Surface Seismic Reflection Data Research Articles

Shallow velocity modelling – the application of high-resolution Full Tensor Gravity Gradiometry data and seismic tomography: a Permian Basin case study

Subsurface imaging from surface seismic reflection data is often complex as a result of issues related to the surface and the near surface including noise (ambient or otherwise), differing coupling of the geophones/geophone arrays and different coupling between the sources and mixed sources and the ground. With the diversity in land seismic exploration and increasingly complex near-surface conditions advanced modelling capabilities are being used for velocity model generation in processing. In this article the use of a multi-physics approach that does not suffer from velocity-depth ambiguity and provides a robust shallow velocity model is presented. This case study is taken from a project in the Permian Basin, USA. It presents the integration of Airborne Full Tensor Gravity Gradiometry (FTG) with tomography and the unique approach applied by the authors for modelling the near surface velocity structure integrated with surface seismic data.

Effective Attributes Quantification to Bridge Gap between Elastic Properties and Reservoir Parameters in Self-Resource Rocks

The successful production of unconventional resources such as shale gas is highly dependent on its two reservoir properties, organic matter and rock brittleness. High resolution spatially characterization of these two unconventional reservoir properties needs surface reflection seismic data. However, to delineate these two parameters on seismic scale is a challenging task because poor correlation is observed between these parameters and elastic properties of the rock. To encounter this adversity in current study we proposed effective attributes method in which organic shale reservoir properties are divided into their hard and soft elastic response. From the analysis of worldwide laboratory dataset, we find that hard and soft components have shown us much better linear correlation with P- and S- wave impedance. The proposed effective attributes, helped us to reduce the gap between unconventional reservoir properties and seismic characteristics. These attributes are the main controlling factor for rock elastic properties and exhibit information about hydrocarbon generation capacity and rock brittleness. A well data example from Sembar shale has also shown successful results for proposed effective attributes methodology. These attributes application on inverted P-wave impedance seismic data of employed organic shale reservoir have shown productive results to quantify its unconventional prospect on seismic scale. The approach used in this study can be confidently employed to assess unconventional reservoir potential in other parts of the world.

A practical implementation of subsalt Marchenko imaging with a Gulf of Mexico data set

Marchenko redatuming allows one to use surface seismic reflection data to generate the seismic response from sources at the surface to any point in the subsurface. Without requiring much information about the earth’s properties, the seismic response generated by Marchenko redatuming contains accurate estimates of not only the primaries but also the internal multiples. A target-oriented imaging method, referred to as Marchenko imaging, was implemented for imaging complex structures of the earth using the seismic response obtained through Marchenko redatuming. Taking account of the contribution of primaries and internal multiples, Marchenko imaging produces images that contain fewer artifacts than the images obtained using conventional imaging methods (e.g., reverse time migration) with the same input data. In this study, we applied Marchenko imaging to a field data set acquired at the Gulf of Mexico to produce an image of a subsalt area. We investigated two important and practical aspects of the Marchenko framework: (1) the missing near offsets in marine shot records and (2) the calibration of the reflection data. Finally, we suggested a workflow for processing the marine towed-streamer field data set acquired at the Gulf of Mexico, and we have developed a complete theoretical and practical framework to produce a target-oriented subsalt image using the Marchenko methods. The images obtained from Marchenko imaging are consistent and comparable, for the most part, with conventional migration methods. However, Marchenko imaging achieves improvements in the continuity of the geologic structures and in suppressing the artifacts that are caused by internal multiples.

An integrated analysis of gravity and well data for deep-seated structural interpretation: a case study, from Ras Budran oil field, Gulf of Suez, Egypt

A combined gravity and well-logging data study was put forward to explore the subsurface structural configurations in Ras Budran oil field area. At first, all available open-hole logging data in the form of porosity tools (density, sonic, and neutron), gamma-ray and resistivity logs (shallow and deep) beside the composite logs were used in determining the lithology and dividing it into three different groups, namely, pre-evaporites, evaporites, and post-evaporites. In addition, the Bouguer anomaly map had been integrated with the available borehole information for interpreting the deep-seated structures which not imaged by surface seismic reflection data. The structural analysis is based essentially on the strip off technique to minimize the ambiguity in the deep gravity data interpretation. This was accomplished through computing the gravity effect of each sedimentary layer of a known density and geometry. Several stages of deep gravity interpretation were done by calculating and removing the gravity effect of the overlying sedimentary section above the stripping datum. The process led to localization and enhancement of the gravity anomalies which give more details about the deep structures/sources. In general, the study reveals that the negative gravity effect of the thick surficial layer (post-evaporite sediments) over all the area is strong enough and superior. The positive gravity effect of the shallow evaporites seriously distorts the negative ones of the pre-Miocene clastics. The gravity effect of the deep sources including the Rudeis–Nubian interval generates the most relevant contribution to the strip off gravity, especially in the southern portion. The stripped gravity of the pre-evaporite strata is distinguished by two local lows in the southern part that could be interpreted in terms of gravity as two local troughs on the basement surface. The density models exhibit the presence of an anticlinal structure correlated with Ras Budran oil field, which is surrounded by two fill-sediment grabens from south direction. This confirm that the near-surface bodies are the most affective to obtain a good fit between the observed and calculated profiles.

Refining the Formation and Early Evolution of the Eastern North American Margin: New Insights From Multiscale Magnetic Anomaly Analyses

AbstractTo investigate the oceanic lithosphere formation and early seafloor spreading history of the North Atlantic Ocean, we examine multiscale magnetic anomaly data from the Jurassic/Early Cretaceous age Eastern North American Margin (ENAM) between 31 and 40°N. We integrate newly acquired sea surface magnetic anomaly and seismic reflection data with publicly available aeromagnetic and composite magnetic anomaly grids, satellite‐derived gravity anomaly, and satellite‐derived and shipboard bathymetry data. We evaluate these data sets to (1) refine magnetic anomaly correlations throughout the ENAM and assign updated ages and chron numbers to M0–M25 and eight pre‐M25 anomalies; (2) identify five correlatable magnetic anomalies between the East Coast Magnetic Anomaly (ECMA) and Blake Spur Magnetic Anomaly (BSMA), which may document the earliest Atlantic seafloor spreading or synrift magmatism; (3) suggest preexisting margin structure and rifting segmentation may have influenced the seafloor spreading regimes in the Atlantic Jurassic Quiet Zone (JQZ); (4) suggest that, if the BSMA source is oceanic crust, the BSMA may be M series magnetic anomaly M42 (~168.5 Ma); (5) examine the along and across margin variation in seafloor spreading rates and spreading center orientations from the BSMA to M25, suggesting asymmetric crustal accretion accommodated the straightening of the ridge from the bend in the ECMA to the more linear M25; and (6) observe anomalously high‐amplitude magnetic anomalies near the Hudson Fan, which may be related to a short‐lived propagating rift segment that could have helped accommodate the crustal alignment during the early Atlantic opening.

Assessment of the Limitations on the Seismic Detectability of Injected CO2 within a Deep Geological Reservoir

Aquistore is a deep saline CO2 storage research and demonstration project located near Estevan, Saskatchewan where CO2 is transported via pipeline and injected into a sandstone reservoir ∼3200 m below the surface. A pre-injection time-lapse analysis performed on two sparse 3D seismic datasets was used to characterise the background time-lapse signal-to-noise level at the storage site. The time-lapse analysis revealed that the lowest global nRMS was 0.07 which was taken to represent the level above which CO2 would be detectable in the reservoir. We investigate the conditions under which the injected CO2 can be detected above the defined minimum noise level through Gassmann fluid substitution and 3D seismic forward modelling. Additionally, Wave Unix was used to simulate the seismic response of the reservoir due to the injected CO2 by generating the synthetic surface reflection seismic data from an explosive surface P-wave source. We generated noise-free synthetic seismograms for the baseline model as well as for the 2-phase fluid replacement of brine with CO2 for CO2 concentrations up to 100% within the target zone – the monitors. The baseline and monitor traces from the 3D seismic survey at Aquistore are used as the noise traces in this study, and were added to their respective baseline and monitor synthetic traces. The nRMS within the reservoir was then computed for the noisy baseline and various noisy monitor surveys and was used in the assessment of the limitation to the detection of the injected CO2 in the reservoir under the background noise level at the site. We are able to conclude that the time-lapse repeatability will not limit the ability to monitor the CO2 induced changes in the reservoir at the Aquistore storage site.

Joint inversion of seismic, vertical seismic profiling, and crosswell data — A case study from China

Three-dimensional poststack and prestack seismic inversion results such as P- and S-impedance are commonly used for reservoir characterization. However, the frequency bandwidth of surface-based reflection seismic surveys usually ranges from 10 to 70 Hz, and these surveys have limited vertical resolution. The frequency bandwidth of vertical seismic profiling (VSP) and crosswell data is much wider than that of surface reflection seismic data, and it can give a detailed illumination of the subsurface around the borehole. We test a joint inversion method that integrated surface reflection seismic, VSP, and crosswell data. To better constrain the inversion results, we further integrate a posteriori information on the reflectivity obtained from petrophysics data into the inversion procedure. The a posteriori distribution we use is a modified-Cauchy distribution obtained from the statistical analysis of petrophysics data. To demonstrate the effectiveness of our algorithm, we applied our inversion strategy to a 2D synthetic model and a real seismic data set, and an uncertainty assessment was also performed. The joint inversion method can detect the thin layers that surface seismic inversion fail to, demonstrating the higher resolution of the method.

Elastic P- and S-wave autofocus imaging with primaries and internal multiples

Conventional seismic imaging methods rely on the single-scattering Born approximation, requiring the removal of multiply scattered events from reflection data prior to imaging. Additionally, many methods use an acoustic approximation, representing the solid earth as an acoustic (fluid) medium. We have developed imaging methods for (solid) elastic media that use primaries and internal multiples, including their PS and SP conversions, thus obviating the need for internal multiple removal and improving handling of internal conversions. Our methods rely on the elastic autofocusing method, which estimates full multicomponent elastodynamic Green’s functions from virtual sources interior to the medium to receivers placed on the surface. They require only surface seismic reflection data and estimates of the direct waves from virtual sources interior to the medium, both of which are commonly available at the imaging step of seismic processing. We tested our methods on a synthetic model with constant P and S velocities and vertical and horizontal density variations, by producing, for the first time to our knowledge, PP and SS images from elastic autofocusing that are compared to reference seismic images based on conventional methods. The effects of multiples are greatly attenuated in the images, with fewer spurious reflectors than are observed when using Born imaging.

Displacement transfer from fault-bend to fault-propagation fold geometry: An example from the Himalayan thrust front

The leading edge of the ENE-trending Himalayan thrust front in Pakistan exhibits along-strike changes in deformational style, ranging from fault-bend to fault-propagation folds. Although the structural geometry is very gently deformed throughout the Salt Range, it becomes progressively more complex to the east as the leading edge of the emergent Salt Range Thrust becomes blind. Surface geology, seismic reflection, petroleum well, and chronostratigraphic data are synthesized to produce a 3-D kinematic model that reconciles the contrasting structural geometries along this part of the Himalayan thrust front. We propose a model whereby displacement was transferred, across a newly-identified lateral ramp, from a fault-bend fold in the west to fault-propagation folds in the east and comparable shortening was synchronously accommodated by two fundamentally different mechanisms: translation vs. telescoping. However, substantially different shortening distribution patterns within these structurally contrasting segments require a tear fault, which later is reactivated as a thrust fault. The present geometry of this S-shaped displacement transfer zone is a combined result of the NW–SE compression of the lateral culmination wall and associated tear fault, and their subsequent modification due to mobilization of underlying ductile salt.

The Application of an Integrated Earth Model in Reservoir Management of a CO2 Plume

Abstract The Illinois Basin – Decatur Project (IBDP) is a large-scale carbon capture and storage (CCS) demonstration project managed by the Midwest Geologic Sequestration Consortium (MGSC). IBDP is injecting 1 million tonnes of carbon dioxide (CO2) in the Cambrian Mt. Simon Sandstone over three years at a rate of 1,000 tonnes per day. At the IBDP site the top of the Mt. Simon Sandstone is overlain by 100 m (300 ft) of tight silt and shale in the Eau Claire Formation that forms the primary seal that prevents possible migration of CO2 into the overlying strata. Below the Mt. Simon Sandstone is a pre-Mt. Simon interval that is characterized by its poor reservoir quality. In the United States, the pore space storage rights are typically owned by the surface landowner. Therefore, it is imperative that CCS projects such as IBDP be able to estimate the size and movement of the CO2 plume within the reservoir. An integrated earth model and subsequent reservoir flow simulations are the first steps to CO2 plume management. An inversion of the 3D surface reflection seismic data was used to create a template for the distribution of the reservoir properties acquired from the well data. Inversion of the 3D seismic data produced two inversion volumes, an acoustic impedance (AI) cube, and a porosity cube. Wireline porosity logs were upscaled to the 3D model and then co-simulated with the seismic inversion porosity. Log permeability was upscaled to the simulation model and co-simulated with porosity. The resulting porosity and permeability fields were rescaled into the simulation grid which was discretized to achieve the required simulation accuracy while maintaining computational efficiency. The reservoir flow simulation based on the integrated earth model suggests that the CO2 will follow the high permeability intervals and there will be very limited vertical migration of the CO2. These flow simulations also will help with plume management in a new planned injection well near the IBDP project site which will be perforated at higher intervals in the Mt. Simon Sandstone than those in the original injection well.

Accurate interval Q-factor estimation from VSP data

Inelastic attenuation, quantified by [Formula: see text], the seismic quality factor, has considerable impact on surface seismic reflection data. A new method for interval [Formula: see text]-factor estimation using near-offset VSP data was based on an objective function minimization measuring the difference between cumulative [Formula: see text] estimates and those calculated through interval [Formula: see text]. To calculate interval [Formula: see text], we used all receiver pairs that provided reasonable [Formula: see text] values. To estimate [Formula: see text] between two receiver levels, we used the equation that links amplitudes at different levels and could provide more accurate [Formula: see text] values than the spectral-ratio method. To improve interval [Formula: see text] estimates, which rely on traveltimes, we used a high-accuracy approach in the frequency domain to determine time shifts. Application of this method to real data demonstrated reasonable correspondence between [Formula: see text] estimates and log data.

Experimental estimation of velocities and anisotropy of a series of Swedish crystalline rocks and ores

ABSTRACTTo provide a guide for future deep (<1.5 km) seismic mineral exploration and to better understand the nature of reflections imaged by surface reflection seismic data in two mining camps and a carbonatite complex of Sweden, more than 50 rock and ore samples were collected and measured for their seismic velocities. The samples are geographically from the northern and central parts of Sweden, ranging from metallic ore deposits, meta‐volcanic and meta‐intrusive rocks to deformed and metamorphosed rocks. First, ultrasonic measurements of P‐ and S‐wave velocities at both atmospheric and elevated pressures, using 0.5 MHz P‐ and S‐wave transducers were conducted. The ultrasonic measurements suggest that most of the measured velocities show positive correlation with the density of the samples with an exception of a massive sulphide ore sample that shows significant low P‐ and S‐wave velocities. The low P‐ and S‐wave velocities are attributed to the mineral texture of the sample and partly lower pyrite content in comparison with a similar type sample obtained from Norway, which shows significantly higher P‐ and S‐wave velocities. Later, an iron ore sample from the central part of Sweden was measured using a low‐frequency (0.1–50 Hz) apparatus to provide comparison with the ultrasonic velocity measurements. The low‐frequency measurements indicate that the iron ore sample has minimal dispersion and attenuation. The iron ore sample shows the highest acoustic impedance among our samples suggesting that these deposits are favourable targets for seismic methods. This is further demonstrated by a real seismic section acquired over an iron ore mine in the central part of Sweden. Finally, a laser‐interferometer device was used to analyse elastic anisotropy of five rock samples taken from a major deformation zone in order to provide insights into the nature of reflections observed from the deformation zone. Up to 10% velocity‐anisotropy is estimated and demonstrated to be present for the samples taken from the deformation zone using the laser‐interferometery measurements. However, the origin of the reflections from the major deformation zone is attributed to a combination of anisotropy and amphibolite lenses within the deformation zone.

3D imaging challenges in steeply dipping mining structures: New lights on acquisition geometry and processing from the Brunswick no. 6 seismic data, Canada

We have analyzed and processed a [Formula: see text] nonorthogonal 3D surface reflection seismic data in the Brunswick no. 6 area to better understand the effect of acquisition geometry on the resultant image and to provide 3D information about the main geologic structures hosting the mineralization. The 3D data were processed using a conventional prestack dip moveout (DMO) and poststack migration algorithm with special focus on refraction static corrections, velocity analysis, and DMO corrections that are important for the data recorded in crystalline environment. However, the nonorthogonal nature of the 3D data combined with its narrow-azimuth, irregular offset distributions, and 2D nature of midpoint distribution in common depth point bins resulted in a lower quality seismic image than those observed on a series of 2D seismic profiles collected in the area prior to the 3D data acquisition. 2D wavenumber spectrum of the data suggests acquisition footprint associated with the data. Most of the noise associated with the acquisition footprint manifested itself as short-length, high-amplitude shallow reflections but largely were attenuated using a dip filter running in the wavenumber domain. Various bin size and geometries were tested, and the best result was obtained using rectangular bins aligned in the orientation of the shot lines. The processing results indicated that the highly prospective and mineralized Brunswick horizon is part of a continuous reflective package that could guide future deep mineral exploration in this mining camp.

Seismic imaging using microseismic events: Results from the San Andreas Fault System at SAFOD

We have developed a new passive seismic imaging approach that consists of two steps. First, the hypocenter of the microseismic event is precisely located. Second, this event is treated as a “pseudo‐active” seismic source and the reflections within the recorded wavefield are processed by using a directional migration algorithm in order to construct a high‐resolution image of the illuminated subsurface region. In this paper we demonstrate the application of our approach to a number of microseismic events recorded by a borehole array in the San Andreas Fault Observatory at Depth main hole. Results obtained were high‐resolution 3‐D images of different SE–NW–oriented reflectors related to the San Andreas Fault (SAF) system in the close vicinity of the borehole. To support our approach, we compared our findings with other active and passive seismic images and analyzed the correlation with borehole lithology. We revealed a predominantly satisfactory agreement for both juxtapositions. Furthermore, the stacked image of several microearthquakes provides a spatial characterization of the complex internal structure of the SAF, with much higher resolution than can be obtained from surface seismic reflection data.

Rock physics modeling for assessing gas hydrate and free gas: a case study in the Cascadia accretionary prism

We investigate the estimation of gas hydrate and free gas concentration using various rock physics models in the Cascadia accretionary prism, which is one of the most intensively studied regions of natural gas hydrate occurrences. Surface seismic reflection data is the most useful and cost-effective in deriving seismic velocity, and hence estimating gas hydrate and free gas across a BSR with depth, if a proper background (without gas hydrate and free gas) velocity is chosen. We have used effective medium theory of Helgerud et al. (EMTH) and, a combination of self-consistent approximation and differential effective medium (SCA-DEM) theory coupled with smoothing approximation for crystalline aggregate. Using the SCA-DEM (non-load-bearing) and EMTH (load-bearing) modeling, we calculate the average saturations of gas hydrate as 17 and 19%, respectively within ~100 m thick sedimentary column using velocity, derived from the surface seismic data. The saturations of gas hydrate are estimated as 15 and 18% using the SCA-DEM, and 20 and 25% using EMTH from the logging-while-drilling and wire-line sonic velocities, respectively. Estimations of gas hydrate from Poisson’s ratio are in average 50% for EMTH and 10% for SCA-DEM theory. We obtain the maximum saturation of free gas as 1–2% by employing the SCA-DEM theory either to seismic or sonic velocities, whereas the free-gas saturation varies between 0.1 and 0.4% for EMTH model. The gas hydrate saturation estimated from the sonic velocity and the free gas saturation derived from both the seismic and sonic velocities using the SCA-DEM modeling match quite well with those determined from the pressure core data in the study region.

Seismic quality factor observations for gas-hydrate-bearing sediments on the western margin of India

Any propagating wave undergoes attenuation, which is primarily governed by the physical properties of the medium, determined in terms of quality factor (Q). Research into the characteristics of both P- and S-wave Q with reference to gas-hydrates exploration remains in its infancy. Presence of gas-hydrates increases the Q, and this again depends on the nature of distribution and amount of hydrates within the sediments. Thus, estimation of Q provides useful input for both the detection and quantitative assessment of gas-hydrates. Here we propose a simple technique of deriving Q from prestack surface seismic reflection data based on the logarithm of spectral ratio (LSR), and apply the method to marine multi-channel seismic (MCS) data collected on the western margin of India where a bottom simulating reflector (BSR), which is a prime marker for gas-hydrates, has already been identified. The Q (256 ± 11) estimated over the region with a strong BSR is found to be more than double the Q (101 ± 9) derived for the region without any BSR or a weak BSR. The anomalously high Q with respect to the background can be used to detect gas-hydrates in areas where the BSR is not very clearly observed on seismic sections.

Wavelet-based detection of singularities in acoustic impedances from surface seismic reflection data

Although the passage of singularity information from acoustic impedance to seismic traces is now well understood, it remains unanswered how routine seismic processing, mode conversions, and multiple reflections can affect the singularity analysis of surface seismic data. We make theoretical investigations on the transition of singularity behaviors from acoustic impedances to surface seismic data. We also perform numerical, wavelet-based singularity analysis on an elastic synthetic data set that is processed through routine seismic processing steps (such as stacking and migration) and that contains mode conversions, multiple reflections, and other wave-equation effects. Theoretically, seismic traces can be approximated as proportional to a smoothed version of the [Formula: see text] derivative of acoustic impedance,where [Formula: see text] is the vanishing moment of the seismic wavelet. This theoretical approach forms the basis of linking singularity exponents (Hölder exponents) in acoustic impedance with those computable from seismic data. By using wavelet-based multiscale analysis with complex Morlet wavelets, we can estimate singularity strengths and localities in subsurface impedance directly from surface seismic data. Our results indicate that rich singularity information in acoustic impedance variations can be preserved by surface seismic data despite data-acquisition and processing activities. We also show that high-resolution detection of singularities from real surface seismic data can be achieved with a proper choice of the scale of the mother wavelet in the wavelet transform. Singularity detection from surface seismic data thus can play a key role in stratigraphic analysis and acoustic impedance inversion.

Seismic imaging of basalts at Glyvursnes, Faroe Islands: hunting for future exploration methods in basalt covered areas

Obtaining good sub-basalt seismic images is known to be problematic (Ziolkowski et al., 2003; White et al., 2003). Although the properties of basalts are quite different from those of most sediments Planke (1999) suggested that seismic energy is transmitted through basalt in much the same way as through sediments so the problem of seismic imaging through basalts amounts to the conventional task of separating primary energy from noise, even though the noise including multiples may be considerable. The physical properties of basalt are markedly different from those of the overlying and underlying sediments. Strong reflections due to high impedance contrasts at the top (and bottom) of the basalts leads to significant loss of transmitted seismic energy (Fruehn et al., 2001). Large variations of intrinsic properties along vertical cross-sections of basalt flows have been demonstrated and quantified by analyses of well-logs from wells penetrating successions of flood basalts (Planke, 1994) and from surface mapping (e.g., Self et al., 1998; Thordarson and Self, 1998). This causes the stratigraphic filtering effects of basalt successions to be more severe than that of sediments (Maresh and White, 2005; Shaw et al., 2004). Lateral variations in the thicknesses of sediments interbedded between basalt flows and in the thickness of the upper porous part of basalt flows have been demonstrated by detailed investigations in exposed flood basalts (Self et al., 1998; Thordarson and Self, 1998). The roughness of inter-beds causes 3D scattering of seismic energy, as demonstrated in studies comparing stratigraphic filtering and the effective quality factor, Q, of basalt successions (e.g., White et al., 2005; Shaw et al., 2005; Shaw et al., 2004). Taking these problems into consideration, experiments have been performed in the last decade using: longer offsets (both synthetic aperture, and longer streamers) to improve the signal-to-noise ratio and NMO resolution and to allow processing of post-critical reflections; larger energy sources to increase the general energy level; low-frequency tuning to allow for better penetration through basalts (characterised by low Q values); and shot-by-shot recording of the source signature and combination of OBS and seismic reflection data to improve velocity estimates. In one experiment all of the above-mentioned techniques were applied, providing considerable improvements in sub-basalt imaging relative to previous work (Spitzer and White, 2005; White et al., 2005). An other parameter for seismic acquisition is the orientation of the seismic line relative to the flow direction of the basalt flows (Reshef et al., 2003). However, poor effective transmission of seismic energy, scattering, strong multiple reflections, multiple mode conversions, and low-pass filtering of the energy that propagates through a layer of stacked basalt flows are still hampering routine imaging for petroleum exploration in sediment basins covered by basalts (Maresh and White, 2005). This was demonstrated by the UK164/07- 01 well where the base of a basaltic succession was found 700 m deeper than anticipated from interpretation of seismic reflection data (Archer et al., 2005). In order to obtain imaging quality and detail comparable to those obtained in other sedimentary basins, further improvements are necessary. The SeiFaBa Project (Seismic and petrophysical properties of Faroes Basalts), sponsored by the Sindri group, aims to create data-derived models for the propagation of seismic energy in basalt to provide a basis for better sub-basalt imaging. The project comprises drilling of the Glyvursnes-1 wells near Tórshavn on the Faroe Islands (Figure 1), core analysis for intrinsic physical parameters, recording of VSP and offset-VSP data in the Glyvursnes-1 and Vestmanna-1 wells, and surface seismic wide-angle and reflection data around the Glyvursnes- 1 and Vestmanna-1 wells (Japsen et al., 2005). At both sites these investigations of the elastic properties of basalts are made at a number of different scales. In this paper we present surface seismic reflection data from SeiFaBa experiment at Glyvursnes in the summer of 2003 illustrating that basalts can be imaged effectively using relatively small energy sources (250 g dynamite; 2.6-litre airgun cluster) and that stratigraphic details of flood-basalt constructions can be identified and characterized based on analysis of seismic data and then correlated to well data. We also demonstrate how different acquisition and processing techniques influence the effective frequency content of seismic reflection data and thus the effective propagation through and imaging of the basalts.

Permeability and porosity images based on P‐wave surface seismic data: Application to a south Florida aquifer

P‐wave surface seismic reflection data, acquired from a carbonate aquifer in southeastern Florida, are used to delineate flow units in a proposed aquifer storage and recovery (ASR) horizon. The impedance image determined by inversion from P‐wave reflection data captures most of the boundaries between hydraulic facies. The hydraulic facies properties are based on integration of the well logs and the lithology, which consists of vuggy carbonate units and sandstones. Within the proposed ASR horizon, located in the upper Floridan aquifer, low‐permeability zones consist of sandstones and highly permeable zones are carbonates with interconnected vuggy porosity. High‐resolution porosity, permeability, and impedance images based on cross‐well reflection and well logs help us evaluate the P‐wave seismic reflection results. We use well logs and cross‐well seismic data to support whether two‐dimensional seismic reflection measurements detect important flow units delineated by cross‐well high‐resolution seismic data. The data analysis demonstrates that major flow units are resolved and imaged with two‐dimensional seismic reflection techniques, although not as clearly as in the high‐resolution cross‐well data. The results suggest that the surface seismic reflection method, integrated with well logs and geology, provides the tools to assess water resources in this south Florida carbonate aquifer. However, we recommend conducting a cross‐well survey in areas where an understanding of the petrophysics is imperative to relating the seismic attributes to rock and fluid properties. This can be a critical step for planning purposes when considering a large surface‐oriented seismic survey.

Advances in seismic imaging through basalts: a case study from the Faroe–Shetland Basin

New seismic reflection data have been used to image intra- and sub-basalt features beneath the Faroe–Shetland Basin in the North Atlantic, where the highly reflective top and base boundaries of flood basalts and their complex internal structure make successful seismic imaging difficult. This study demonstrates that appropriate acquisition and processing of marine seismic data from hydrophone streamers and ocean-bottom seismometers (OBS) has the potential to enhance significantly imaging of intra-basalt and sub-basalt seismic reflections. The intersection of a new seismic reflection profile recorded in 2002 with a seismic profile recorded in 1998 allows a direct comparison of advances in sub-basalt imaging over this period and an interpretation of geological structures and seismic velocities at the intersection. To achieve better resolution of sediments below basaltic layers, surface seismic reflection data using a broad-band, low-frequency source have been recorded. By using a source wavelet with a significantly enhanced frequency spectrum centred at 10 Hz, generated from a large (167 l) airgun array tuned to the first bubble pulse, a wavelet is produced that is capable of penetrating through thick basaltic sequences. Furthermore, recording the low-frequency reflections to large offsets along a 12 000 m long streamer with 3.125 m single sensor spacing allowed effective suppression of multiple reflections, enhanced the precision of velocity analyses and produced better migration results. Seismic data along the same profile were also recorded on a dense deployment of 85 four-component OBS at 2 km or 6 km station spacing. These complementary ocean-bottom data allowed the large-scale velocity variations within and beneath the basalt to be constrained by recording at large offsets the termination of the diving rays ( c . 18 km) propagating through the basalt. Integration of seismic reflection data and OBS recordings yields good seismic velocity estimates and, therefore, increased confidence in the interpretation of intra- and sub-basalt features and lithology. Four major stratigraphic units characterized by their seismic velocities and reflection characters were identified: (a) post-basalt sediments; (b) basalt sequence; (c) sub-basalt unit; and (d) the top of presumed Cretaceous basement. The top boundary of the basalt sequence is defined by a strong reflection with a steep velocity increase from 3.5 km s −1 to 4.5 km s −1 in the c . 200 m thick layer above it. The basalt sequence here is c . 2.5 km thick. Beneath the base of the basalt there is a velocity decrease from 5.8 km s −1 to 5.2 km s −1 . The sub-basalt unit is subdivided further into hyaloclastites mixed with sediments and sills, and sub-basalt sediments separated by another velocity inversion from 5.2 km s −1 to 4.5 km s −1 . The Cretaceous basement with velocities above 5.5 km s −1 lies underneath these sub-basalt sediments.