estimated heat flow, hydrate-related gas volume and their implications for methane seepage and gas hydrate in the Dongsha region, northern South China Sea

To investigate the relationship among methane seepage, subsurface gas hydrate, and heat flow and estimate hydrate-related gas amount in the Dongsha region, northeastern South China Sea, in this study we first identified Bottom Simulating Reflector (BSR) using multi-channel seismic data collected in 2004. Subsequently, the thickness of gas hydrate stability zones (GHSZ) was calculated from the observed BSR using time–depth conversion, ranging from 60 to 300 m in seawater depths of 600–1200 m. Additionally, a seismic feature of double BSRs is locally observed in this region, which is possibly caused by either the presence of mixed gas source (thermogenic and biogenic) or migration of a pre-existing base of GHSZ. Geothermal gradient, heat flow and hydrate-related gas volume are calculated based on the thickness of GHSZ. The estimated geothermal gradient and heat flow varies from 20 to 100 °C/km with an average of 48 °C/km, and from 20 to 120 mW/m2 with an average of 54 mW/m2, respectively. Both estimated geothermal gradient and heat flow are generally consistent with those from measurements. High geothermal gradient (up to 60 °C/km) and heat flow anomaly (up to 80 mW/m2) are present in one methane seepage site, where BSR is clear and reliable. High heat flow could result in thin GHSZ in the continental slope. In this case, gas hydrate could form in the shallow sediments, making dissolved gas from hydrate and/or free deep-seated gas easy to migrate upwards to seafloor through faults, and thus resulting in the occurrence of methane venting. The hydrate-related gas volume is estimated as ∼6 × 1011 m3 over an area of ∼400 km2 with possible occurrence of BSR. This finding would shed lights on understanding potential amount of subsurface methane in the Dongsha region and place useful constraints for modeling GHSZ from measured heat flow data, especially in regions where no BSRs are present.

A source data-driven method for 3D geological modeling in coal mines

Building accurate 3D geological models relies heavily on large numbers of data. Nowadays, there are plenty of data including geological, 3D seismic, and roadway data in coal mines, and some specialized softwares allow modeling complex geological bodies using these data. However, there are still two main problems: the first is how to use such information in one modeling system because of the characteristics of the heterogeneity between seismic and other geological data and the second is how to use the latest data because all the data in coal mines change frequently with the production and organization activities. A solution is presented in this paper based on a source data-driven method for 3D geological modeling using geological data, roadway, and seismic interpretation results in a complete process system. The processing flows include: data integration, time–depth conversion, and surfacebased modeling. The method not only effectively takes advantage of various data but also will not lose the correcting information from geologic cognition. Meanwhile, the application in Pingshuo shows that the method is in accord with the dynamic production situation of the coal mine.

Orthoscopic real image reconstruction in integral imaging by rotating an elemental image based on the reference point of object space.

We propose a new approach for depth conversion of three-dimensional (3D) reconstruction from pseudoscopic to orthoscopic real images in resolution priority integral imaging. In integral imaging, depth of field is recorded in an elemental image array. In the proposed method, the depth information is converted by a 180° rotation of each elemental image in an elemental image array based on a reference point of conversion, which is caused by a reference point of object space. Orthoscopic real images can be reconstructed in 3D space by using the depth conversion of an elemental image array. The feasibility of the proposed method has been confirmed through preliminary experiments as well as ray optical analysis.

Receiver functions and crustal structure of the northwestern Andean region, Colombia

AbstractWe used the receiver function technique to deduce crustal thickness beneath the northwestern Andean system, using data from the permanent seismic network of Colombia, combined with some of the IRIS and CTBTO stations in Colombia and Ecuador. The estimation of crustal thickness was made using the primaryPtosconversion and crustal reverberations. The bulk crustalVP/VSratio was constrained using a crustal thickness versusVP/VSstacking method, in addition to estimations using a time to depth conversion technique based on results of a modified Wadati diagram analysis. We observed a wide range of crustal thicknesses, including values around 17 km beneath the Malpelo Island on the Pacific Ocean, 20 to 30 km at the coastal Pacific and Caribbean plains of Colombia, 25 to 40 km beneath the eastern plains and foothills, 35 km beneath the Western Cordillera, 45 km at the Magdalena River intermountain valley, 52 to 58 km under the northern Central Cordillera, and reaching almost 60 km beneath some of the volcanoes of the Southern Cordilleran system of Colombia; crustal thickness can be slightly greater than 60 km beneath the plateau of the Eastern Cordillera. The values ofVP/VSare particularly high for some of the stations on the volcanic centers, reaching values above 1.79, probably related to the addition of mafic materials to the lower crust, and in the plateau of the Eastern Cordillera near Bogota, where we speculate about the possibility of crustal seismic anisotropy associated with shear zones.

Geological analysis and updating velocity model by restoration of a cross section in Zagros fold–thrust belt in the southwest of Iran

The Zagros region (SW Iran) contains dominant parts of the remaining Iranian natural resources. Covering Abadan Plain, Dezful Embayment, and Izeh zone, this paper studies a structural cross section constructed across the Zagros belt using 2D and 3D seismic data, well data, surface and subsurface geological maps, satellite images, and field reconnaissance. Restoration assists us for validation and modification of the constructed cross section, better understanding of geology and stratigraphy of the Zagros basin, and computing an updated velocity model in seismic time to depth conversion. In the area of interest, Hormuz basal decollement and Gachsaran detachment play the most significant role in structural style and deformation of the Zagros. More complication links with interval decollements such as Triassic evaporites, Albian shales, and Eocene marls. Estimation of shortening is complex because of a variety of lithotectonic units and detachment surfaces. Here, we are about to introduce a new approach to update seismic velocity model by means of deduced balanced depth model by restoration and seismic two-way time. This is valuable in the case of thick sedimentary cover and steep dip of faults, where acquired seismic images are ambiguous and unreliable. The seismic velocity of Asmari and Gachsaran formations mainly depends on lateral facies and thickness variations within the overburden sediments and generally increases across the Zagros belt from foreland towards hinterland.

Evaluation of an airborne triple-pulsed 2 μm IPDA lidar for simultaneous and independent atmospheric water vapor and carbon dioxide measurements

Water vapor and carbon dioxide are the most dominant greenhouse gases directly contributing to the Earth's radiation budget and global warming. A performance evaluation of an airborne triple-pulsed integrated path differential absorption (IPDA) lidar system for simultaneous and independent monitoring of atmospheric water vapor and carbon dioxide column amounts is presented. This system leverages a state-of-the-art Ho:Tm:YLF triple-pulse laser transmitter operating at 2.05 μm wavelength. The transmitter provides wavelength tuning and locking capabilities for each pulse. The IPDA lidar system leverages a low risk and technologically mature receiver system based on InGaAs pin detectors. Measurement methodology and wavelength setting are discussed. The IPDA lidar return signals and error budget are analyzed for airborne operation on-board the NASA B-200. Results indicate that the IPDA lidar system is capable of measuring water vapor and carbon dioxide differential optical depth with 0.5% and 0.2% accuracy, respectively, from an altitude of 8 km to the surface and with 10 s averaging. Provided availability of meteorological data, in terms of temperature, pressure, and relative humidity vertical profiles, the differential optical depth conversion into weighted-average column dry-air volume-mixing ratio is also presented.

An integrated approach to geostatistical depth conversion and gross rock volume estimation

Static, dynamic, and geomechanical models are typically built in depth because this is required when planning and drilling wells. These models are usually driven by seismic data via structural interpretation and seismic attributes, whose position is fixed in the time domain. To incorporate seismically derived data, or interpretations, time objects must be transformed to the depth domain using the process of depth conversion. The depth-conversion model should dynamically evolve during project life, from exploration to maturation. As more data become available and the geologic understanding improves, an evolving model can accommodate requirements for specific deliverables for various disciplines, e.g., gross rock volume (GRV) estimates, drilling uncertainty windows, and structural base case models. To meet the above specifications, we used a fully geostatistical method that enabled the incorporation of a large variety of data types and their associated uncertainties. We performed depth modeling simultaneously for all layers in the model, in which all layers were coupled. This imposed a dependency on depth surfaces, such that a depth surface was influenced by surfaces above and below, moving away from a classical top-down approach of depth conversion. A scriptable and mechanized method allowed for fast scenario modeling in which various geologic and geophysical concepts or model parameterization choices can be tested to provide discrete models and avoid anchoring at a single preferred model and associated probabilistic range. Depth results from multiple scenarios can be combined to provide common confidence intervals. The proposed method including scenario modeling has been successfully implemented in the UK Culzean HPHT gas field. The model successfully delivered input to the various disciplines in the asset team, providing depth structures for static model build, GRV estimates for each reservoir, and localized drilling uncertainty estimates.

Hydrocarbon reservoir volume estimation using 3-D seismic and well log data over an X-field, Niger Delta Nigeria

This study delineates and maps the hydrocarbon-bearing reservoir HD2000 from surface seismic sections and well logs within the depth interval of 5,700 ft (1,737 m) and 6,200 ft (1,890 m). The objective is to establish the geometry, reservoir distribution, delineate hydrocarbon-bearing reservoirs from surface seismic sections and well logs. In this process, a 3-D structural interpretation and estimation of the volume of hydrocarbon in place of the reservoirs was carried out. Well-to-seismic tie revealed that hydrocarbon-bearing reservoirs were associated with direct hydrocarbon indicators (bright spots and dim spots) on the seismic sections. Two horizons were studied (HD2 and HD2_version2) and several faults mapped for the purpose of carrying out 3-D subsurface structural interpretation. This was used in generating the time structure maps. From the maps, it was observed that the principal structure responsible for hydrocarbon entrapment in the field was the anticlinal structure at the center of the field which tied to the crest of the rollover structure seen on the seismic sections. Check shots from the control well were used to create a velocity model from which the time to depth conversion was made. Horizon slice taken shows the reservoir spans a thickness of 400 ft. Direct hydrocarbon indicators were used to map the reservoir boundary. They were seen on the reflection amplitude maps as high amplitude zones (bright spots) and low amplitude zones (dim spots). Reservoir area extent estimated by square grid template method revealed that reservoir HD2000 had an area estimate of 5.29 km2. The results show the effectiveness of the estimation techniques in the lateral prediction of reservoir properties, discriminating litho-fluid and determining the porosity, saturation, net-to-gross ratio, and moreover the reserve volume. Hydrocarbon saturation varied between 0.64 and 0.65, while effective porosity varied between 0.31 and 0.32. Estimation of the volume of hydrocarbon in place revealed that the delineated reservoir HD2000 contained an estimate of 776,545,418.22 barrels (123,460,855.4 cm3) of hydrocarbon which shows great potential of considerable size.

Receiver function images of the mantle transition zone beneath NE China: New constraints on intraplate volcanism, deep subduction and their potential link

In order to better understand the deep subduction geometry of the Pacific plate and genesis of intraplate volcanism in northeast China (NE China), we computed a total of 45,505 receiver functions from 788 teleseismic events recorded by 255 stations (NECESSArray temporal and permanent stations) in NE China. We used a common-conversion-point stacking (CCP) method to generate a 3D reflectivity volume beneath the study area. To position the P-to-S conversions to the correct depths, we employed 3D crustal and mantle models as references to make time to depth conversion. The 3D reflectivity volume was generated in an area between 115°–135°E and 40°–49°N, in the depth range of 300 to 800 km. We found significant topographic relief on the 660-km discontinuity across the study area. In particular, in a westward Pacific plate subduction section between 40°N and ∼45.5°N, the 660-km discontinuity is depressed by as much as ∼30–40 km along the western extension of the deep seismicity. The depression is elongated along the strike of the deep seismicity and is confined to a 200–300 km region in the E–W direction of subduction. To the west of this depression the 660-km discontinuity is uplifted by 5–10 km in a rectangular area of ∼100 km by 200 km centered at about 125°E and 43°N. In the north, the 660-km discontinuity is moderately depressed (∼20 km) in a broad area that extends further west. The high and low regions in the 660-km topographic map correlate, respectively, with low- and high-velocity anomalies in the P- and S-wave tomographic velocity images at the same depth. Our results suggest that slab stagnation might not be occurring in the southern part of the NE China, where the Changbaishan volcanic complex is located, thus the magmatism is unlikely caused by dehydration of the flat-lying Pacific slab in the transition zone. The low velocity mantle upwelling arising from a gap of stagnant slabs is a likely source that feeds the volcanic complex in NE China.

North Sea case study: Heavy oil reservoir characterization from integrated analysis of Towed Streamer EM and dual-sensor seismic data

Integrated analysis of geophysical data can provide valuable information on reservoir properties, on the basis of which exploration, appraisal, and development decisions can be made. Hence, we have introduced a quantitative interpretation workflow that integrates dual-sensor seismic and Towed Streamer controlled-source electromagnetic (CSEM) data. The workflow was designed to facilitate a reliable extraction of the complementary information from the two datasets. The seismic contribution starts with a depth-converted sparse horizon model to initialize the EM inversion, but it is not placed rigidly. This makes good sense when taking into account the uncertainties in seismic data, in the time to depth conversion, and more importantly, the fact that a reservoir can be hydrocarbon-charged to an unknown degree corresponding to the spill-point or less. We show how this approach enables a robust and reliable workflow for integrating EM and 3D seismic data with data examples acquired in an area with the complex geology of the Bressay, Bentley and Kraken (BBK) fields in the North Sea. The three heavy oil reservoirs are injectites, located in close proximity to other high resistivity settings, such as the shallow gas in the overburden, regional Balder Tuff and granite intrusions, resulting in challenging imaging issues.

Normal fault growth and segment linkage in a gravitationally detached delta system: evidence from 3D seismic reflection data from the Ceduna Sub-basin, Great Australian Bight

The authors used three-dimensional (3D) seismic reflection data from the central Ceduna Sub-Basin, Australia, to establish the structural evolution of a linked normal fault assemblage at the extensional top of a gravitationally driven delta system. The fault assemblage presented is decoupled at the base of a marine mud from the late Albian age. Strike-linkage has created a northwest–southeast oriented assemblage of normal fault segments and dip-linkage through Santonian strata, which connects a post-Santonian normal fault system to a Cenomanian-Santonian listric fault system. Cenomanian-Santonian fault growth is on the kilometre scale and builds an underlying structural grain, defining the geometry of the post-Santonian fault system. A fault plane dip-angle model has been created and established through simplistic depth conversion. This converts throw into fault plane dip-slip displacement, incorporating increasing heave of a listric fault and decreasing in dip-angle with depth. The analysis constrains fault growth into six evolutionary stages: early Cenomanian nucleation and radial growth of isolated fault segments; linkage of fault segments by the latest Cenomanian; latest Santonian Cessation of fault growth; erosion and heavy incision during the continental break-up of Australia and Antarctica (c. 83 Ma); vertically independent nucleation of the post-Santonian fault segments with rapid length establishment before significant displacement accumulation; and, continued displacement into the Cenozoic. The structural evolution of this fault system is compatible with the isolated fault model and segmented coherent fault model, indicating that these fault growth models do not need to be mutually exclusive to the growth of normal fault assemblages.

Subtle oil fields along the Western Flank of the Cooper/Eromanga petroleum system

Oil production from Cooper/Eromanga started in 1978, peaked in the 1980s and began a steady decline. Oil production from the Western Flank commenced in 2002 and has steadily increased. In the year until July 2014, a total of 8.6 million BBL of oil was produced from 16 active fields along the Western Flank, bringing the cumulative total to 24 million BBL. Western Flank oil has underpinned a ten-fold growth in market capitalisation in four listed Australian companies: Beach Energy, Drillsearch Ltd, Senex Energy and Cooper Energy. Two sandstone plays dominate the Western Flank petroleum geology: the Namur Sandstone low-relief structural play and the mid-Birkhead stratigraphic play. The use of 3D seismic has improved the definition of both plays, increased exploration success and optimised field appraisal and development drilling. Success rates have improved despite most Namur structural closures being close to the resolution margin for depth conversions (less than 8 m). Seismic attribute mapping is being refined in the more difficult search for mid-Birkhead stratigraphic traps with recent exploration discoveries indicating improved success. Reservoir properties in the Namur are excellent with multi-Darcy permeability, unlimited aquifer strength, low gas/oil ratio (GOR) and low residual oil saturation. This combination leads to an oil recovery factor greater than 75%. Initial free-flow production rates commonly exceed 6,000 BBL per a day. The mid-Birkhead reservoir is also of high quality but the lack of a strong aquifer drive reduces primary recovery. New and re-processed 3D seismic and water-flood projects are expected to drive further discoveries, reserve and production growth.

Technology Focus: Formation Evaluation (August 2014)

Technology Focus Any oil and gas deal, whether it is acquisition, divestment, greenfield, brownfield, or out of left field, usually involves a data room, where rich seams of proprietary information can be mined for a limited period. However, this physical data room, as opposed to a virtual version where data are downloadable, is the bane of due diligence. Some may recall caffeine-fueled assignments to paw through boxes brimming with confidential documents that allegedly held the financial secrets of a potential acquisition. The process is expensive; can be a logistical nightmare of travel arrangements, visas, and inoculations; and is always stressful because of time pressure. Data rooms are migraine inducing. The priority of the petrophysicist on a data-room team is to access digital data for all wells—for example, logs, wireline formation pressures, and deviation surveys for depth conversion. Often neglected are the log headers themselves, which provide data on tool setup, bottomhole temperature, mud properties, and the casing program and present the logging engineer’s comments. Gathering data is less straightforward if they are in an unfamiliar language or are vintage (noncombinable tools with unfocused measurements requiring more runs). These factors require translation, tool response modeling, and increased depth matching, adding time, cost, and potential for error. Petrophysicists audit the hydrocarbons in place by reviewing previous studies to confirm the all-important trinity of porosity, water saturation, and net pay claimed by the seller. The appropriateness of the saturation-height function in the geomodel is checked for the assumed hydrocarbon and rock type, or merely to ascertain that one was used at all. Proposed fluid contacts must be verified, no mean feat when faced with structure maps showing only oil-down-to (ODT) and water-up-to (WUT) depths, with the contact possibly being anywhere in between. Some companies assume that the fluid contact is halfway between ODT and WUT, which is a completely arbitrary approach. In these digital days, almost everything needed to carry out petrophysical due diligence can be downloaded for off-site analysis. However, there are places where it is illegal to take data out of the country, so physical-data-room attendance is compulsory. In these instances, as in the past, the petrophysicist’s briefcase might hold color pencils, a ruler, tracing paper, chart books, and maybe a planimeter, a strange device for measuring areas within mapped contours. Yet painful memories suggest that the most important piece of equipment one should take along to a data room is probably a bottle of aspirin. JPT

Constraining snowmelt in a temperature-index model using simulated snow densities

Current snowmelt parameterisation schemes are largely untested in warmer maritime snowfields, where physical snow properties can differ substantially from the more common colder snow environments. Physical properties such as snow density influence the thermal properties of snow layers and are likely to be important for snowmelt rates. Existing methods for incorporating physical snow properties into temperature-index models (TIMs) require frequent snow density observations. These observations are often unavailable in less monitored snow environments. In this study, previous techniques for end-of-season snow density estimation (Bormann et al., 2013) were enhanced and used as a basis for generating daily snow density data from climate inputs. When evaluated against 2970 observations, the snow density model outperforms a regionalised density-time curve reducing biases from −0.027 g cm−3 to −0.004 g cm−3 (7%). The simulated daily densities were used at 13 sites in the warmer maritime snowfields of Australia to parameterise snowmelt estimation. With absolute snow water equivalent (SWE) errors between 100 and 136 mm, the snow model performance was generally lower in the study region than that reported for colder snow environments, which may be attributed to high annual variability. Model performance was strongly dependent on both calibration and the adjustment for precipitation undercatch errors, which influenced model calibration parameters by 150–200%. Comparison of the density-based snowmelt algorithm against a typical temperature-index model revealed only minor differences between the two snowmelt schemes for estimation of SWE. However, when the model was evaluated against snow depths, the new scheme reduced errors by up to 50%, largely due to improved SWE to depth conversions. While this study demonstrates the use of simulated snow density in snowmelt parameterisation, the snow density model may also be of broad interest for snow depth to SWE conversion. Overall, the study responds to recent calls for broader testing of TIMs across different snow environments, improves existing snow modelling in Australia and proposes a new method for introducing physically-based constraints on snowmelt rates in data-poor regions.

Architecture of the western margin of the North Adriatic foreland:the Schio-Vicenza fault system

In the Italian Southern Alps, the Lombardic and Venetian chains and related foreland are separated by the Lessini-Berici-Euganei foreland block, which is unaffected by the Neogene-Quaternary shortenings. However, the Veneto-Friuli alluvial plain to the east of this foreland block has been affected by a poly-phase evolution since the Mesozoic era and represents the foreland of three surrounding chains. The area was affected by several flexural cycles related to the diachronous build-up of the External Dinarides to the east (Late Cretaceous-Late Eocene), the Eastern Southern Alps to the north (Late Oligocene-Quaternary), and the Northern Apennines to the southwest (Middle Miocene-Quaternary). The last two chains are currently active, although at different rates. The western margin of the foreland is marked by the Schio-Vicenza fault, which divides the deformed foreland (Veneto-Friuli alluvial plain) from the undeformed foreland (Lessini and Berici Mountains and Euganei Hills). The aim of this work is to unravel the architecture and evolution of this boundary using 2D seismic sections and deep wells acquired by ENI for hydrocarbon exploration. Approximately 1,000 km of seismic lines were interpreted and 10 wells were used to calibrate the seismic interpretation. Seven seismic sections that are sub-orthogonal to the main faults were selected to obtain geological cross-sections through a depth conversion process. The collected data display a complex buried fault system (Schio-Vicenza fault system) that extends with a NW-SE trend from the foot of the Prealps to the Po Delta. In the cross sections, the movement of the fault shows a vertical component with down-throw of its eastern side (hanging wall block). Some faults display a Mesozoic extensional displacement in accordance with the Mesozoic basin and swell architecture of the area. In addition, the Pliocene throw increases from the southeast to the northwest.Therefore, the Schio-Vicenza fault system can be interpreted as an inherited Mesozoic structure that reactivated during the Neogene shortening of the area. In particular, this fault system appears to have been mainly active during the Pliocene-Quaternary flexural cycle that is related to the Northern Apennines subduction, which is when the fault system accommodated a scissor movement between the Veneto-Friuli foreland and the Lessini-Berici-Euganei block.

Velocity analysis from common offset GPR data inversion: theory and application to synthetic and real data

We implemented a procedure to estimate the electromagnetic (EM) velocity using common offset ground penetrating radar (GPR) data. The technique is based on the inversion of reflection amplitudes to compute the series of reflection coefficients used to estimate the velocity in each interpreted layer. The proposed method recursively calculates the incident angles at any interface, taking into account the offset between antennas, and needs as input, in addition to the picked amplitudes values, a reference amplitude for each analysed GPR trace and a velocity value for the first (shallowest) layer. The latter two parameters can be estimated directly from the available data or can be better constrained by further dedicated GPR acquisitions or by additional direct measurements. We critically evaluated the performances for both synthetic and real data acquired with different antenna frequencies and we demonstrated that the new method can be applied in several real situations. Despite the necessary approximations and simplifying hypotheses, the velocity values calculated for each layer are consistent with direct information and with cross-validations obtained considering profiles acquired using different antennas and various path directions. Tests of the method on synthetic and real data sets show that the errors in the calculated velocity fields are quite low and comparable with more demanding velocity analysis techniques. The obtained EM velocity field is crucial in many processing steps, such as, for example, true amplitude recovery, depth conversion and imaging, and provide essential information to characterize the subsurface materials.

ProbGRV: a workflow for geostatistical depth conversion and probabilistic assessment of gross rock volume

Understanding and estimating the probability of geological success (Pg) and original hydrocarbon-in-place (OHIP) are an integral part of the geoscientist’s role from exploration though to development. This paper presents a workflow developed at Santos for the systematic assessment of two key elements in this process: the probability of structural closure (Pcl); and, the estimation of the uncertainty of gross rock volume (GRV). There are two distinct parts to the workflow. Firstly, Kriging and sequential Gaussian simulation (SGS) are used in map-based, seismic time-to-depth conversion to generate multiple equi-probable depth maps (in the order of 1,000+ realisations). Numerous depth conversion methods are considered where appropriate. Secondly, every depth map realisation is validated for structural closure around a target location (e.g. a proposed or existing well). Primary outputs from the workflow include estimates of Pcl, and probability distributions of GRV and pool area for valid maps, which may then be used directly in estimating geological risk and OHIP. Secondary outputs from the workflow include basic statistics associated with depth conversion uncertainty, such as target depth prognosis and the distribution of the depth to lowest closing contour. The ultimate objective is to provide the interpreter with a greater appreciation of the range of possible outcomes given the uncertainty in structural geometry. This workflow, named probGRV, has been designed and developed within Santos using commercially available geophysical software and in-house proprietary software. During the past few years, the workflow has been implemented on key assets at various stages of the prospect/field life cycle and in both pre- and post-drilling campaigns for audit purposes. Case studies from the Cook Field in the onshore Eromanga Basin and the Finucane South Field in the offshore Carnarvon Basin are shown to illustrate the methods and results.