EAGE workshop on Developments in Land Seismic Acquisition for Exploration

Summary not available

Vibroseis is the principal source of seismic energy in land seismic exploration. It seems that this method has been extended to its limits as we continue the search for energy resources. The vibroseis method uses the vibrator to generate a controlled force at the source point. It has been recognized that the hydraulic vibrator is not perfect and does not perform exactly as we expect. In spite of significant success using vibroseis technology in land seismic exploration, many practical issues arising from field operations have remained theoretically unexplained. Many people believe that these problems are attributed to the nonlinearities in the complex system involving the vibrator and the ground. This paper will focus on a few such examples.

In October 2008, BP awarded a large 3D Seismic Survey in Blocks 230 and 231, Algeria, to Global Geophysical Services. The objective of the survey was to acquire 2560 Sq Km of high fold seismic (495 at 4km offset) in 8 months using a technique called ISS which had never been previously deployed full scale in Algeria. A further requirement of the survey was that the recorded seismic data would be fast track processed in the field, up to and including post-stack time migration and delivered periodically to the interpreter’s workstation in a reduced time frame to enable a strategic drilling program. This paper describes the ISS technique and its implementation in Algeria where data acquisition was completed in October 2009 after just 8 months of recording operations at productivity rates some ten times greater than those previously achieved in Algeria , with a fully processed data volume delivered within 30 days of last shot point. The paper demonstrates that BP’s ISS seismic method is a viable and commercially feasible technique which enabled a large, high quality, very high fold seismic survey to be acquired in difficult terrain, and fast-tracked data delivered for Well Planning

Seismic acquisition is a trade-off between economical and quality considerations. Generally seismic data is recorded with large time delays between illuminating sources in order to avoid interference in time. The consequence is a poorly sampled shot domain. However, in the concept of blended acquisition, the time delay between sources is reduced significantly. Moreover, the sources may transmit encoded signals. Depending on the acquisition objectives, blended acquisition significantly improves the economics or quality or both by adding additional degrees of freedom in the acquisition design. By a deblending procedure, the individual source responses are retrieved. However, the deblended result contains residual noise due to the interference from other sources. The level of this noise depends on the choice of blending parameters. E.g., in the case of a simple code like linear phase encoding (i.e., applying time delays), it is larger than in the case of a more sophisticated code like transmitting sweeps as in vibroseis technology. In this paper an iterative approach is proposed for deblending, based on the estimation and subsequent adaptive subtraction of the interference noise. The type of coding that is applied is one of the factors that determines the required number of iterations.

Summary not available

This paper describes a high-productivity vibroseis acquisition and separation method termed dithered slip-sweep acquisition. It combines two techniques, slip-sweep and dithered acquisition, that have been previously, but individually, used in land vibroseis and marine acquisition, respectively. The constraints on the start time of the sweeps keep the interference noise under control such that the separation algorithms can attenuate the interference more effectively than when the vibrators sweep without synchronizing their activities. An example of dithered slip-sweep acquisition generated from a conventional dataset demonstrates the method.

Noise caused by the interference of seismic sources in simultaneous shooting is a barrier to the acceptance of this type of seismic acquisition. This paper presents a method that produces pre-stack data from blended acquisition data that are comparable to data from conventional acquisition. While filtering interference noise from the shots and the subsequent stacking of the data are effective in attenuating the interference on high fold wide azimuth ISS(TM) surveys, a high-quality separation of the interference from the pre-stack data would make the data more suitable for amplitude dependent analysis such as amplitude-versus-offset, time-lapse measurements and fracture detection. The high quality interference attenuation method presented here is achieved by using a sparse inversion process that solves a modified version of Berkhout’s matrix system, accurately separating source responses for good quality amplitude measurements. This produces seismic records in which the interference is attenuated to the point that it is well below the background noise. The success of this source separation step in the data processing of blended simultaneous source acquisition therefore further improves the data quality while retaining the lower cost and higher productivity of ISS(TM) acquisition compared with conventional acquisition.

Summary not available

The Western Desert of Egypt challenges the exploration for oil and gas both from the reservoir as well as from the surface. The reservoir rocks are often intensely faulted and fractured. Mapping these reservoirs requires high quality 3D seismic data of broad bandwidth with frequencies above 40 Hz to provide sufficient vertical resolution and lateral discrimination of reservoir features. This paper addresses the challenges of the Western Desert through the introduction of point-receiver based seismic technology. Conventional receiver arrays, which attenuate surface waves and high frequency signal, are replaced by point-receivers. Coherent noise attenuation can be carried out on point-receiver traces, whilst additional signal enhancement is taken care of in subsequent steps. The result is often a substantial increase in high frequency content of the desired signal. Reducing multiple contamination is one of the greatest challenges in land seismic processing. Demultiple algorithms and innovative workflows are presented, specifically designed to handle 3D land geometries and attack multiples in a 3D sense. In practice, these methods, relating to both surface and interbed multiples may be combined and cascaded to obtain the optimum solution. We will illustrate the benefits of point-receiver technology through examples from Egypt and the Middle East.