Vertical Seismic Profiling Experiment Research Articles

Characterizing shallow fault zones by integrating profile, borehole and array measurements of seismic data and distributed acoustic sensing

AbstractWithin the framework of the Intercontinental Scientific Drilling Programme (ICDP) ‘Drilling the Eger Rift’ project, five boreholes were drilled in the Vogtland (Germany) and West Bohemia (Czech Republic) regions. Three of them will be used to install high‐frequency three‐dimensional (3D) seismic arrays. The pilot 3D array is located 1.5 km south of Landwüst (Vogtland). The borehole, with a depth of 402 m, was equipped with eight geophones and a fibre optic cable behind the casing used for distributed acoustic sensing (DAS) measurements. The borehole is surrounded by a surface array consisting of 12 seismic stations with an aperture of 400 m. During drilling, a highly fractured zone was detected between 90 m and 165 m depth and interpreted as a possible fault zone. To characterize the fault zone, two vertical seismic profiling (VSP) experiments with drop weight sources at the surface were conducted. The aim of the VSP experiments was to estimate a local 3D seismic velocity tomography including the imaging of the steep fault zone. Our 3D tomography indicates P‐wave velocities between 1500 m/s and 3000 m/s at shallow depths (0–20 m) and higher P‐wave velocities of up to 5000 m/s at greater depths. In addition, the results suggest a NW–SE striking low‐velocity zone (LVZ; characterized by = 1500–3000 m/s), which crosses the borehole at a depth of about 90–165 m. This LVZ is inferred to be a shallow non‐tectonic, steep fault zone with a dip angle of about . The depth and width of the fault zone are supported by logging data as electrical conductivity, core recovery and changes in lithology. In this study, we present an example to test and verify 3D tomography and imaging approaches of shallow non‐tectonic fault zones based on active seismic experiments using simple surface drop weights as sources and borehole chains as well as borehole DAS behind casing as sensors, complemented by seismic stand‐alone surface arrays.

Effect of Source Mispositioning on the Repeatability of 4D Vertical Seismic Profiling Acquired with Distributed Acoustic Sensors.

Vertical seismic profiling (VSP) with distributed acoustic sensing (DAS) is an increasingly popular evolving technique for reservoir monitoring. DAS technology enables permanent fibre installations in wells and simultaneous seismic data recording along an entire borehole. Deploying the receivers closer to the reservoir allows for better detectability of smaller signals. A high level of repeatability is essential for the robust time-lapse monitoring of geological reservoirs. One of the prominent factors of repeatability degradation is a shift between source/receiver locations (mispositioning) during baseline and monitor surveys. While the mispositioning effect has been extensively studied for surface 4D seismic, the number of such studies for VSP is quite limited. To study the effects of source mispositioning on time-lapse data repeatability, we performed two VSP experiments at two on-shore sites with vibroseis. The first study was carried out at the Otway International Test Centre during Stage 3 of the Otway project and showed that the effect of source mispositioning on repeatability is negligible in comparison with the effect of temporal variations of the near-surface conditions. To avoid these limitations, we conducted a same-day controlled experiment at the Curtin University site. This second experiment showed that the effect of source mispositioning on repeatability is controlled by the degree of lateral variations of the near-surface conditions. Unlike in marine seismic measurements, lateral variations of near-surface properties can be strong and rapid and can degrade the repeatability for shifts of the source of a few meters. The greater the mispositioning, the higher the chance of such significant variations. When the near-surface conditions are laterally homogeneous, the effect of typical source mispositioning is small, and in all practical monitoring applications its contribution to non-repeatability is negligible.

Quantification of DAS VSP Quality: SNR vs. Log-Based Metrics.

The initial quantification of data quality is an important step in seismic data acquisition design, including the choice of sensing strategy. The signal-to-noise ratio (SNR) often drives the choice of distributed acoustic sensing (DAS) parameters in vertical seismic profiling (VSP). We compare this established approach for data quality assessment with metrics comparing DAS data products to available well logs. First, we create kinematic and dynamic data products derived from original seismic data, such as the interval velocity and amplitude of P-wave arrivals. Next, we quantify the quality of derived data products using well log data by calculating various statistical metrics. Using a large dataset of 220 different VSP experiments with a fixed source location and various DAS acquisition parameters, such as gauge length (GL), conveyance type, and lead-in length, we analyzed the statistical distribution of various metrics. The results indicate the decoupling between seismic-based and log-based metrics as well as between the quality of dynamic and kinematic data-products for the same record. Therefore, we propose using fit-for-purpose metrics to optimize the acquisition cost. In particular, for ray-based tomographic processing, it is sufficient to use traveltime-based metrics. On the other hand, for advanced dynamic analysis, amplitude-based metrics define the quality of final processing products. Hence, it is crucial to use fit-for-purpose metrics to optimize DAS VSP acquisition.

Orthotropic anisotropy analysis and parameter estimation from 3-D vertical seismic profile data

SUMMARY Orthotropic velocity models are commonly considered as to have the lowest complexity (or highest symmetry) that can realistically describe the combined effect of Earth’s velocity heterogeneity in the vertical and in the horizontal (radial) directions. Velocity heterogeneity in the vertical direction is commonly caused by horizontal stratification of subsurface rock layers with different elastic properties or caused intrinsically by microlayered rocks such as shales. In the horizontal direction, azimuthal variation of the Earth’s stress field can initially lead to preferential deformation of pore space in the rocks and eventual development of aligned (vertical) fractures that are commonly regarded as the main cause for seismic azimuthal anisotropy. Building an appropriate orthotropic velocity model that can explain seismic velocity variations in vertical planes and along any azimuthal direction can therefore enhance the quality of seismic data processing and imaging. This, however, requires knowledge of several anisotropy parameters defined in the three symmetry planes of the orthotropic model and also the orientation of these planes. A combination of P- and S-wave slowness and polarization data derived from vertical seismic profile (VSP) measurements has been routinely used to estimate parameters of less complex anisotropic media such as transverse isotropy with vertical axes of symmetry (VTI). The accuracy of anisotropy parameter estimation by these methods depends primarily on the availability of data that are used in the inversion and the size of anisotropy parameters. In this study, we develop a benchmark for the accuracy of orthotropic parameters that are estimated from 3-D VSP measurements using P-wave slowness only method. Through numerical analysis, we model a wide range of fracture-induced orthotropic symmetries by adding vertical fractures to a background VTI media and show that the accuracy of our orthotropic parameter estimation method is dependent on anisotropy in the host media, intensity of fracturing, noise level in slowness data and slowness data coverage in both the dip and azimuthal planes. We then expand our accuracy analysis work to multilayer media by examining the accuracy of P-wave slowness method for orthotropic anisotropy parameter estimation by inverting finite-difference data generated in a pseudo 3-D VSP experiment. We show that, for a typical 3-D VSP experiment, reasonable estimates of orthotropic parameters in the model can only be obtained with slowness data taken from Xmax/Z ≥ 1 and line azimuth interval ϕ ≤ 10° where Xmax is the maximum source to receiver offset and Z is the receiver depth. We finally describe how field VSP measurements can be inverted for orthotropic anisotropy parameters.

Practical concept of traveltime inversion of simulated P-wave vertical seismic profile data in weak to moderate arbitrary anisotropy

We have developed a practical concept of compressional wave (P-wave) traveltime inversion in weakly to moderately anisotropic media of arbitrary symmetry and orientation. The concept provides sufficient freedom to explain and reproduce observed anisotropic seismic signatures to a high degree of accuracy. The key to this concept is the proposed P-wave anisotropy parameterization (A-parameters) that, together with the use of the weak-anisotropy approximation, leads to a significantly simplified theory. Here, as an example, we use a simple and transparent formula relating P-wave traveltimes to 15 P-wave A-parameters describing anisotropy of arbitrary symmetry. The formula is used in the inversion scheme, which does not require any a priori information about anisotropy symmetry and its orientation, and it is applicable to weak and moderate anisotropy. As the first step, we test applicability of the proposed scheme on a blind inversion of synthetic P-wave traveltimes generated in vertical seismic profile experiments in homogeneous models. Three models of varying anisotropy are used: tilted orthorhombic and triclinic models of moderate anisotropy (approximately 10%) and an orthorhombic model of strong anisotropy (>25%) with a horizontal plane of symmetry. In all cases, the inversion yields the complete set of 15 P-wave A-parameters, which make reconstruction of corresponding phase-velocity surfaces possible with high accuracy. The inversion scheme is robust with respect to noise and the source distribution pattern. Its quality depends on the angular illumination of the medium; we determine how the absence of nearly horizontal propagation directions affects inversion accuracy. The results of the inversion are applicable, for example, in migration or as a starting model for inversion methods, such as full-waveform inversion, if a model refinement is desired. A similar procedure could be designed for the inversion of S-wave traveltimes in anisotropic media of arbitrary symmetry.

Sparse 3D Seismic Imaging in the Kylylahti Mine Area, Eastern Finland: Comparison of Time Versus Depth Approach

A 10.5 km2 3D seismic survey was acquired over the Kylylahti mine area (Outokumpu mineral district, eastern Finland) as a part of the COGITO-MIN (COst-effective Geophysical Imaging Techniques for supporting Ongoing MINeral exploration in Europe) project, which aimed at the development of cost-effective geophysical imaging methods for mineral exploration. The cost-effectiveness in our case was related to the fact that an active-source 3D seismic survey was accomplished by using the receiver spread originally designed for a 3D passive survey. The 3D array recorded Vibroseis and dynamite shots from an active-source 2D seismic survey, from a vertical seismic profiling experiment survey, as well as some additional “random” Vibroseis and dynamite shots made to complement the 3D source distribution. The resulting 3D survey was characterized by irregular shooting geometry and relatively large receiver intervals (50 m). Using this dataset, we evaluate the effectiveness of the standard time-imaging approach (post-stack and pre-stack time migration) compared to depth imaging (standard and specialized Kirchhoff pre-stack depth migration, KPreSDM). Standard time-domain processing and imaging failed to convincingly portray the first ~1500 m of the subsurface, which was the primary interest of the survey. With a standard KPreSDM, we managed to obtain a good image of the base of the Kylylahti formation bordering the extent of the mineralization-hosting Outokumpu assemblage rocks, but otherwise the image was very noisy in the shallower section. The specialized KPreSDM approach (i.e., coherency-based Fresnel volume migration) resulted in a much cleaner image of the shallow, steeply dipping events, as well as some additional deeper reflectors, possibly representing repetition of the contact between the Outokumpu assemblage and the surrounding Kalevian metasediments at depth.

Characterization and imaging of a hydrothermally active near-vertical fault zone in crystalline rocks based on hydrophone VSP data

A shallow, hydrothermally active, fault zone embedded in fractured crystalline rocks of the Central Swiss Alps has been drilled and geophysically explored in view of its potential analogies to planned petrothermal reservoirs in the Alpine Foreland. Vertical seismic profiling (VSP) experiments have been performed using a 24-channel borehole hydrophone array with a sensor spacing of 1 m in order to image the fault core and to detect hydraulically open fractures. The dense sensor spacing allows for an accurate sampling of the wavefields. This, in turn, enables their separation through standard seismic processing methodologies, thus allowing for the generation of seismic body wave VSP profiles and a tube wave corridor stack, which maps brittle deformation in the surrounding rock mass.Multiple offset hydrophone VSP data were acquired with a crooked-line survey geometry and processed with 3D methods. Hereby, a laterally changing velocity cube, representative of the vertical structural variations associated with the fault zone, is generated and utilized for Kirchhoff pre-stack-depth-migration (PSDM) imaging. The resulting seismic image delineates various vertical structures, notably the target fault zone. The interpretation of the VSP data is complemented by and corroborated through a suite of geophysical borehole logs, which characterize the fine-scale petrophysical variations in response to brittle deformation in the fault core and the surrounding damage zones.This study exemplifies typical targets of hardrock seismic exploration, where the structures are frequently near-vertical, discontinuous and where acoustic impedance contrasts are commonly associated with deformation. As such, it illustrates the benefits of using borehole seismic methods in such environments and the use of borehole hydrophone arrays for targeting fractures and shear zones.

Underground Vertical Seismic Profiling with Conventional and Fiber-Optic Systems for Exploration in the Kylylahti Polymetallic Mine, Eastern Finland

Seismic reflection methods have been used for the exploration of mineral resources for several decades. However, despite their unmatched spatial resolution and depth penetration, they only have played a minor role in mineral discoveries so far. Instead, mining and exploration companies have traditionally focused more on the use of potential field, electric and electromagnetic methods. In this context, we present a case study of an underground Vertical Seismic Profiling (VSP) experiment, which was designed to image a (semi-)massive sulfide deposit located in the Kylylahti polymetallic mine in eastern Finland. For the measurement, we used a conventional VSP with three-component geophones and a novel fiber-optic Distributed Acoustic Sensing (DAS) system. Both systems were deployed in boreholes located nearby the target sulfide deposit, and used in combination with an active seismic source that was fired from within the underground tunnels. With this setup, we successfully recorded seismic reflections from the deposit and its nearby geological contrasts. The recording systems provided data with a good signal-to-noise ratio and high spatial resolution. In addition to the measurements, we generated a realistic synthetic dataset based on a detailed geological model derived from extensive drilling data and petrophysical laboratory analysis. Specific processing and imaging of the acquired and synthetic datasets yielded high-resolution reflectivity images. Joint analysis of these images and cross-validation with lithological logging data from 135 nearby boreholes led to successful interpretation of key geological contacts including the target sulfide mineralization. In conclusion, our experiment demonstrates the value of in-mine VSP measurements for detailed resource delineation in a complex geological setting. In particular, we emphasize the potential benefit of using fiber-optic DAS systems, which provide reflection data at sufficient quality with less logistical effort and a higher acquisition rate. This amounts to a lower total acquisition cost, which makes DAS a valuable tool for future mineral exploration activities.

Imaging the high-temperature geothermal field at Krafla using vertical seismic profiling

Geophysical exploration and in particular active-source seismic imaging of geothermal fields is important to assess and optimize the exploitation of natural heat sources for energy production and direct use. The first multi-offset (moving-source) vertical seismic profiling (VSP) experiment over the high-temperature geothermal field in Krafla (Iceland) was carried out in spring 2014 with the aim to test whether VSP is a suitable method to map volcanic stratigraphy, fractures, dykes, steam zones and magmatic bodies at this site and for volcanic environments in general. In this study, we present a workflow for processing the sparse Krafla VSP dataset recorded with receivers in either of two boreholes. The analysis involved first-arrival traveltime inversion and seismic reflection processing. The seismic velocity model obtained by traveltime tomography reveals structural information between the two boreholes and can be linked to an existing geological model, showing that the seismic velocities are mainly controlled by lithology. The zero-offset seismic reflection data were processed into two corridor stacks. Walk-away VSP reflection data were migrated with a novel multicomponent Kirchhoff migration algorithm that includes P- and S-wave isolation to obtain separate PP, PS and SS migrated images. The reflections imaged in the corridor stacks can be linked to the main lithological units known from borehole logging information. Migrated images from the walk-away data reveal reflectors below and to the sides of the two boreholes. Considering à priori information, such as hypocenter locations from earthquake seismology studies, the reflectors can be related to changes in lithology, fault zones, dykes and possibly the top of the Krafla magma chamber. We found that VSP is potentially a useful method to image the key lithological boundaries and volcanic stratigraphy in the complex magmatic environment at Krafla, but à priori information proved to be essential to constrain the processing and interpretation of the sparse array dataset.

VSP polarization angles determination: Wysin-1 processing case study

In this paper, we present an analysis of borehole seismic data processing procedures required to obtain high-quality vertical stacks and polarization angles in the case of walkaway VSP (vertical seismic profile) data gathered in challenging conditions. As polarization angles are necessary for more advanced procedures like anisotropy parameters determination, their quality is critical for proper media description. Examined Wysin-1 VSP experiment data indicated that the best results can be obtained when rotation is performed for each shot on data after de-noising and vertical stacking of un-rotated data. Additionally, we proposed a procedure of signal matching that can substantially increase data quality.

Vertical seismic profiling using a daisy‐chained deployment of fibre‐optic cables in four wells simultaneously – Case study at the Ketzin carbon dioxide storage site

ABSTRACTThe geological storage of carbon dioxide is considered as one of the measures to reduce greenhouse gas emissions and to mitigate global warming. Operators of storage sites are required to demonstrate safe containment and stable behaviour of the storage complex that is achieved by geophysical and geochemical monitoring, combined with reservoir simulations. For site characterization, as well as for imaging the carbon dioxide plume in the reservoir complex and detecting potential leakage, surface and surface‐borehole time‐lapse seismic monitoring surveys are the most widespread and established tools. At the Ketzin pilot site for carbon dioxide storage, permanently installed fibre‐optic cables, initially deployed for distributed temperature sensing, were used as seismic receiver arrays, demonstrating their ability to provide high‐resolution images of the storage formation. A vertical seismic profiling experiment was acquired using 23 source point locations and the daisy‐chained deployment of a fibre‐optic cable in four wells as a receiver array. The data were used to generate a 3D vertical seismic profiling cube, complementing the large‐scale 3D surface seismic measurements by a high resolution image of the reservoir close to the injection well. Stacking long vibro‐sweeps at each source location resulted in vertical seismic profiling shot gathers characterized by a signal‐to‐noise ratio similar to gathers acquired using geophones. A detailed data analysis shows strong dependency of data quality on borehole conditions with significantly better signal‐to‐noise ratio in regions with good coupling conditions.

Microseismic reflection imaging of the Central Andean crust

The coda of passive seismic recordings is often rich in arrivals that are coherent across several stations. If reflections can be extracted, then they may be used for seismic reflection subsurface imaging. With the objective to image the upper crust of the North Chilean Precordillera (Central Andes; approximate location 21°S 69°W), we developed a workflow to process passive seismic data into subsurface reflection images. We analysed the waveform recordings of several hundred microseismic events using signal processing and imaging techniques adapted from active (controlled source) seismic imaging as used in the oil industry. Key processing steps involved precise arrival time picking and hypocentre determination, removing signal amplitude variations due to varying source radiation patterns, identification and separation of reflections from coherent noise, and transformation of the processed waveforms into images of the subsurface reflectivity. When designing our microseismic reflection imaging workflow, we took advantage of the fact that the passive seismic recording geometry with the hypocentres located at depth and the receivers positioned at the surface resembles a reverse vertical-seismic profiling experiment. The resultant P- and S-wave reflection images reveal several reflective features, such as an approximate 15° westward dipping reflector over the 5–25 km depth range that largely coincides with a distinct seismicity boundary. We interpret the imaged interface as the brittle-ductile transition zone boundary, possibly enhanced by a tectonic shear zone. For the area of the North Chilean Precordillera, the deduced microseismic reflection sections with horizontal extensions of about 50 km represent the first high-resolution images of the shallow crust, which could not be obtained from previous active-source seismic-reflection data.

Borehole study of compressional and shear attenuation of basalt flows penetrated by the Brugdan and William wells on the Faroes shelf

ABSTRACTWe investigated the seismic attenuation of compressional (P‐) and converted shear (S‐) waves through stacked basalt flows using short‐offset vertical seismic profile (VSP) recordings from the Brugdan (6104/21–1) and William (6005/13–1A) wells in the Faroe‐Shetland Trough. The seismic quality factors (Q) were evaluated with the classical spectral ratio method and a root‐mean‐square time‐domain amplitude technique. We found the latter method showed more robust results when analysing signals within the basalt sequence. For the Brugdan well we calculated effective Q estimates of 22–26 and 13–17 for P‐ and S‐waves, respectively, and 25–33 for P‐waves in the William well. An effective QS/QP ratio of 0.50–0.77 was found from a depth interval in the basalt flow sequence where we expect fully saturated rocks. P‐wave quality factor estimates are consistent with results from other VSP experiments in the North Atlantic Margin, while the S‐wave quality factor is one of the first estimates from a stacked basalt formation using VSP data. Synthetic modelling demonstrates that seismic attenuation for P‐ and S‐waves in the stacked basalt flow sequence is mainly caused by one‐dimensional scattering, while intrinsic absorption is small.

Shallow vertical seismic profiling experiments with a surface impact source and a cemented 3-C geophone string; Perth Basin, Western Australia.

Analysis of first arrivals recorded by a total of 24 cemented geophone from a surface source provides insight into the propagation of acoustic waves. A set of surface to hole experiments have been carried out to measure variations of transit times, frequency content and amplitudes of the downgoing (transmitted) compressional waves. The geology of the test site is tightly constrained by wire-line and geological logs from more than 20 close spaced drill holes. One experiment consisted of 274 repeat blows from a surface impact source 25 m away from a vertical string of 3-C geophones cemented in a drill hole. The geophones spanned a depth range from 74 to 284 m. A detailed comparison of total travel time and interval travel times demonstrate a surprisingly high level of repeatability across 274 repeated shots at the same position. Experiments with repeat radially symmetric source positions located around the cemented 3-C geophone string were also carried out. These experiments indicated that even a slight change in source location or condition can significantly change the shape of the waveform generated at the surface. This was likely the result on slight variations at the source ground interface.

IODP Expedition 319, NanTroSEIZE Stage 2: First IODP Riser Drilling Operations and Observatory Installation Towards Understanding Subduction Zone Seismogenesis

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a major drilling project designed to investigate fault mechanics and the seismogenic behavior of subduction zone plate boundaries. Expedition 319 is the first riser drilling operation within scientific ocean drilling. Operations included riser drilling at Site C0009 in the forearc basin above the plate boundary fault, non-riser drilling at Site C0010 across the shallow part of the megasplay fault system – which may slip during plate boundary earthquakes – and initial drilling at Site C0011 (incoming oceanic plate) for Expedition 322. At Site C0009, new methods were tested, including analysis of drill mud cuttings and gas, and in situ measurements of stress, pore pressure, and permeability. These results, in conjunction with earlier drilling, will provide (a) the history of forearc basin development (including links to growth of the megasplay fault system and modern prism), (b) the first in situ hydrological measurements of the plate boundary hanging wall, and (c) integration of in situ stress measurements (orientation and magnitude) across the forearc and with depth. A vertical seismic profile (VSP) experiment provides improved constraints on the deeper structure of the subduction zone. At Site C0010, logging-while-drilling measurements indicate significant changes in fault zone and hanging wall properties over short (< 5 km) along-strike distances, suggesting different burial and/or uplift history. The first borehole observatory instruments were installed at Site C0010 to monitor pressure and temperature within the megasplay fault zone, and methods of deployment of more complex observatory instruments were tested for future operations. doi:10.2204/iodp.sd.10.01.2010

IODP Expedition 319, NanTroSEIZE Stage 2: First IODP Riser Drilling Operations and Observatory Installation Towards Understanding Subduction Zone Seismogenesis

The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) is a major drilling project designed to investigate fault mechanics and the seismogenic behavior of subduction zone plate boundaries. Expedition 319 is the first riser drilling operation within scientific ocean drilling. Operations included riser drilling at Site C0009 in the forearc basin above the plate boundary fault, non-riser drilling at Site C0010 across the shallow part of the megasplay fault system &amp;ndash; which may slip during plate boundary earthquakes &amp;ndash; and initial drilling at Site C0011 (incoming oceanic plate) for Expedition 322. At Site C0009, new methods were tested, including analysis of drill mud cuttings and gas, and &lt;i&gt;in situ&lt;/i&gt; measurements of stress, pore pressure, and permeability. These results, in conjunction with earlier drilling, will provide (a) the history of forearc basin development (including links to growth of the megasplay fault system and modern prism), (b) the first &lt;i&gt;in situ&lt;/i&gt; hydrological measurements of the plate boundary hanging wall, and (c) integration of &lt;i&gt;in situ&lt;/i&gt; stress measurements (orientation and magnitude) across the forearc and with depth. A vertical seismic profile (VSP) experiment provides improved constraints on the deeper structure of the subduction zone. At Site C0010, logging-while-drilling measurements indicate significant changes in fault zone and hanging wall properties over short (&lt; 5 km) along-strike distances, suggesting different burial and/or uplift history. The first borehole observatory instruments were installed at Site C0010 to monitor pressure and temperature within the megasplay fault zone, and methods of deployment of more complex observatory instruments were tested for future operations. &lt;br&gt;&lt;br&gt; doi:&lt;a href="http://dx.doi.org/10.2204/iodp.sd.10.01.2010" target="_blank"&gt;10.2204/iodp.sd.10.01.2010&lt;/a&gt;

Near-wellbore VSP imaging without overburden

In seismic exploration a complex overburden can cause defocused and distorted seismic images of deeper target areas. While in surface seismic imaging it is difficult to overcome this problem, vertical seismic profile (VSP) borehole seismic imaging with its well-oriented recording geometry has proven able to circumvent the overburden’s detrimental impact. Recently, VSP near-wellbore salt flank imaging without knowledge of the overburden has shown encouraging results. In this paper, we introduce another method for a more common imaging purpose: imaging near-wellbore horizons without knowledge of the overburden. A key component of our method is a reverse ray-tracing technique that uses actual VSP direct-wave traveltime picks and extrapolates through a local velocity model to obtain the direct-wave traveltimes in the vicinity of the well. The extrapolated traveltimes then serve as references for migration of the VSP reflected data from near-wellbore horizons. The merits of this method are that all source-side effects such as shot position and triggering time statics are avoided, and it can efficiently and robustly image the near-wellbore horizons because it does not need an estimated overburden velocity model prone to errors. However, the image is limited to a diamond-shaped area around the well whose size is proportional to the VSP array. A synthetic VSP experiment on the SEG/EAGE 2D overthrust model shows that this method, with a local (even an estimated 1D) velocity model, effectively images structural features, such as dip direction and discontinuities of the near-wellbore horizons.

Resolution analysis for stereotomography in media with elliptic and anelliptic anisotropy

Stereotomography is a tomographic method that uses slowness-vector components to improve traveltime inversion. We extend it to general anisotropic media and implement it for media with elliptic and anelliptic anisotropy. Such media depend on only three elastic parameters, which makes stereotomography less sensitive to ambiguity resulting from limited coverage of surface seismic experiments than it would be in transversely isotropic or orthorhombic media. The corresponding approximations of the slowness surface restrict the validity of the present approach to qP events and mild anisotropy. Our numerical analysis demonstrates that stereotomography can be used to estimate macrovelocity models in the presence of anisotropy. For elliptic anisotropy, the parameter that is most difficult to estimate is the orientation of the symmetry axis. For anelliptic anisotropy, estimation of stereotomography parameters is possible in principle; however, it will be mostly ill conditioned in practice, even for low noise levels. Better results are achieved, if transmission events from multiple-offset vertical seismic profiling experiments are included in the inversion.

Seismic attenuation in Faroe Islands basalts

ABSTRACTWe analysed vertical seismic profiling (VSP) data from two boreholes at Glyvursnes and Vestmanna on the island of Streymoy, Faroe Islands, to determine the magnitude and causes of seismic attenuation in sequences of basalt flows. The work is part of SeiFaBa, a major project integrating data from vertical and offset VSP, surface seismic surveys, core samples and wireline log data from the two boreholes. Values of effective seismic quality factor (Q) obtained at Glyvursnes and Vestmanna are sufficiently low to significantly degrade the quality of a surface reflection seismic image. This observation is consistent with results from other VSP experiments in the North Atlantic region. We demonstrate that the most likely cause of the low values of effective Q at Glyvursnes and Vestmanna is a combination of 1D scattering and intrinsic attenuation due to seismic wave‐induced fluid flow within pores and micro‐cracks. Tests involving 3D elastic wave numerical modelling with a hypothetical basalt model based on field observations, indicate that little scattering attenuation is caused by lateral variations in basalt structure.

Waveform tomography at a groundwater contamination site: VSP-surface data set

Application of 2D frequency-domain waveform tomography to a data set from a high-resolution vertical seismic profiling (VSP) experiment at a groundwater contamination site in Hill Air Force Base (HAFB), Utah, reveals a surprisingly complicated shallow substructure with a resolution of approximately 1.5 m. Variance in the waveform misfit function is reduced 69.4% by using an initial velocity model from first-arrival traveltime tomography. The waveform tomography model suggests (1) a low-velocity layer at 1 to 4 m depth, (2) a high-vertical-velocity gradient of 80 m/s/m on average, and (3) severe lateral variations — velocity contrasts as large as about 200 m/s occur in a distance as short as 1.5 m. The model is well correlated with lithologic logs and is interpreted geologically. A Q-value of 20 is estimated for the target area. The extreme lateral and vertical variations of the subsurface compromise many standard seismic processing methods.