Low-frequency Vibroseis Research Articles

Characteristic analysis and data comparative of linear and nonlinear low-frequency sweep in vibroseis

Abstract Low-frequency seismic data plays a crucial role in seismic data processing and seismic wave inversion. At present, there are two methods to realize the low-frequency excitation of vibrators: one is exciting low-frequency vibrators by linear sweep signals, and the other is exciting conventional vibrators by nonlinear low-frequency sweep signals. The cost of exploration using low-frequency vibroseis is high, and it is challenging to obtain sufficient low-frequency information using traditional vibrators. To this end, this paper comparatively studies the low-frequency sweep signal characteristics and data effects of low-frequency and traditional vibrators. Therefore, three kinds of linear and nonlinear low-frequency sweep signal are designed. Theoretical analysis shows that there are certain differences between linear and nonlinear signal in design methods, signal shapes, etc., but after correlation calculations the signal spectra reflecting the seismic response and the related wavelet shapes are basically consistent. Besides, the actual force signal data shows that the linear and nonlinear harmonic distortion are basically equivalent. Finally, based on the forward simulation of three sweep signals and the comparative analysis of field test data, it can be considered that the linear and nonlinear low-frequency sweep signals of vibrators have almost the same denoising ability under the basic conditions of the spectrum and wavelet. Both can achieve low-frequency excitation and obtain rich low-frequency information, and the quality of seismic data is basically the same, so they can be applied in practical production.

Data processing of a wide-azimuth, broadband, high-density 3D seismic survey using a low-frequency vibroseis: a case study from Northeast China

Seismic exploration employing wide-azimuth, broadband, high-density data (i.e. double-width single-height; “double-width” means wide-azimuth and broadband; “single-height” means high-density data, generally referring to small-bin-size data (less than 10 × 10 m); and double-width single-height is an abbreviation) enables more complete wave field information to be recorded, reduces aliasing, and produces abundant low-frequency information, which is conducive to broadband processing and the anisotropic study of seismic data. Based on the acquired double-width single-height seismic data, in this paper, we analyse the wave field characteristics, signal-to-noise ratio and frequency of seismic data. We also design a procedure for processing double-width single-height seismic data. The key techniques in the proposed procedure focus on the high-resolution amplitude-preserved characteristics of double-width single-height seismic data obtained with a low-frequency vibroseis sweep. Faults and sand bodies are characterised and described by using the final imaging results, which embody the advantages of double-width single-height seismic exploration.

Experiments on excitation and data processing of low-frequency vibroseis in permafrost area of the tibetan plateau

Few seismic exploration work was carried out in Tibetan Plateau due to the characteristics of alpine hypoxia and harsh environmental protection needs. Complex near surface geological conditions, especially the signal shielding and static correction of permafrost make the quality of seismic data is not ideal, the signal to noise ratio (SNR) is low, and deep target horizon imaging is difficult. These data cannot provide high quality information for oil and gas geological survey and structural sedimentary research in the area. To solve the issue of seismic exploration in Tibetan Plateau, this test used low frequency vibroseis wide-line and high-density acquisition scheme. In view of the actual situation of the study area, the terrain, the source and the different observation system were simulated, and the processing technique was adopted to improve the quality of seismic data. Low-frequency components with a minimum of 1.5Hz of vibroseis ensure the deep geological target imaging quality in the area, the seismic profile wave group is clear, and the SNR is relatively high, which can meet the needs of oil and gas exploration. Seismic data can provide the support for the development of oil and gas survey in the Tibet plateau.

Research of Using Geophysical method to detect Natural Gas Hydrate-Application in Hala Lake area of Qinghai Province

Hara Lake is located in the south of Qilian Mountains. According to the previous geological survey in this area, the study area has a good prospect of gas hydrate prospecting. As a new type of clean energy, natural gas hydrate is seldom explored in our country. In order to find gas hydrate, two-dimensional reflection seismic exploration is carried out in this area, and the props are selected. Multiple coverage observation system and low frequency vibroseis. High quality 2-D seismic data are obtained by reflection seismic exploration, and the characteristics of velocity inversion anomaly and low frequency amplitude of gas hydrate are obtained, which provides a theoretical basis for gas hydrate acquisition.

Low-frequency Vibroseis: current achievements and the road ahead?

Extending the seismic signal frequency towards low frequencies has become almost standard on seismic projects, owing to the benefits it provides in terms of vertical resolution, signal penetration, inversion workflow results or ease of interpretation. Vibroseis is preferred to explosives not only for its productivity, but also for the control it allows over frequencies generated. While Vibroseis equipment has been improved in recent years, discrepancies are observed in the equipment and methods used, in particular in the design of low-frequency sweeps. A desire for lower frequencies seems also to guide the development of some vibrators, at the expense of technical complexity, higher cost and reduced performance at higher frequencies. However, if the need for broader frequencies is not questioned, the right level of low-frequency energy necessary for imaging and interpretation purposes is still not clearly defined by the industry, at the risk that the current focus for more powerful lower frequencies could be a temporary trend. This paper discusses achievements in equipment and methods, and speculates on the future of low-frequency Vibroseis.

On the generation of low frequencies with modern seismic vibrators

Acquiring low-frequency seismic data using Vibroseis techniques has gained attention in recent years. Successful application of low-frequency Vibroseis acquisition requires evaluating each element of the data acquisition system and ensuring that each part of the system contributes to the success of the method. We focus on generating low frequencies with Vibroseis sources rather than recording and preserving them. To generate the low-frequency signals using seismic vibrators, it is better to know (1) the hydraulic and mechanical system of the vibrator; (2) the design of a low-frequency sweep that can deliver the vibrator ground force maximally; and (3) the vibrator control system that can minimize the harmonic distortion of the vibrator ground force output at low frequencies. Field tests show that hydraulic pump flow is not a limiting factor for the generation of the low frequencies for most modern seismic vibrators. In addition, with harmonic distortion reduction control, the vibrator performance at low frequencies is improved.

A Geophysical Investigation of Shallow Deformation Along an Anomalous Section of the Wasatch Fault Zone, Utah, USA

We report the results of a geophysical study of the Wasatch fault zone near the Provo and Salt Lake City segment boundary. This area is anomalous because the fault zone strikes more east-west than north-south. Vibroseis was used to record a common mid-point (CMP) profile that provides information to depths of ∼500 m. A tomographic velocity model, derived from first breaks, constrained source and receiver static corrections; this was required due to complex terrain and significant lateral velocity contrasts. The profile reveals an ∼250-m-wide graben in the hanging wall of the main fault that is associated with both synthetic and antithetic faults. Faults defined by apparent reflector offsets propagate upward toward topographic gradients. Faults mapped from a nearby trench and the seismic profile also appear to correlate with topographic alignments on LiDAR gradient maps. The faults as measured in the trench show a wide range of apparent dips, 20°–90°, and appear to steepen with depth on the seismic section. Although the fault zone is likely composed of numerous small faults, the broad asymmetric structure in the hanging wall is fairly simple and dominated by two inward-facing ruptures. Our results indicate the feasibility of mapping fault zones in rugged terrain and complex near-surface geology using low-frequency vibroseis. Further, the integration of geologic mapping and seismic reflection can extend surface observations in areas where structural deformation is obscured by poorly stratified or otherwise unmappable deposits. Therefore, the vibroseis technique, when integrated with geological information, provides constraints for assessing geologic hazards in areas of potential development.