Gregory S. Baker

Stabilizing feedbacks in glacier-bed erosion.

Glaciers often erode, transport and deposit sediment much more rapidly than nonglacial environments, with implications for the evolution of glaciated mountain belts and their associated sedimentary basins. But modelling such glacial processes is difficult, partly because stabilizing feedbacks similar to those operating in rivers have not been identified for glacial landscapes. Here we combine new and existing data of glacier morphology and the processes governing glacier evolution from diverse settings to reveal such stabilizing feedbacks. We find that the long profiles of beds of highly erosive glaciers tend towards steady-state angles opposed to and slightly more than 50 per cent steeper than the overlying ice–air surface slopes, and that additional subglacial deepening must be enabled by non-glacial processes. Climatic or glaciological perturbations of the ice–air surface slope can have large transient effects on glaciofluvial sediment flux and apparent glacial erosion rate.

Seismic reflections from depths of less than two meters

Three distinct seismic reflections were obtained from within the upper 2.1 m of flood-plain alluvium in the Ar- kansas River valley near Great Bend, Kansas. Reflections were observed at depths of 0.63, 1.46, and 2.10 m and confirmed by finite-difference wave-equation modeling. The wavefield was densely sampled by placing geophones at 5-cm intervals, and near-source nonelastic deformation was minimized by us- ing a very small seismic impulse source. For the reflections to be visible within this shallow range, low seismic P-wave ve- locities (<300 m/s) and high dominant-frequency content of the data (-450 Hz) were essential. The practical implementa- tion of high-resolution seismic imaging at these depths has the potential to complement ground-penetrating radar (GPR), chiefly in areas where materials exhibiting high electrical con- ductivity, such as clays, prevent the effective use of GPR. Potential applications of these results exist in hydrogeology and environmental, Quaternary, and neotectonic geology.

Toward the Autojuggie: Planting 72 geophones in 2 sec

This is the publishers version, also available electronically from “http://onlinelibrary.wiley.com”.

Applying AVO analysis to GPR data

For reflected-wave phenomena in both seismology and ground-penetrating radar (GPR), the amplitudes of the reflected waves depend on the incidence angles and the physical properties of the media above and below an interface. Amplitude variation with offset (AVO) analysis employs knowledge of reflected-wave phenomena to constrain the physical properties of two media at an interface. Ostrander (1984) demonstrated that AVO analysis can be used on seismic reflection data in certain instances to obtain information about the subsurface not revealed by traveltime-offset relationships (i. e., traditional seismic cross sections). Quantifiable information about the physical properties of the subsurface may then be extracted. AVO analysis, proven for a decade to work well in reflection seismology, may be useful in near-surface hydrogeological situations when applied carefully to GPR data to better constrain the presence or absence of nonaqueous phase liquid contaminants (NAPLs) in the subsurface.

In‐situ, high‐frequency P-wave velocity measurements within 1 m of the earth’s surface

Seismic P-wave velocities in near‐surface materials can be much slower than the speed of sound waves in air (normally 335 m/s or 1100 ft/s). Difficulties often arise when measuring these low‐velocity P-waves because of interference by the air wave and the air‐coupled waves near the seismic source, at least when gathering data with the more commonly used shallow P-wave sources. Additional problems in separating the direct and refracted arrivals within ∼2 m of the source arise from source‐generated nonlinear displacement, even when small energy sources such as sledgehammers, small‐caliber rifles, and seismic blasting caps are used. Using an automotive spark plug as an energy source allowed us to measure seismic P-wave velocities accurately, in situ, from a few decimeters to a few meters from the shotpoint. We were able to observe three distinct P-wave velocities at our test site: ∼130m/s, 180m/s, and 300m/s. Even the third layer, which would normally constitute the first detected layer in a shallow‐seismic‐...

Near‐surface imaging using coincident seismic and GPR data

In many near-surface applications, detailed subsurface characterization is important. Characterization often is obtained using ground-penetrating radar (GPR) or shallow seismic-reflection (SSR) imaging methods, depending upon depth of interest and surficial geology. Each method responds to different physical properties; thus, each may produce different images of the same near-surface volume. By incorporating the two methods, we generated a cross-section of the subsurface at an alluvial test site and identified the depths of three interfaces accurately to ±5 cm. We present here experimental results and examples of SSR and GPR images obtained along the same traverse, showing coincident and noncoincident reflections from multiple interfaces within 3 m of the surface.

Muting the noise cone in near-surface reflection data: An example from southeastern Kansas

A 300-m near-surface seismic reflection profile was collected in southeastern Kansas to locate a fault(s) associated with a recognized stratigraphic offset on either side of a region of unexposed bedrock. A substantial increase in the S/N ratio of the final stacked section was achieved by muting all data arriving in time after the airwave. Methods of applying traditional seismic data processing techniques to near-surface data (200 ms of data or less) often differ notably from hydrocarbon exploration-scale processing (3-4 s of data or more). The example of noise cone muting used is contrary to normal exploration-scale seismic data processing philosophy, which is to include all data containing signal. The noise cone mute applied to the data removed more than one-third of the total data volume, some of which contains signal. In this case, however, the severe muting resulted in a higher S/N ratio in the final stacked section, even though some signal could be identified within the muted data. This example supports the suggestion that nontraditional techniques sometimes need to be considered when processing near-surface seismic data.

Geophones on a board

We examined the feasibility of using seismic reflections to image the upper 10 m of the earth’s surface quickly and effectively by rigidly attaching geophones to a wooden board at 5-cm intervals. The shallow seismic reflection information obtained was equivalent to control‐test data gathered using classic, single‐geophone plants with identical 5-cm intervals. Tests were conducted using both a .22-caliber rifle source and a 30.06-rifle source. In both cases, the results were unexpected: in response to our use of small, high‐resolution seismic sources at offsets of a few meters, we found little intergeophone interference that could be attributed to the presence of the board. Furthermore, we noted very little difference in a 60-ms intra‐alluvial reflection obtained using standard geophone plants versus that obtained using board‐mounted geophones. For both sources, amplitude spectra were nearly identical for data gathered with and without the board. With the 30.06 source, filtering at high‐frequency passbands...

The time dependence of shallow reflection data

To demonstrate the variation in data quality over time of three shallow‐seismic sources, we present data collected at the same site using a sledgehammer, a downhole .22 rifle, and a downhole 30.06 rifle in the spring of 1997. During this period two significant rainfalls occurred, and soil conditions varied from relatively dry to extremely soft and water saturated. Based on our experiment, we conclude that reflection data quality in the upper 75 ms varies, depending on soil moisture, for the three sources. Specifically, at our site, the hammer yields the highest quality during damp conditions, the .22 during saturated and wet conditions, and the 30.06 under dry conditions.

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

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 d...