Far-offset detection of normal modes and diving waves: a case study in Valhall, southern North Sea

The use of permanent arrays for continuous reservoir monitoring has become a reality in the past decades, with Ekofisk and Valhall being its flagships. One of the possibilities when such solution is available is to passively record data while acquisitions with an active source are ongoing in nearby areas. These recordings might contain ultrafar-offset data (over 30 km), which are hardly used in standard reservoir exploration and monitoring, as they are mostly a combination of normal modes, deep reflections and diving waves. We present here data from the Valhall Life of Field Seismic array, recorded while an active seismic survey was being acquired in Ekofisk, in April 2014. Despite the lack of control on source firing time and position, analysis of the data shows that the normal modes are remarkably clear, overcoming the ambient noise in the field. The normal modes can be well explained by a two-layer acoustic model, while a combination of diving waves and refracted waves can be fairly well reproduced with a regional 1D velocity model. We suggest a method to use the far-offset recordings to monitor changes in the shallow sediments between source and receivers, both with and without a coherent seismic source in the area.

Technique for the imaging crystalline basement according to the DSS data

The method of deep seismic sounding (DSS), the observation systems in which are characterized by an irregular arrangement of both sources and receivers along the profile, a significant step between receivers, as well as maximum source-receiver distances exceeding several hundred kilometers, makes it possible to obtain an image of the crystalline basement using seismic migration fields of reflected/refracted waves. The main part of the existing migration methods, the use of which makes it possible to form an image of the deep structure of the study area in the dynamic characteristics of the recorded wave field, is focused on processing seismic data obtained by the method of reflected waves with multiple overlap observation systems (MOV—CDP). And, as a rule, these migration methods are designed for a smooth change in speed with depth. At the same time, at the boundary of the crystalline basement, the speed changes very sharply, which must be taken into account when processing data using migration. The proposed method for constructing an image of the crystalline basement is based on the use of finite-difference migration of the field of reflected/refracted waves, which was developed at the Institute of Geophysics named after S. I. Subbotin National Academy of Sciences of Ukraine. This migration method is designed to isolate supercritically reflected and refracted waves recorded from the basement in the far zone of the source and takes into account the full trajectory of waves passing through a two-layer medium, at the boundary of which there is a significant jump in velocity. Thus, the migration of the field of reflected/refracted waves makes it possible to obtain a correct image of the structure of the refractive layer of the crystalline basement. The article describes in detail the algorithm of the technique for constructing an image of the crystalline basement using finite-difference migration of the field of reflected/refracted waves and its difference from similar methods of migration. The advantages and disadvantages of the proposed method are shown when solving problems of regional seismic research. Explained and illustrated the features of constructing the image of violations on the border of the foundation. The effectiveness of the technique is demonstrated on a model example and real seismic data observed by the DSS method on the territory of Ukraine.

Results of studying the azimuthal anisotropy of the paleozoic basement with vsp technique at the Bystrovka vibroseismic test site

At the Bystrovka vibroseismic test site (Novosibirsk region) 3-component refracted waves profiling was performed at three intersecting lines. Shear waves analysis made possible to detect anisotropy of the Paleozoic basement occurring at depth of about 10 m and to suggest symmetry elements of this medium along with their orientation. Compressional waves data were used to construct depth sections estimating head waves velocities. These velocities demonstrate significant variation in lines of different orientation. The results obtained agree with previously performed VSP.

Research on the influence of bubble curtains on shock wave fields of the underwater explosion

Shock waves, which derive from explosions, generate reflected and refracted waves when propagating in the layered medium with various acoustic stiffness. Depending on the acoustic characteristic of each layer of the medium, properties of reflected and refracted waves will increase or decrease pressures/stresses at the investigated point of medium, compared to influences of explosive shock waves (incident waves) propagated in a homogeneous and isotropic medium. Based on this mechanical physical property, scientists have studied a diversity of solutions decreasing effects of explosive shock waves in various medium such as rock and soil, water, air. However, currently there have not been any comprehensive theoretical studies on the reduction in intensity of the underwater explosion shock wave when interacting with bubble curtain. By using the analytical method and the virtual explosive method, the paper presents the propagation rule of new waves formed when the underwater explosion shock wave interacts with the bubble curtain. The results showed that the more the thickness of the bubble curtain or the higher the bubble content or the longer the distance from the explosive to the curtain, the weaker the intensity of the shock wave when passing through the curtain.

Comparison of estimates parameters of earthquakes in the Kamchatka region from different catalogs

Для землетрясений Камчатского региона и землетрясений в районе западной части Алеутско – Командорского желоба рассчитаны невязки между временем пробега волн P исходя из оценки времени в очаге и временем пробега в соответствии с моделью Земли AK135. Выявлено систематическое расхождение значений времен пробега, полученных этими способами, причем величина невязки меняется в зависимости от применяемого метода оценки параметров гипоцентров. Результаты сравнения отражают реальную оценку точности координат гипоцентров в каталогах. Проверка качества привязки сейсмических трасс к точному времени показало наличие сбоев синхронизации на сейсмограммах почти всех комплектов сейсмографов, что может быть одной из причин невязок. he article presents the results of work with such devices as the digital engineering seismic station «TELSS — 3» (LLC «Geosignal»), complete with seismic streamer SM — 20 and seismic receivers GS — 20 DX in order to clarify soil conditions in the valley of the Avacha river in Yelizovsky urban district. The method of refracted waves of seismic exploration was performed for the first time. It is important to note the importance of the seismic station for obtaining primary acoustic data for further refinement of seismic increments. Thanks to the data obtained, it becomes possible to make the right decisions when designing construction objects.

STUDY OF NEAR-SURFACE ANISOTROPY FOR THE KLUCHI CITE BY REFRACTED WAVES SUTVEYING

Refracted waves are often used in neat-surface exploration. A limited observation system in the form of unidirectional profiles is often used. In our work, we selected an object with a known anisotropic upper part of the cross-section. The results of refracted waves processing show the anisotropy of one of the layers of the medium, the azimuthal anisotropy of which led to the observation that the refraction on its top ceases from the first arrivals, for the direction of the profile aligned with the axis of symmetry of the azimuthally anisotropic layer.

SEISMIC MODEL OF THE UPPER EARTH`s CRUST ON NORTH-EAST PART OF TRAVERSE 3-DV BASED ON RESULTS OF HEAD WAVES DIGITAL PROCESSING

Digital processing of refracted waves data, which are registered on North-East part of profile3-DV, is done. Time sections and velocity model are formed. It is proved, that refraction horizons on depth interval of 0-1.5 km are geologic boundaries in sedimentary cover on Ayan-Yuryakh tectonic block. Refraction boundary on depth of ~1.0 km is not lithologic border on Inyaly-Debin block. Layers, which are potentially productive for ore mineral resources, are substracted by low values of V/V (1.66-1.70) on depth below 1.0 km, on Inyaly-Debin block and Orotukan-Balygychan elevation.

Shear and compressive wave data acquisition using multicomponent vibrating source and landstreamer

<p>In the vicinity of Ottawa, Ontario, Canada, we have recorded many multicomponent seismic data sets using an in-house multicom­ponent vibrator source named Microvibe and a landstreamer receiver array with 48 3-C 28-Hz geophones at 0.75-m intervals. The receiver spread length was 35.25 m, and the near-offset was 1.50 m. We used one, two or three source and three receiver orientations — vertical (V), inline-horizontal (H1), and transverse-horizontal (H2). We identified several reflection wave modes in the field records — PP, PS, SP, and SS, in addition to refracted waves, and Rayleigh-mode and Love-mode surface waves. We computed the semblance spectra of the selected shot records and ascertained the wave modes based on the semblance peaks. We then performed CMP stacking of each of the 9-C data sets using the PP and SS stacking velocities to compute PP and SS reflection profiles.</p><p>Despite the fact that any source type can generate any combination of wave modes — PP, PS, SP, and SS, partitioning of the source energy depends on the source orientation and VP/VS ratio. Our examples demonstrate that the most prominent PP reflection energy is recorded by the VV source-receiver orientation, whereas the most prominent SS reflection energy is recorded by the H2H2 source-receiver orientation with possibility to obtain decent shear wave near surface data in all other vibrating and receiving directions.</p><p>Pugin, Andre and Yilmaz, Öz, 2019. Optimum source-receiver orientations to capture PP, PS, SP, and SS reflected wave modes. The Leading Edge, vol. 38/1, p. 45-52. https://doi.org/10.1190/tle38010045.1</p>

On the efficiency of integrated analysis of parameters of ref- racted and reflected waves in prediction of oil-gas bearing of section

Based on the comparative analysis of results of researches carried out via the methods of reflected and refracted waves with the purpose of prediction of geological section, the paper justifies that one of the major evidences in the field of seismic wave in oil-gas fields of geological section is the differentiation of elastic properties of oil-gas bearing reservoirs from the environment. Based on mutual comparison of results of conducted researches, it is shown that in the prediction of geological section the integ- rated analysis of changes of dynamic parameters of the waves reflected from the surface of oil-gas bearing reservoir and transmitted through this surface may further increase the reliability of obtained results. The research surveys carried out in Naftalan-Shimali Naftalan areas via the methods of reflected and refracted waves were simultaneously analyzed. With the aim of comparative analysis, the deposit type anomalies revealed through the method of refracted waves were compared to the results of attributed analysis of seismic cross-sections obtained with the method of common depth point. The correspondence of the signs associated with the oil-gas bearing content in change of dynamic parameters of refracted and reflected waves has been specified. With the purpose of prediction of geological section, a complex research is proposed.