Pre-stack Processing Research Articles

Combined onshore and offshore wide-scale seismic data acquisition and imaging for carbon capture and storage exploration in Havnsø, Denmark

Strong global actions for climate change include carbon capture and storage (CCS) as a feasible solution to reach carbon neutrality and raise opportunities for detailed subsurface investigations. An acquisition set-up designed for onshore-offshore zones was maximized for wide-scale high-resolution structural imaging and implemented to cover a domal structure of interest for CCS utilization close to the town of Havnsø (Denmark). The challenges of the combined acquisition and processing of land and marine multisensor data along a 42 km seismic profile are analyzed, the suggested solutions are applied, and the limitations are discussed. On the onshore side, a nodal array and a seismic landstreamer system were simultaneously used, whereas along the transition zone, a marine seismic streamer and ocean-bottom seismometers were added to record the seismic response generated by two seismic vibrator sources. The adopted sensing domains (velocity, acceleration, and pressure) were studied, and different processing steps were evaluated to enable their processing and subsequent data set merging. Results suggest, as the best approach, a separate prestack processing of the different data sets and the computation of new geometries and new surface-consistent residual static correction after their merging. The data acquired in the transition zone illuminate, for the first time, the subsurface geology of the region, delineating an expected domal closure. The final seismic section shows high continuity of the reflections with good resolution along the entire profile, identifying the main reservoir structure and the surrounding areas, which are important to ensure reservoir integrity and safe exploitation over longer time scales. Shallow and deep reflections are consistent with the stratigraphic column from a well log near the profile. The presented study shows a comprehensive workflow for processing such a multisensor data set in onshore and transition zone settings.

Fluid Accumulation Zone by Seismic Attributes and Amplitude Versus Offset Analysis at Solfatara Volcano, Campi Flegrei, Italy

The imaging of volcanic systems is a challenging topic that attracts the scientific community’s attention. The characterization of structures and rock properties by means of seismic active methods is becoming fundamental for providing ultra-high-resolution images of the structures of interest. The Solfatara Volcano is a quiescent volcano in the Campi Flegrei resurgent nested caldera that is continuously under investigation and monitoring for its shallow activity, such as fumaroles. The purpose of this work is to characterize the fluid accumulation zone in the first 150 m depth in the middle of the crater, using several post-stack seismic attributes and Amplitude Versus Offset (AVO) analysis to characterize the contact between the CO2 and condensed water in the shallower accumulation zone. The two 400 m-long profiles to which we refer in this work have been acquired during the active Repeated InduCed Earthquakes and Noise experiment. The profiles were deployed along with the NNE-SSW and WNW-ESE directions across the whole surface of the crater including the main surface anomalies of the fumaroles, in the eastern area, and the mud-pool of Fangaia, located in the western area. The seismic pre-processing, pre-stack processing, and post-stack analysis previously applied on the NNE-SSW profile are here performed for the first time on the WNW-ESE profile, while partial-stack AVO analysis is performed for both profiles. The post-stack attributes including time gain, envelope, energy, and root mean square have been computed and extracted for determining the maximum and minimum values of amplitude zones on the migrated post-stack seismic profiles. Such anomalies are provided by complex and geometrical attributes embedding information on faults and chaotic zones. The AVO technique has also been used as a direct gas indicator to enhance fluid discrimination and identification. Finally, the analysis of the profile, seismic attributes, and near-surface structural interpretation related to the Solfatara Volcano has been incorporated into the proposed analysis. The multi-2D image depicts fluids trapped in the Solfatara Volcano at depths ranging from 10 to 50 m below the crater’s surface, as well as their migration paths up to 150 m deep: this evidenced contact between the fluids has been probably due to the solfataric alteration of the minerals, caused by the arising plume and the abovecondensed water which decreases the permeability of the rocks and forms an argillic phase working as cap-rock and trapping the gases. The application of the AVO analysis, coupled with the seismic attribute’s investigation, provides a very detailed multi-2D image of the shallower Solfatara Volcano, which outperforms in terms of accuracy the ones obtained with different tools in previous works, and that evidences the presence and the position of the liquid and the gases in the north-east area of the Solfatara Volcano.

Pre-stack Time Migration Processing for Block A of Putaohua Oilfield

According to the reservoir and structural characteristics of Heidimiao, this paper combines fine pre-stack pre-processing and small-panel pre-stack time migration, and optimizes targeted methods and parameters to obtain the seismic data with higher resolution and better amplitude retention. Compared with the previous processing results, the migration imaging of the new data has been improved, and the geological phenomenon of the whole area is clear. The fracture system is reasonable, the fracture points are crisp, and the imaging of small faults is clear. The wave group characteristics of the new treatment are obvious, and the transverse resolution, longitudinal resolution and signal-to-noise ratio are improved obviously. By using relative amplitude preserving and VTI pre-stack time migration, the signal to noise ratio and resolution of seismic processing results have been greatly improved, and the interlayer information is more abundant, which can meet the needs of target interpretation.

On the Nature of Higher‐Order Ambient Seismic Field Correlations

AbstractWe find that higher‐order ambient seismic field correlations, specifically C3 (correlation of the coda of correlations) of long‐duration data sets from Southern California are symmetric despite the fact that the background wavefield is strongly directional. We demonstrate that this symmetry arises primarily from the structure of the C3 time series analysis rather than the diffuse character of the coda. For that reason, the resulting ambient‐field Green's function symmetry may not be diagnostic of correlation quality. We propose an alternative prestack processing scheme that constructs C3 correlations from each C1 (cross correlation) coda time window, but stacks the results only at the final step of processing. Prestack C3 leads to faster convergence and extracts arrivals at higher frequencies than standard poststack processing.

Attribute-assisted footprint suppression using a 2D continuous wavelet transform

Acquisition footprint manifests itself on 3D seismic data as a repetitive pattern of noise, anomalously high amplitudes, or structural shifts on time or horizon slices that is correlated to the location of the sources and receivers on the earth’s surface. Ideally, footprint suppression should be handled by denser seismic acquisition and more careful prestack processing prior to seismic imaging. In the case in which only legacy data exist, or when economic and time constraints preclude more expensive acquisition and more careful processing, interpreters must deal with data contaminated by footprint. Although accurate time-structure maps can be constructed from footprint-contaminated data, the effect of footprint on subsequent attributes, such as coherence, curvature, spectral components, and P-wave impedance will be exacerbated. We have developed a workflow that uses a 2D continuous wavelet transform to suppress coherent and incoherent noise on poststack seismic data. The method involves decomposing time slices of amplitude and attribute data into voices and magnitudes using 2D wavelets. We exploit the increased seismic attribute sensitivity to the acquisition footprint to design a mask to suppress the footprint on the original amplitude data. The workflow is easy to apply and improves the interpretability of the data and improves the subsequent attribute resolution.

ExSeisDat: A set of parallel I/O and workflow libraries for petroleum seismology

Seismic data-sets are extremely large and are broken into data files, ranging in size from 100s of GiBs to 10s of TiBs and larger. The parallel I/O for these files is complex due to the amount of data along with varied and multiple access patterns within individual files. Properties of legacy file formats, such as the de-facto standard SEG-Y, also contribute to the decrease in developer productivity while working with these files. SEG-Y files embed their own internal layout which could lead to conflict with traditional, file-system-level layout optimization schemes. Additionally, as seismic files continue to increase in size, memory bottlenecks will be exacerbated, resulting in the need for smart I/O optimization not only to increase the efficiency of read/writes, but to manage memory usage as well. The ExSeisDat (Extreme-Scale Seismic Data) set of libraries addresses these problems through the development and implementation of easy to use, object oriented libraries that are portable and open source with bindings available in multiple languages. The lower level parallel I/O library, ExSeisPIOL (Extreme-Scale Seismic Parallel I/O Library), targets SEG-Y and other proprietary formats, simplifying I/O by internally interfacing MPI-I/O and other I/O interfaces. The I/O is explicitly handled; end users only need to define the memory limits, decomposition of I/O across processes, and data access patterns when reading and writing data. ExSeisPIOL bridges the layout gap between the SEG-Y file structure and file system organization. The higher level parallel seismic workflow library, ExSeisFlow (Extreme-Scale Seismic workFlow), leverages ExSeisPIOL, further simplifying I/O by implicitly handling all I/O parameters, thus allowing geophysicists to focus on domain-specific development. Operations in ExSeisFlow focus on prestack processing and can be performed on single traces, individual gathers, and across entire surveys, including out of core sorting, binning, filtering, and transforming. To optimize memory management, the workflow only reads in data pertinent to the operations being performed instead of an entire file. A smart caching system manages the read data, discarding it when no longer needed in the workflow. As the libraries are optimized to handle spatial and temporal locality, they are a natural fit to burst buffer technologies, particularly DDN’s Infinite Memory Engine (IME) system. With appropriate access semantics or through the direct exploitation of the low-level interfaces, the ExSeisDat stack on IME delivers a significant improvement to I/O performance over standalone parallel file systems like Lustre.

Phase characteristic analysis of continuous depth air-gun source wavelet

Air guns are important sources for marine seismic exploration. Far-field wavelet of air gun arrays, as a necessary parameter for pre-stack processing and source models, plays an important role during marine seismic data processing and interpretation. When an air gun fires, it generates a series of air bubbles. Similar to onshore seismic exploration, the water forms a plastic fluid near the bubble; the farther the air gun is located from the measurement, the more steady and more accurately represented the wavelet will be. In practice, hydrophones should be placed more than 100 m from the air gun; however, traditional seismic cables cannot meet this requirement. On the other hand, vertical cables provide a viable solution to this problem. This study uses a vertical cable to receive wavelets from 38 air guns and data are collected offshore Southeast Qiong, where the water depth is over 1000 m. In this study, the wavelets measured using this technique coincide very well with the simulated wavelets and can therefore represent the real shape of the wavelets. This experiment fills a technology gap in China.

Practical considerations and quality control for an FWI workflow

Prestack processing skills are essential in full-waveform inversion (FWI). In practical applications, without careful guidance and continuous monitoring of intermediate data and model updates, the output of the inversion process can diverge and produce nongeologic output models and inversion artifacts. A synthetic case study demonstrates challenges in real-life FWI projects. The FWI workflow is described in terms used by seismic processors. Detailed understanding of the input data set and the selection of starting models, combined with creative analysis tools and quality-control workflows, are paramount for a successful inversion project.

Improving homomorphic wavelet estimation by compensating for residual NMO stretching on stack section

Wavelet estimation is a common step in seismic data processing and inversion. Homomorphic wavelet estimation has long utilized as a method that uses a seismic stack section with no phase presumption. Forming a stack section, normal move-out (NMO) correction must be applied on common midpoint (CMP) gathers, although it introduces NMO stretching. After stacking, residual of the NMO stretching may affect the stack section even after muting the highly stretched zone of the NMO corrected CMP gather. Presence of significant residual NMO stretching changes the spectral characteristics of data in time direction, by different degrees. Considering that in homomorphic process the wavelet is estimated based on the spectral characteristics of data, compensating for the residual NMO stretching, can improve the accuracy of the process. Here, we introduce a fast method of calculating the amount of residual NMO stretching and compensating for its effect on wavelet estimation. The proposed method needs limited prestack information like offsets and velocity function and include no prestack processing. We apply the proposed method on synthetic and real datasets and demonstrate the improvement of the estimated wavelet.

Interpolating wide-aperture ground-penetrating radar beyond aliasing

Most of the data acquisition in ground-penetrating radar is done along fixed-offset profiles, in which velocity is known only at isolated points in the survey area, at the locations of variable offset gathers such as a common midpoint. We have constructed sparse, heavily aliased, variable offset gathers from several fixed-offset, collinear, profiles. We interpolated those gathers to produce properly sampled counterparts, thus pushing data beyond aliasing. The interpolation methodology estimated nonstationary, adaptive, filter coefficients at all trace locations, including at the missing traces’ corresponding positions, filled with zeroed traces. This is followed by an inversion problem that uses the previously estimated filter coefficients to insert the new, interpolated, traces between the original ones. We extended this two-step strategy to data interpolation by employing a device in which we used filter coefficients from a denser variable offset gather to interpolate the missing traces on a few independently constructed gathers. We applied the methodology on synthetic and real data sets, the latter acquired in the interior of the Antarctic continent. The variable-offset interpolated data opened the door to prestack processing, making feasible the production of a prestack time migrated section and a 2D velocity model for the entire profile. Notwithstanding, we have used a data set obtained in Antarctica; there is no reason the same methodology could not be used somewhere else.

Assessment of 4D seismic repeatability and CO2 detection limits using a sparse permanent land array at the Aquistore CO2 storage site

Two 3D time-lapse seismic surveys were acquired in 2012 and 2013 at the Aquistore [Formula: see text] storage site prior to the start of [Formula: see text] injection. Using these surveys, we determined the background time-lapse noise at the site and assessed the feasibility of using a sparse areal permanent receiver array as a monitoring tool. Applying a standard processing sequence to these data, we adequately imaged the reservoir at 3150–3350 m depth. Evaluation of the impact of each processing step on the repeatability revealed a general monotonic increase in similarity between the data sets as a function of processing. The prestack processing sequence reduced the normalized root mean squared difference (nrms) from 1.13 between the raw stacks to 0.13 after poststack time migration. The postmigration cross-equalization sequence further reduced the global nrms to 0.07. A simulation of the changes in seismic response due to a range of [Formula: see text] injection scenarios suggested that [Formula: see text] was detectable within the reservoir at the Aquistore site provided that zones of greater thickness than 6–13 m have reached [Formula: see text] saturations of greater than 5%.

A review on time-lapse seismic data processing and interpretation

Time-lapse (TL) seismic or 4D seismic has great potential in monitoring and interpreting time-varying variations in reservoir fluid properties during hydrocarbon production. The TL surveys interpret dynamic changes in reservoir parameters by detecting differences in seismic responses from different vintages, which are obtained by repeated seismic surveys over the same area. For a proper and more precise quantitative interpretation of the TL seismic responses, TL seismic processing enhances repeatability of the TL-data sets, and thus reduces artificial differences that come from unavoidable differences in repeated-seismic-survey environments. This study aims to bring an insight to the state of the art in both TL seismic data processing and interpretation by reviewing previously-published researches. TL data processing is a sequential process conducting 4D binning and simultaneous 4D pre-stack processing; 4D binning improves spatial repeatability while “simultaneous 4D pre-stack processing” derives common processing operators employing cross-equalization (XEQ) as a core technique. TL seismic interpretation is categorized by the characteristics of data being interpreted into two strategies: interpretation of data sets from all TL vintages and interpretation of differences in seismic data between vintages.

Analyze the Seismic Attributes in HSX Volcanic Fracture Based on Forward Modeling

This paper uses the Hassan Carboniferous fractured reservoirs as the goal, firstly it analyses the reservoir characteristics of the cracks. On this basis, it uses two-dimensional random fractured media modeling method to build three different fracture models with different fracture parameters (fracture density, dip and speed). Then it uses finite difference wave equation forward and pre-stack depth migration processing of these models, and analyses seismic attribute of the migrated data, finally it finds a variety of seismic attributes sensitive to cracks, lays the foundation for fracture prediction with the seismic multi-attributes.

Simultaneous sources separation via multidirectional vector-median filtering

Simultaneous source acquisition technology, also referred to as “blended acquisition,” involves recording two or more shots simultaneously. Despite the fact that the recorded data has crosstalk from different shots, conventional processing procedures can still produce acceptable images for interpretation. This is due to the power of the stacking process using blended data with its increased data redundancy and inherent time delays between various shots. It is still desirable to separate the blended data into single shot gathers and reduce the crosstalk noise to achieve the highest seismic image quality and for standard prestack processing, such as filtering, statics computation, and velocity analysis. This study introduced a new and simple multidirectional vector-median filter (MD-VMF) to separate the blended seismic shot gathers. This method extended the well-known conventional median filter from a scalar implementation to a vector version. More specifically, a vector median filter was applied in many trial directions and the median vector was chosen from among these. We demonstrated the effectiveness of our proposed MD-VMF on simulated data generated by blending synthetic and real marine seismic data.

3D symmetric sampling of sparse acquisition geometries

3D symmetric sampling introduced in the 1990s is characterized by dense sampling of two of the four spatial coordinates. The two sparsely sampled coordinates determine the periodicity of the geometry and the dimension of the offset-vector tiles that can be used to generate pseudocommon-offset-vector gathers. These gathers turn out to be useful for prestack processing applications, such as regularization, migration velocity analysis, and azimuthal anisotropy analysis. Although single-point acquisition is the ideal acquisition method, it is not necessarily better than array-based acquisition. Field arrays are still useful in suppressing noise and need not harm signal in most practical cases. In hybrid geometries three spatial coordinates are sampled densely. In all published cases at least two of the three are sampled quite coarsely and may not provide the best quality for the given trace density. Coil geometry (sailing in circles) is a special case of wide-azimuth towed streamer acquisition. It is essentially a random geometry that should be modifiable into a geometry with regularly sampled midpoints, absolute offsets and azimuths. Despite recent technological developments, the basic idea of 3D symmetric sampling still is a highly useful principle for the design of land and marine 3D seismic surveys.

New approach to common reflection surface and depth imaging: Tembungo Field, Sabah, Malaysia

The Tembungo Field area is located about 100 km offshore Sabah, in eastern Malaysia (Figure 1). Geologically, it is a folded anticlinal structure with intense flower-type faulting. The exploration target is a deep turbidite complex in Miocene basin slope fan system. The velocity field in the overburden is severely complicated by the presence of near-surface reef carbonates, gas wipeout and complex faulting. These complications result in strong energy scattering, low signal to noise (S/N) ratio and strong multiple energy that lead to a significant deterioration of the seismic image over the field and across the whole general area. Processing, including the use of prestack depth migration (PreSDM) with a view to extracting adequate signal under these circumstances, has been fraught with difficulties. The commercial importance of the Tembungo Field was a major incentive for seeking ways of resolving the above imaging issues. A new approach that we devised and adopted proved to be immensely successful. The approach consisted of processing the data with Common Reflection Surface (CRS) technology followed by post-stack depth migration (PostSDM) in which the provelocity model was obtained through a pre-stack depth migration (PreSDM) process. The term ‘provelocity’ will be used in the present work specifically to denote the parameter derived from seismic processing as ‘velocity’ because this is a modelling parameter that can be quite different from the true propagation velocity in the ground (Al-Chalabi, 1994). Unlike conventional PreSDM, which aims at enhancing the image of the subsurface, the process in the present case was exclusively employed to produce an optimum PostSDM provelocity model. This is perhaps the most significant ‘innovation’ in the present work. It is driven by the fact that CRS is more amenable to post-stack than to pre-stack processes. Figure 2 shows the overall workflow of the approach. CRS has already established itself as an effective technique for enhancing S/N ratio. The CRS technique was introduced by Müller (1998) as a data driven zero-offset simulation method. Hubral (1999) then refined the technique and followed the strategy of a macro model independent imaging principle. The technique uses larger stacking surfaces rather than relying on a single CMP stack location as in conventional stacking processes. This improves the emergence angle and wavefront curvatures approximation, utilizing seismic wavefield and travel time information. The parameter derivations are based on second order travel time estimations which utilize multi-dimensional global maximum of the coherency function.

Surface-consistent deconvolution using reciprocity and waveform inversion

Source and receiver responses must be equalized when their behavior or coupling changes with location within a given survey. Existing surface-consistent deconvolution techniques that account for these effects assume that common-midpoint (CMP) gathering is valid — the seismic trace is decomposed into a source function, a receiver response, a normal-incidence reflectivity term, and an offset-related component that is laterally shift invariant. As a result, the performance of existing surface-consistent deconvolution techniques is best when applied to primary reflection data only, since the offset dependency of ground roll and multiples varies laterally in media with lateral variations. We have developed an alternative method for surface-consistent deconvolution that is applicable to the entire seismic trace and is therefore essentially a raw-data preprocessing step. The method is based on reciprocity of the medium response. Assuming that conditions for applicability of reciprocity are met, we can attribute differences between normal and reciprocal recordings to the source and receiver perturbations. Contrary to existing surface-consistent deconvolution methods, this approach uses the full description of the wavefield and is therefore ideally suited for prestack processing. We have applied this technique to single-sensor data acquired in Manistee County, Michigan. At this site, near-surface conditions vary, and this significantly affects data quality. The application of the new deconvolution procedure substantially improves S/N ratio on both prestack and poststack data, and these results compare favorably to those obtained using existing surface-consistent deconvolution techniques, since they require subjective data scaling to obtain acceptable results. The obtained source corrections are correlated to changes in near-surface conditions — in this case, to changes in water-saturation levels. We do not observe such a correlation for the receiver corrections, which vary rapidly along the spread. Finally, the receiver response does not agree with the generally accepted damped harmonic oscillator model. For frequencies below 100 Hz, the retrieved receiver variations are larger than predicted by this model, and we cannot explain the receiver response using a single resonant frequency for the geophone-ground coupling.

The Global Offset seismic approach: advantages and limitations

Seismic acquisition and processing in thrust belt areas represent challenging tasks in geophysics. Rough topography, geological complexity and sharp velocity variations increase the difficulties of recording seismic data of good quality and decrease the possibility of obtaining satisfactory imaging. In these circumstances, application of pre-stack depth migration (PSDM) can help in improving the imaging. However, the success of PSDM depends on the range of offset used and on the accuracy of the adopted background velocity model (Jin and Madariaga, 1994). When significant lateral velocity variations are predominant (Lynn and Claerbout, 1982), the derivation of an accurate velocity model by standard velocity analysis can fail. In these cases, accurate tomographic techniques can be extremely helpful, especially if applied to redundant data sets recorded in an extended range of offsets. In this framework, the advantages and difficulties of recording useful data at offsets larger than 15-20 km, or more, are well known from refraction seismic widely applied for studies of the middle-deep portion of the terrestrial crust. Super-critical reflections and long offset turning rays generally contain a lot of information about the elastic properties of the investigated medium. On the other hand, the risk of introducing artefacts when stacking and migrating wide angle reflections can be high, especially in the case of complex geological settings. Typical artefacts can be produced by lateral events introduced in the final migrated section, or by reflected energy migrated in the wrong position due to wrong velocity field definition. Other artefacts can be created when the wide angle reflections are not properly distinguished and separated from refracted energy. How to reduce these risks without renouncing the benefits offered by long offset data is still an open question. During the last decade many experiments have been performed using the Global Offset seismic approach (Buia et al., 2002; Colombo et al., 2004a; Colombo et al., 2004b; Colombo et al., 2003; Dell’Aversana, 2003), in order to verify the contribution offered by this methodology for improving the velocity field definition and seismic imaging. The Global Offset acquisition layout is set in order to guarantee the same shot and receiver spacing typical of the standard near-vertical reflection seismic, but for an extended range of offsets. Experiments with offsets larger than 30 km and shotreceiver density comparable with that of the industrial reflection seismic have been performed in the last few years (Dell’Aversana et al., 2001). In this sense Global Offset bridges the gap between commercial reflection seismic (commonly used for exploration in the hydrocarbon industry) and long offset refraction seismic (often applied for academic studies, focused mainly on the middle-deep portions of the terrestrial crust). Near-critical and post-critical reflections and long-offset turning rays are included in the Global Offset data set. The advantage is that the wide-angle reflections can show a high signal-to-noise ratio even in those cases where complex overburden can prevent to record good near-vertical reflection data. Moreover, using long offsets, it is possible to undershoot high velocity ‘shallow’ layers that can produce a shield effect for the propagating wave-field (Dell’Aversana et al. 2003). In general, massive application of transmission-reflection tomography is the first step aimed at producing a reliable velocity model (Dell’Aversana et al., 2003; Improta et al., 2000a, 2000b; Operto et al., 2004; Ravaut et al., 2004;). This can be successively used for improving the pre-stack depth migration process (Dell’Aversana et al, 2002). In this paper we show, with real examples, how including high-fold long offset data can really improve the seismic imaging and interpretation process. At the same time we also discuss the pitfalls that can be caused if long offset data are used indiscriminately without the necessary limitations. Finally, we explain how these problems can be controlled and avoided using a series of ‘consistency’ criteria.

Analysis of multi‐offset GPR data: a case study in a coarse‐grained gravel aquifer

ABSTRACTMulti‐offset ground‐penetrating radar (GPR) data were collected in a coarse‐grained gravel aquifer located in a glacial delta environment within dipping foresets (at the Tettnang aquifer test site, SW Germany). We apply prestack processing techniques and normal‐moveout (NMO) velocity analysis in preparation for stacking. The estimation of propagation velocities is of considerable importance for converting time‐domain radargrams into depth‐sections and for an interpretation in terms of petrophysical properties. In our case, interval velocity determination is difficult because reflector dip angles are variable, and stacking and NMO velocities differ significantly since the antenna offset is comparable to the reflector depth. Using a synthetic two‐layer model, we systematically study possible errors in interval velocity determination. We compare the cases of decreasing velocity with depth (typical for ground‐penetrating (GPR) surveys of aquifers) and increasing velocity with depth (typical for seismic surveys). In the case of decreasing velocity with depth, the discrepancy between stacking and NMO velocity is considerable and, consequently, interval velocities calculated with the Dix equation or the more accurate 2D NMO approximation for dipping layers show unacceptably high errors. The errors are much smaller in comparable models with increasing velocity with depth. Thus, the small‐offset approximation inherent in the NMO concept is not appropriate for the first case. Consequently, it is necessary to model the true common‐midpoint (CMP) raypaths to determine realistic interval velocities from our GPR field data.Stacked GPR sections of multi‐offset data show significant improvements in imaging of reflectors and depth of investigation compared with a standard constant‐offset section. This allows a more reliable interpretation of geological structures. Simultaneous acquisition of four channels, which is possible with some commercial GPR systems, yields significantly better results than constant‐offset standard acquisition without increasing the efforts for data acquisition. Major reflectors in the GPR section can be correlated with distinct contrasts of porosity, represented by the base of open‐framework gravels and sand beds within poorly sorted gravels.

The revolution in seismic resolution: High density 3D spatial sampling developments and results

Recent marine case studies have demonstrated that a significant component of the seismic "noise" contaminating 3D images actually arises during processing, as an unfortunate, and inescapable artefact from poor 3D spatial sampling. When the cross-line acquisition dimension is sampled at an equally small interval as the inline dimension, a much larger frequency bandwidth than typical of standard 3D acquisition can preserved throughout all stages of processing, free of aliasing, and free of artefacts. Hence, it is observed that once the random noise component is suppressed below a certain threshold, other factors than mere fold are clearly contributing to the quality of a seismic image. It is quite poorly established how more complicated acquisition parameters, such as multi-streamer spread dimensions and shooting templates, influence the "S/N ratio" of seismic data - particularly after the application of multichannel pre-stack processing algorithms, notably pre-stack migration.Historically, efforts at towing the source and streamer at shallower depths rather fruitlessly delivered higher dominant signal frequencies, at the cost of degraded lower frequency amplitudes, increased survey noise, and with minimal perceivable improvements in target resolution. Even if means can be found to reduce the inherent noise incurred, resolution remains frustratingly restricted, and the emphasis upon higher frequencies during acquisition was largely wasted. The solution is to sample densely in both the shot and receiver domains, particularly in the cross-line direction.Several case study examples demonstrate significant improvements in resolution and signal-to-noise content are routinely achieved by high-density seismic acquisition. Depending upon local geological conditions, high frequency amplitudes can be increased by up to 15 dB, frequency bandwidth can be doubled, 3D steep dip imaging can be significantly improved, and overall signal-to-noise ratio is improved, further contributing to better resolution. Hence, a powerful demonstration is made that tight 3D spatial sampling must be the foundation for all high resolution seismic acquisition.