Shear Wave Seismic Data Research Articles

Reverse Time Migration Imaging Using SH Shear Wave Data

In this paper, we discussed the reverse time migration imaging of compressional wave (P-wave) and horizontally polarized shear wave (SH shear wave) seismic data, together with P- and SH shear wave constrained velocity model building. In the Sanhu area in Qaidam Basin, there are large areas of gas clouds, which leads to poor P-wave seismic imaging. The P and SH shear wave seismic data of a co-located survey line with the same acquisition geometry were used to access their imaging capability using reverse time migration. We first estimated the change in P-wave and SH shear wave velocity ratio using pre-stack time migration (PSTM) for constraining the overall depth domain velocity model. Additionally, we then used an eighth-order finite difference scheme for P-wave reverse time migration on a variable grid and used the sixth-order combined compact difference (CCD) wave field simulation method for SH shear wave reverse time migration on a regular grid. The results show that the constrained velocity model produces a good match in the overall geological structure shown in the P-wave and SH shear wave reverse time migration results. However, in the gas cloud areas, SH shear wave reverse time migration has obvious imaging advantages, which can clearly image the structure inside the gas clouds.

High-resolution shear-wave seismics across the Carlsberg Fault zone south of Copenhagen — Implications for linking Mesozoic and late Pleistocene structures

The Carlsberg Fault zone (CFZ) is a NNW-SSE striking structure close to the transition zone between the Danish Basin and the Baltic Shield. We examine the fault evolution by combining very-high-resolution onshore shear-wave seismic data, one conventional onshore seismic profile and marine reflection seismic profiles. The faulting geometry indicates a strong influence of Triassic subsidence and rifting in the Central European Basin System. Growth strata within the CFZ surrounding Höllviken Graben reveal syntectonic sedimentation in the Lower Triassic, indicating the opening to be a result of Triassic rifting. In the Upper Cretaceous growth faulting documents continued rifting. These findings contrast the Late Cretaceous to Paleogene inversion tectonics in neighboring structures, such as the Tornquist Zone. The high-resolution shear-wave seismic method was used to image faulting in Quaternary and Danian layers in the CFZ. The portable compact vibrator source ElViS III S8 was used to acquire a 1150m long seismic section on the island Amager, south of Copenhagen. The shallow subsurface in the investigation area is dominated by Quaternary glacial till deposits in the upper 5–11m and Danian limestone below. In the shear-wave profile, we imaged the uppermost 30m of the western part of CFZ. The complex fault zone comprises normal block faults and one reverse block fault. The observed faults cut through the Danian as well as the Quaternary overburden. Hence, there are strong indicators for ongoing faulting, like mapped faulting in Quaternary sediments and ongoing subsidence of the eastern block of the CFZ as interpreted by other authors. The lack of earthquakes localized in the fault zone implies that either the frequency of occurring earthquakes is too small to be recorded in the observation time-span, or that the movement of the shallow sub-surface layers may be due to other sources than purely tectonic processes.

Sinkholes in the city of Hamburg—New urban shear-wave reflection seismic system enables high-resolution imaging of subrosion structures

Abstract The investigated roof region of a salt diapir in Hamburg, northern Germany, suffers sinkhole activity that was accompanied lately by microseismic events in the Gross Flottbek quarter. Thus, a high geohazard potential is present which can only be evaluated if highly resolved structural data are available. The urban environment and high building density required adapted and new, non-invasive geophysical methods for shallow applications (e.g., urban geophysics). Our shear-wave seismic system, under development at LIAG, consists of a small, horizontal vibrator source (ELVIS, v. 5.0) and a 120 m long, mobile land streamer equipped with 120 SH-geophones. Thereby, a cost-effective, complete shear-wave seismic survey is possible, which is dedicated to urban applications and accounts for sealed surfaces. This high-resolution system is especially useful on sealed terrain because of the absence of suppressed (Love) surface waves, and it provides in the near surface an up to 10-times higher resolution than a compressional-wave survey. Across the Wobbe See sinkhole in Gross Flottbek, we acquired 500 profile m of high-resolution shear-wave seismic data that enabled urban subsurface imaging with 1 m vertical and 5–10 m horizontal resolution. Small-scale structures in the sediments and salt are resolved down to 100 m depth. We show that it is possible (1) to classify a sinkhole type structurally and (2) to map subrosion areas by the physical property of lowered shear-wave velocities and a chaotic reflection character. Collapse depth is found here at ca. 60–80 m depth below surface, and the fault structures observed coincide well with the focal depth and mechanism of the 2009 microseismic events. Additionally, the diapir model is refined locally in terms of rock salt depth suggested here lying as shallow as 80–100 m below surface.

Response of a proglacial delta to rapid high-amplitude lake-level change: an integration of outcrop data and high-resolution shear wave seismics

In this paper, we will present the stratigraphic evolution, internal facies architecture and geomorphology of the Middle Pleistocene Emme delta, controlled by rapid high-amplitude lake-level change. The Emme delta was deposited on the northern margin of glacial Lake Weser, located in north-west Germany. Rates of lake-level rise were probably >50 mm year−1 and rates of lake-level fall 30–50 m within a few days or weeks, due to the opening of lake outlets. We use digital elevation models, sedimentology and shear wave seismics to improve earlier reconstructions and investigate the influence of rapid base-level change on delta development. Shear wave seismic data resolve architectural elements in the range of metres and bridge the common gap between outcrop and conventional compression wave seismic data. The radial delta complex is about 2 km long, 1.8 km wide and up to 70 m thick, overlying a concave, up to 13° steep dipping ramp surface. It consists of vertically and laterally stacked delta lobes, caused by lobe switching during base-level change. During the lake-level rise, vertically stacked (Gilbert-type) delta systems formed. The decrease in thickness and lateral extent indicates a rapid upslope shift of depocentres. A high rate and magnitude of lake-level fall (50 m) promoted the development of a single incised valley and the deposition of forced regressive coarse-grained delta lobes in front of the valley. The incised valley was filled during decreasing rates of lake-level fall and low base-level, because the alluvial gradient was larger than the emergent lake profile. Attached sand-rich forced regressive aprons formed during lower magnitudes of lake-level falls in the range of 30–35 m. Valley incision occurred, but was limited to the uppermost portion of the delta, controlled by the steep slope. The incised valley related to the final lake drainage is associated with long-wavelength (60–90 m) bedforms at the downslope end, attributed to the formation of standing waves as a result of a hydraulic jump. Estimated palaeoflow depth during standing wave formation was ∼9–14 m and flow velocity was 10–12 m s−1. Because subsidence, waves or tides did not play a major role, the Emme delta can be used as an analogue-based predictive stratigraphical and sedimentological model for steep glacigenic deltas controlled by rapid base-level change and can help to understand better the facies distribution and three-dimensional geometry of these depositional systems.

Exploration methods for late Quaternary shallow biogenic gas reservoirs in the Hangzhou Bay area, eastern China

Commercial accumulations of shallow biogenic gas have been widely found in the world. The successful development and exploitation of shallow gas is dependent on using suitable technology to effectively prospect for this kind of resource. Shallow biogenic gas reservoirs, buried less than 120 m (393.7 ft), have been discovered and successfully exploited in the Hangzhou Bay area, northern Zhejiang Province, eastern China. In this study, we describe the methods used for the exploration of shallow gas, including cone penetration test (CPT), shallow shear-wave seismic data, soil-gas radon analysis, microbiological prospecting, and electromagnetic surveying. The CPT is effective in helping to determine stratigraphic divisions and correlations, especially reservoir identification. Dense CPTs are useful for both the preliminary exploration and enlarging a known gas field. Shallow shear-wave seismic profiles identify the top surface of a gas-bearing sand bed, which shows a strong reflecting boundary. The reflection will sharply decline where the gas-bearing sand body pinches out. Thus, a gas-bearing sand body can be outlined. The area of gas reservoirs can further be determined by high soil-gas radon content over the boundary of the gas accumulation. Methane-consuming bacteria can act as an indicator for the presence of methane within the gas-bearing area. Electromagnetic methods can aid in determining whether there is gas in the sand bodies and in determining the thickness of the gas layer.

Constraints on the composition of the crust and uppermost mantle in northwestern Canada: Vp/Vs variations along Lithoprobe's SNorCLE transect

Shear-wave seismic data recorded along four profiles during the SNoRE 97 (1997 Slave – Northern Cordillera Refraction Experiment) refraction – wide-angle reflection experiment in northwestern Canada are analyzed to provide S-wave velocity (Vs) models. These are combined with previous P-wave velocity (Vp) models to produce cross sections of the ratio Vp/Vs for the crust and upper mantle. The Vp/Vs values are related to rock types through comparisons with published laboratory data. The Slave craton has low Vp/Vs values of 1.68–1.72, indicating a predominantly silicic crustal composition. Higher values (1.78) for the Great Bear and eastern Hottah domains of the Wopmay orogen imply a more mafic than average crustal composition. In the western Hottah and Fort Simpson arc, values of Vp/Vs drop to ∼1.69. These low values continue westward for 700 km into the Foreland and Omineca belts of the Cordillera, providing support for the interpretation from coincident seismic reflection studies that much of the crust from east of the Cordilleran deformation front to the Stikinia terrane of the Intermontane Belt consists of quartzose metasedimentary rocks. Stikinia shows values of 1.78–1.73, consistent with its derivation as a volcanic arc terrane. Upper mantle velocity and ratio values beneath the Slave craton indicate an ultramafic peridotitic composition. In the Wopmay orogen, the presence of low Vp/Vs ratios beneath the Hottah – Fort Simpson transition indicates the presence of pyroxenite in the upper mantle. Across the northern Cordillera, low Vp values and a moderate-to-high ratio in the uppermost mantle are consistent with the region's high heat flow and the possible presence of partial melt.

Tullich Field

Tullich Field, operated by Kerr-McGee Oil (UK) plc with 100% interest, lies in UK block 9/23a, to the southeast of Gryphon, and to the east of the Harding fields. The field is a subsea tie-back from a central manifold to the Gryphon ‘A’ FPSO (floating production, storage and offloading) facility. The southern part of the field was discovered by Hamilton in 1991–1992. Well 9/23a-27 and its sidetrack encountered thin oil and gas bearing sandstones of Lower Eocene age from the Balder Formation, which tested 570 bpd of 25.8 degree API oil. Kerr-McGee recorded a 3D seismic survey over the area in 1990, and in 1999 acquired a 3D OBC seismic survey over the Gryphon Field and the northern part of block 9/23a. The similarity of the inverted shear wave seismic data over the Gryphon Field and the northern part of block 9/23a was the catalyst for the further exploration and appraisal of the area. This led to an exploration drilling programme in 2001, which included the acquisition of full waveform shear sonic data and core. In July 2001, a separate discovery was made by Kerr-McGee with the drilling of 9/23a-29A, to the north of the 9/23a–27 wells. Thick oil bearing sandstones of excellent reservoir quality were encountered in the Balder Formation. The field was delineated between July and December 2001 by sidetracking 9/23a–29A and drilling the 9/23a–31 well. From a windowed extraction of peak amplitudes of the shear seismic data, it was noted that at Gryphon the high amplitudes corresponded to the presence of thick reservoir sandstones, whilst at Tullich it appeared that the high amplitudes related to a non-reservoir argillaceous sandstone sequence rather than the much thinner reservoir sandstones. These reservoir sandstones lie at different stratigraphic levels in the B2 unit of the Balder Formation but appear to be sufficiently in communication to act as a single reservoir unit. They are currently interpreted as sandstones deposited by turbidity currents which pinch out against the plunging nose of the Crawford Ridge. In order to take advantage of synergy with other work in the Gryphon area, a tight timeframe was imposed on the project. A four-well development drilling programme began immediately once the Department of Trade and Industry had given their approval of the Field Development Plan (FDP) in March 2002 and was completed by October 2002, with first oil achieved at the end of August 2002.

Shear Wave VSP in Amarume Oil Field

Japan National Oil Corporation (JNOC) acquired the following shear wave seismic data around the TRC-AMR1 (JNOC research well) in the Amarume oil field which is located in the northern part of Japan: 1) 1989 Shear wave VSP by a single, coupled source 2) 1990 Shear wave surface seismic by a single, coupled source 3) 1992 Shear wave VSP by dual coupled sources Rotation analysis for VSP data of the first survey showed remarkable shear wave splitting. The surface seismic data of the second survey did not show this effect. In both surveys, the motion of the shear wave source was limited to a direction perpendicular to the source-receiver line. In general, rotation analysis requires data from dual coupled sources whose motions are perpendicular to each other. The last VSP data (3) were acquired in order to investigate the conflicting results of both surveys. Data from this survey confirmed the existence of an anisotropic layer shallower than 700 m. The direction of anisotropy was found to be almost east-west, that is, in line with the receiver direction of the second survey. It was concluded that this was the reason why the second survey did not show shear wave splitting. The time difference between S1-wave (the faster shear wave) and S2-wave (the slower one) was over 100 ms at 700 m depth. This survey also detected the location of a gas reservoir at around 870 m depth.

Geophysical evidence for fluids in the crust beneath the Black Forest, SW Germany

We summarize and discuss the evidence for fluids in the deep crust in the Black Forest, SW Germany. Joint interpretation of wide-angle and near-vertical compressional and shear wave seismic data provide a valuable tool for the detection of fluids in the crystalline crust. Together with other geophysical informations — conductivity, thermal properties and an established petrological model, we propose a model that interpretes a pronounced low-velocity zone at mid-crustal level as a layer enriched in fluids and influenced by retrograde metamorphism because of these fluids. The water is expelled during prograde metamorphism in the lower crust caused by intrusions of hot basaltic material related to Tertiary rift processes.

Method of seismic exploration including processing and displaying shear seismic data

A method of seismic exploration including processing and displaying shear wave seismic data for identifying gaseous hydrocarbon-containing formations and for inferring changes in the geological character of subterranean formations. A measure of the shear wave reflection coefficient is obtained for selected seismic events in the shear wave seismic data. Measures of formation property contrasts represented by the selected seismic event are obtained from the shear wave reflection coefficients.

Comparison of P- and S-Wave Seismic Data: A New Method for Detecting Gas Reservoirs

Compressional waves are sensitive to the type of pore fluid within rocks, but shear waves are only slightly affected by changes in fluid type. This suggests that a comparison of compressional- and shear-wave seismic data recorded over a prospect may allow an interpreter to discriminate between gas-related anomalies and those related to lithology. This case study documents that where a compressional-wave 'bright spot' or other direct hydrocarbon indicator is present, such a comparison can be used to verify the presence of gas. In practice, the technique can only be used for a qualitative evaluation. However, future improvement of shear-wave data quality may enable the use of more quantitative methods as well.

A case study of stratigraphic interpretation using shear and compressional seismic data

This paper describes the use of surface recorded compressional and horizontal shear wave seismic data to detect lateral changes in the physical properties of a clastic unit. Shear and compressional wave transit times were measured across a selected interval from CDP stacked sections derived from data collected along coincident shear and compressional seismic lines. At each surface position the ratio of the shear to compressional transit time across the target horizon is calculated. It is shown that lateral variations in this ratio, coupled with the behavior of the individual transit time curves, can be used to infer changes in the physical properties of a formation. The horizon selected for this case study was the lower Pennsylvanian Morrow formation which produces gas from channel sand bodies at the Empire Abo field, New Mexico. A detailed geologic section of the producing horizon was mapped along a seismic line oriented so that it crossed productive and nonproductive regions of the field. Shear and compressional Vibroseis® surveys were conducted along this surface profile using data acquisition parameters designed to produce comparable signal‐to‐noise ratios and resolution in both sets of field data. After processing, the shear and compressional interval transit times through the Morrow formation decreased in going from nonproductive to productive thicknesses of sand. Furthermore there is a proportionately greater decrease in the shear wave transit time than in the compressional transit time resulting in an overall decrease in the ratio of shear to compressional transit times. While several possible physical changes in the lateral properties of the reservoir could explain these observations, it is concluded that the primary mechanism causing these ratio changes is variation in the sand‐shale ratio within the Morrow formation.