Near‐surface seismic reflection profiling of the Matanuska Glacier, Alaska

Geophysics ◽  
2003 ◽  
Vol 68 (1) ◽  
pp. 147-156 ◽  
Author(s):  
Gregory S. Baker ◽  
Jeffrey C. Strasser ◽  
Edward B. Evenson ◽  
Daniel E. Lawson ◽  
Kendra Pyke ◽  
...  
Keyword(s):  
High Resolution ◽  
Seismic Reflection ◽  
Longitudinal Axis ◽  
Horizontal Distribution ◽  
P Wave ◽  
Near Surface ◽  
Surface Reflection ◽  
Reflection Data ◽  
Basal Ice ◽  
Matanuska Glacier

Several common‐midpoint seismic reflection profiles collected on the Matanuska Glacier, Alaska, clearly demonstrate the feasibility of collecting high‐quality, high‐resolution near‐surface reflection data on a temperate glacier. The results indicate that high‐resolution seismic reflection can be used to accurately determine the thickness and horizontal distribution of debris‐rich ice at the base of the glacier. The basal ice thickens about 30% over a 300‐m distance as the glacier flows out of an overdeepening. The reflection events ranged from 80‐ to 140‐m depth along the longitudinal axis of the glacier. The dominant reflection is from the contact between clean, englacial ice and the underlying debris‐rich basal ice, but a strong characteristic reflection is also observed from the base of the debris‐rich ice (bottom of the glacier). The P‐wave propagation velocity at the surface and throughout the englacial ice is 3600 m/s, and the frequency content of the reflections is in excess of 800 Hz. Supporting drilling data indicate that depth estimates are correct to within ± 1 m.

scholarly journals Avoiding pitfalls in shallow seismic reflection surveys

Geophysics ◽  
1998 ◽  
Vol 63 (4) ◽  
pp. 1213-1224 ◽  
Author(s):  
Don W. Steeples ◽  
Richard D. Miller
Keyword(s):  
Seismic Reflection ◽  
Seismic Waves ◽  
Oil And Gas ◽  
Digital Processing ◽  
Petroleum Exploration ◽  
Near Surface ◽  
Surface Reflection ◽  
Oil And Gas Exploration ◽  
Reflection Data ◽  
Shallow Seismic

Acquiring shallow reflection data requires the use of high frequencies, preferably accompanied by broad bandwidths. Problems that sometimes arise with this type of seismic information include spatial aliasing of ground roll, erroneous interpretation of processed airwaves and air‐coupled waves as reflected seismic waves, misinterpretation of refractions as reflections on stacked common‐midpoint (CMP) sections, and emergence of processing artifacts. Processing and interpreting near‐surface reflection data correctly often requires more than a simple scaling‐down of the methods used in oil and gas exploration or crustal studies. For example, even under favorable conditions, separating shallow reflections from shallow refractions during processing may prove difficult, if not impossible. Artifacts emanating from inadequate velocity analysis and inaccurate static corrections during processing are at least as troublesome when they emerge on shallow reflection sections as they are on sections typical of petroleum exploration. Consequently, when using shallow seismic reflection, an interpreter must be exceptionally careful not to misinterpret as reflections those many coherent waves that may appear to be reflections but are not. Evaluating the validity of a processed, shallow seismic reflection section therefore requires that the interpreter have access to at least one field record and, ideally, to copies of one or more of the intermediate processing steps to corroborate the interpretation and to monitor for artifacts introduced by digital processing.

scholarly journals Static corrections from shallow‐reflection surveys

Geophysics ◽  
1990 ◽  
Vol 55 (6) ◽  
pp. 769-775 ◽  
Author(s):  
D. W. Steeples ◽  
R. D. Miller ◽  
R. A. Black
Keyword(s):  
Layer Thickness ◽  
Seismic Reflection ◽  
P Wave ◽  
Thickness Variation ◽  
Near Surface ◽  
Weathered Zone ◽  
Reflection Data ◽  
Weathered Layer ◽  
Static Corrections ◽  
Thickness Variations

Shallow seismic reflection surveys can assist in determination of velocity and/or thickness variations in near‐surface layers. Static corrections to seismic reflection data compensate for velocity and thickness variations within the “weathered zone.” An uncompensated weathered‐layer thickness variation on the order of 1 m across the length of a geophone array can distort the spectrum of the signal and result in aberrations on final stacked data. P-wave velocities in areas where the weathered zone is composed of unconsolidated materials can be substantially less than the velocity of sound in air. Weathered‐layer thickness variation of 1 m in these low‐velocity materials could result in a static anomaly in excess of 3 ms. Shallow‐reflection data from the Texas panhandle illustrate a real geologic situation with sufficient variability in the near surface to significantly affect seismic signal reflected from depths commonly targeted by conventional reflection surveys. Synthetic data approximating a conventional reflection survey combined with a weathered‐layer model generated from shallow‐reflection data show the possible dramatic static effects of alluvium. Shallow high‐resolution reflection surveys can be used both to determine the severity of intra‐array statics and to assist in the design of a filter to remove much of the distortion such statics cause on deeper reflection data. The static effects of unconsolidated materials can be even more dramatic on S-wave reflection surveys than on comparable P-wave surveys.

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