Field Comparison of High-frequency Seismic Sources for Imaging Shallow (10–250m) Structures

Abstract

Several high-frequency seismic sources (i.e., sledgehammer, pipegun, weightdrop, and small- and intermediate-size vibrators) have been tested at a site characterized by a surface layer of very recent sediments overlying unconsolidated glacial and glaciolacustrine units. Reflections from very shallow (10m) to moderate (250m) depths were recorded. Evaluation of the different sources involved analyzing prominent features (e.g., reflections and signal-generated noise that includes complex patterns of interfering direct, guided, refracted, and surface waves) on source gathers, frequency spectra and stacked sections. Our experiments demonstrated that minor changes in local conditions (e.g., changes in ground-water table depth) may influence significantly the characteristics of the resulting data. Of the tested sources, the simple sledgehammer provided the highest resolution images at shallow depths (10–100m) and comparably good images at greater depths (100–250m). Local changes in surficial sediments affected the quality of data generated by the small (60kg reaction mass) vibrator; at many locations, the input energy was insufficient to illuminate the important geological structures. By comparison, the intermediate-size (2,300kg reaction mass) vibrator provided high quality images to depths as great as 250m (corresponding to the deepest reflection from near the base of unconsolidated sediments), but relatively strong airwaves and imprecise vibroseis correlation precluded the resolution of reflections from depths shallower than 40–50m. Despite recent advances in vibratory systems, the humble, but cost-effective sledgehammer continues to be an attractive source for high-resolution seismic experiments.