Shallow seismic reflection data were recorded along two long (>1.6 km) intersecting profiles in the glaciated Suhre Valley of northern Switzerland. Appropriate choice of source and receiver parameters resulted in a high‐fold (36–48) data set with common midpoints every 1.25 m. As for many shallow seismic reflection data sets, upper portions of the shot gathers were contaminated with high‐amplitude, source‐generated noise (e.g., direct, refracted, guided, surface, and airwaves). Spectral balancing was effective in significantly increasing the strength of the reflected signals relative to the source‐generated noise, and application of carefully selected top mutes ensured guided phases were not misprocessed and misinterpreted as reflections. Resultant processed sections were characterized by distributions of distinct seismic reflection patterns or facies that were bounded by quasi‐continuous reflection zones. The uppermost reflection zone at 20 to 50 ms (∼15 to ∼40 m depth) originated from a boundary between glaciolacustrine clays/silts and underlying glacial sands/gravels (till) deposits. Of particular importance was the discovery that the deepest part of the valley floor appeared on the seismic section at traveltimes >180 ms (∼200 m), approximately twice as deep as expected. Constrained by information from boreholes adjacent to the profiles, the various seismic units were interpreted in terms of unconsolidated glacial, glaciofluvial, and glaciolacustrine sediments deposited during two principal phases of glaciation (Riss at >100 000 and Würm at ∼18 000 years before present).

Shallow 3-D seismic reflection techniques have been used to map glacial deposits in a Swiss mountain valley. A dense distribution of source and receiver positions resulted in a small subsurface sampling of 1.5 m × 1.5 m and a high fold of >40. Common processing operations that included pseudotrue amplitude scaling, deconvolution, and band‐pass filtering successfully enhanced shallow reflections relative to source‐generated noise. Careful top muting helped avoid erroneous stacking of direct and guided waves. Azimuth‐dependent velocity analyses proved to be unnecessary. Three‐dimensional (3-D) migration of the stacked data yielded the final high‐resolution images of the shallow subsurface (15–170 m). Because most reflections and diffractions were migrated to their correct subsurface locations, confident interpretations of 3-D structures were possible. Time slices and cross‐sections along arbitrary directions proved to be powerful analysis tools. Even small‐scale features (<20 m wide), such as subglacial channels and troughs, could be mapped. Five major lithologic units separated by four principal reflecting boundaries were distinguished on the basis of their characteristic seismic facies. The principal reflecting boundaries were semiautomatically tracked through the 3-D data volume. Borehole information allowed the uppermost boundary at 15–27 m to be identified as the top of a 68–80-m-thick sequence of basal and reworked tills characterized by high‐amplitude discontinuous to quasi‐continuous reflections. Low reflectivity of seismic units above and below the till units was associated with finely layered or massive glaciolacustrine clay/silt deposited during and after two principal phases of glaciation (Würm at 28 000 to 10 000 and Riss at 200 000 to 100 000 years before the present). Top of Tertiary Molasse basement was delineated by prominent east‐dipping reflections at variable depths of 85–170 m.

A comprehensive strategy of 3-D seismic reflection data acquisition and processing has been used in a study of glacial sediments deposited within a Swiss mountain valley. Seismic data generated by a downhole shotgun source were recorded with single 30-Hz geophones distributed at 3 m x 3 m intervals across a 357 m x 432 m area. For most common-midpoint (CMP) bins, traces covering a full range of azimuths and source-receiver distances of -2 to -125 m were recorded. A common processing scheme was applied to the entire data set and to various subsets designed to simulate data volumes collected with lower density source and receiver patterns. Comparisons of seismic sections extracted from the processed 3-D subsets demonstrated that high-fold (>40) and densely spaced (CMP bin sizes < 3 m x 3 m) data with relatively large numbers (>6) of traces recorded at short (<20 m) source-receiver offsets were essential for obtaining clear images of the shallowest (<100 ms) reflecting horizons. Reflections rich in frequencies > 100 Hz at traveltimes of ^20 to -170 ms provided a vertical resolution of 3 to 6 m over a depth range of ^-15 to ^-150 m. The shallowest prominent reflection at 20 to 35 ms (^-15 to 27 m depth) originated from the boundary between a near-surface sequence of clays/silts and an underlying unit of heterogeneous sands/gravels.

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 [Formula: see text] to moderate [Formula: see text] 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 [Formula: see text] and comparably good images at greater depths [Formula: see text]. Local changes in surficial sediments affected the quality of data generated by the small ([Formula: see text] reaction mass) vibrator; at many locations, the input energy was insufficient to illuminate the important geological structures. By comparison, the intermediate-size ([Formula: see text] reaction mass) vibrator provided high quality images to depths as great as [Formula: see text] (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 [Formula: see text]. Despite recent advances in vibratory systems, the humble, but cost-effective sledgehammer continues to be an attractive source for high-resolution seismic experiments.

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