Exploration Geophysics - Volume 42, Issue 2, 2011

P-wave reflection-statics solutions typically incorporate P-wave refraction data, derived from the first breaks of the production data. Similarly, converted-wave refractions, taken from inline-component recordings, can be exploited to yield S-wave receiver statics, required in the processing of converted-wave reflection data. This methodology requires extensions to well known P-wave refraction analysis methods. This paper outlines extensions of the slope-intercept method and the reciprocal method, required to analyse converted-wave refractions. We discuss the computation of S-wave time-depths and describe how the observed ratio of S-wave to P-wave time-depths can provide a useful estimate of the near-surface VP/VS ratio, which is of interest in the analysis of engineering rock strengths.

We also include discussion of several related practical issues, with particular reference to dynamite sources. When the source is buried in the refractor, the required reciprocal times cannot be directly measured from the raw travel-time data. They can, however, be easily derived via correction using measured intercept times. Often converted-wave refractions are of poorer quality than conventional P-wave refractions, such that reversed refractions may not be available over some parts of the spread. In this situation, the preferred time-depth quantity cannot be computed. However, delay-times derived from single-ended data can be substituted, particularly if lateral variations in refractor velocity are allowed for.

The concepts outlined here are used in a companion paper to correct S-wave receiver statics in a coal-scale dataset from the Bowen Basin in central Queensland.

Converted-wave refraction statics is an algorithm that incorporates both P-wave and S-wave refraction events to correct S-wave static errors in multicomponent seismic data. Conventional (PPP) and converted (PPS) refractions are picked on vertical and inline-horizontal shot records respectively. These picks are then analysed using the reciprocal method to create a near-surface model from which S-wave receiver statics are derived.

The derived PPS refraction statics have a similar short-wavelength character to S-wave statics obtained via statistical analysis of converted-wave reflections. Based on standard P-wave practice, we believe that an optimal production approach will include converted-refraction analysis, followed by converted-wave residual statics.

Although the thrust of this work has been towards derivation of S-wave statics, an interesting auxiliary output is also available. Based on theoretical modelling, the observed S-to-P time-depth ratios can be tuned to provide P-to-S velocity ratios (and hence dynamic Poisson’s ratios) for the near-surface. This has interesting implications for lithological and rock strength analysis in mining, geotechnical and environmental investigations.

The volume of shale calculation based on naturally occurring gamma rays frequently overestimates shale volume when radioactive material other than shale is present, for example where sand appears to be shale. In this situation, shale volume calculations from other methods are highly recommended in order to avoid overestimation or underestimation of shale volume. This paper introduces an equation relating shale volume to porosity logs (neutron, density and acoustic logs), which takes into account the effect of matrix, fluid and shale parameters. This equation, which is based on the effective porosity definition and the Dresser Atlas (1982) equation, has been successfully applied to many shaly formations, regardless of the type and distribution of shale. Solved examples are used to test and compare this equation and the results come close to what actually exists, with the amount of error ranging from –5 to +5%.

The advantages of the proposed equation can be summarised as: (1) it is a function of several parameters that affect the determination of shale volume in one formula; (2) it collects the three porosity tools for a more accurate calculation; and (3) it works best where radioactive material other than shale is present.

To determine the continuity of known Fe-rich emery horizons and to explore new deposits in the Elmacik area (Yatagan, Turkey), a geophysical survey was carried out using magnetic and electrical methods. Magnetic measurements were taken in the target area of 5 km² and a vertical electrical sounding technique was applied at 15 locations in the alluvial/eluvial (A/E) part of the area in order to explore possible placer emery horizons, and to investigate the thickness of the A/E assembly and any probable faults.

Significant magnetic anomalies occur in the vicinity of old and abandoned emery pits in the marbles of Mt Ismail. The anomalies in the marbles were caused by Fe-rich emery bodies, which did not crop out and were not more than 10 m deep. The magnetic anomalies in the A/E part of the area were weak in amplitude and may suggest small new placer emery deposits.

The result of the vertical electrical soundings indicated two fault zones, one in a N–S direction, and the other approximately in an E–W direction. The thickness of the A/E assembly varies from 2–3 m to 60–100 m. A low resistivity zone, which is located in the mid-east of the A/E part of the study area, correlates well with the long-wavelength magnetic anomaly. According to the survey results, further exploration activities should take place around abandoned emery pits.