Joel D. Brewer

Load Cell System Test Experience: Measuring the Vibrator Ground Force On Land Seismic Acquisition

Many methods have been developed in an attempt to expand the recorded bandwidth associated with Vibroseis surveys, particularly to boost the signal at high frequencies for improving seismic image resolution. However, the dominant frequency band still remains in the range of 10– 60Hz and no significant improvement has been achieved at high frequencies in conventional 3D seismic surveys. Ground attenuation is one of possible limitations, and another is that during the course of the seismic acquisition, the vibrator may not actually be generating as much energy at high frequencies as expected. In this paper the letter possibility is addressed by using a Load Cell System to measure the force actually being generated by the servo hydraulic vibrator. We find that these measurements are not consistent with the drive signal from the vibrator electronic controller (weighted-sum ground force estimate) therefore the conventional weighted-sum ground force signal is deemed questionable.

A Decade of Field Trials in the Panhandle of Oklahoma and the Development of ZenSeis

Summary ConocoPhillips has spent the last decade in the field developing a high production Vibroseis acquisition technique based upon the concepts of point source, point receiver acquisition combined with high fold and simultaneous sourcing. This technology is the result of many field experiments and production 3D’s. The methodology has now been used to acquire over one thousand square miles of very high fold 3D seismic data. The purpose of our paper is to document some of the field experiments and touch upon the lessons and conclusions learned in the field. This paper will also set the stage for additional papers that will deal with the analysis of the test results and the production 3D’s that were acquired with this new method.

Geologic-to-seismic Modelling for Eldfisk SOA Reservoir Characterization - An Integrated Study

Seismic imaging for the Eldfisk SOA requires a good understanding of overburden geology and detailed reservoir structure. In Eldfisk, we found that, based on well logs and seismic amplitude attributes, gas zones represented as pancake-shaped geobodies in the overburden, produce significant time sags in modeled seismic sections that are similar to sags observed on field datasets. Amplitude decays observed on the acquired seismic data also suggest that the pancake-shaped geobodies are heterogeneous, causing scattering and energy loss. The main contribution of this paper is to develop a workflow to convert geologic modeling property (e.g. porosity) to seismic velocity for 3D seismic modeling and RTM imaging. High-resolution seismic modeling and imaging are important for well planning and field development at Eldfisk. The integrated velocity model contain both geobodies constructed from the overburden and detailed geologic features such as small faults in the reservoirs. Results from the full 3D RTM and decimation studies have illustrated that receiver spacing greater than 300x300 m would produce significant artifacts in the reservoirs. Target-oriented visibility studies based on the integrated velocity model suggested that dominant energy around ~5-10km may provide imaging uplift to reservoir structures in the Eldfisk SOA.

Vibroseis Source Signature Uncertainty and its Impact on Simultaneous Sourcing

Knowledge of the source wavelet in a controlled source seismic experiment significantly improves our ability to extract information from the resulting seismic data. The radiated source signature in Vibroseis field experiments is found to deviate from the pilot sweep and the ground force estimate (GFE) signal put out by the controller, especially at higher frequencies. The Vibroseis source signature uncertainty is a problem for simultaneous sweeping techniques that require reliable phase control and an accurate GFE in order to separate simultaneous sweeps.

Validating land data quality of simultaneous multiple vibrator acquisition

Summary In 2007, ConocoPhillips conducted field experiments designed to evaluate the data quality of multi-offset VSPs acquired by a single vibrator and simultaneous multiple vibrators. To check the repeatability of vibrator sources, we recorded 8 repeated sweeps at the same source location for both acquisitions. The data quality is consistent from sweep to sweep at the same source location showing good repeatability of vibrator sources. Inverting 8 repeated sweeps simultaneously by a least-squares approach produces a solution that is very comparable to an average solution derived from inverting each sweep separately. In some cases, the least-squares solution tends to handle the ambient noise better and gives a slightly better solution than the average solution. The analyses of downgoing and upgoing VSPs demonstrates that simultaneous multiple vibrator acquisition yields equivalent data quality when compared with a single vibrator and cross-talk artifacts generated by simultaneous multiple vibrators are minimal in this case.

Modelling and Imaging Through a Gas-Obscured Zone in Malaysia

Summary We demonstrate using viscoelastic modeling followed by dual velocity RTM imaging that ocean bottom node (OBN) survey is needed in a gas-obscured zone in Malaysia. Scattering caused by “geobodies” filled or partially filled with gas will distort the P-wave propagation. S-wave, on the other hand, is less sensitive to the geobodies, and therefore has a good chance for imaging through gasobscured zones. However, when intrinsic Q (or attenuation) is present in the overburden, the ability of PSconverted wave imaging will be degraded. The reliability and resolution of PS-converted wave imaging also depends on the accuracy of velocity models used for RTM imaging.

Improved quality control of vibroseis sources using vibrator control system generated files

Increased productivity of Vibroseis crews has made it increasingly difficult to perform detailed quality control analysis on the sources on a daily basis. Many of the new vibrator control systems available provide tools to help fully QC the sources and gain an understanding of surface conditions through files that are generated and output on a daily basis. Using these files it is easy to track vibrator attributes and statistics. The files can also be used to track the timing of the crew down to the second and understand the efficiency of the crew’s operations. These files provide a simple tool to help perform quality control analysis on the vibrators.