Seismic velocity anomalies in the infilling of tunnel valleys: influence on the interpretation of seismic data. An example from western Lithuania

Abstract

Buried tunnel valleys, developed by glacial meltwater below former continental ice-sheets, are often hardly recognisable during deep land seismic surveys and may act as obstructions in the investigation of deeper geological strata both onshore and offshore. Unique, detailed high-resolution onshore seismic investigations in western Lithuania revealed strong or marked anomalies of seismic velocities within the infilling of a tunnel valley. Such an anomaly produces a pull-up effect of seismic reflections from deeper geological interfaces, which in turn may produce inaccuracies in seismic data interpretation giving false pull-up structures, false faults and even false oil traps. In this paper, a method for improving depth mapping of horizons subject to high-velocity pull-ups in time-domain images is discussed. Using seismic and well-log data, improved geological sections were obtained by properly accounting for seismic velocities of valley-infill (lenses) obtained via well-logs.

Keywords:

• Seismic survey
• seismic velocity anomaly
• tunnel valley
• western Lithuania

Acknowledgements

The authors would like to thank the AB LOTOS “Geonafta” company and Igoris Šeštokas for providing the seismic and well-logging data and OJSC “Kaliningradgeofizika” for the seismic-data processing. We thank Professor Algimantas Grigelis and the unforgettable Professor Tom Flodèn (1941–2016) for consulting and profound advice. We acknowledge Dr. Dainius Michelevičius for support with the seismic-data interpretation and his review. Thanks to Dmitrij Novik for geophysical-geological consulting and Will Brew for corrections. A special thanks to Gustaf Peterson and an anonymous scientist for this paper review.