James A. Hunter

Comparing shear-wave velocity profiles inverted from multichannel surface wave with borehole measurements

Recent field tests illustrate the accuracy and consistency of calculating near-surface shear (S)-wave velocities using multichannel analysis of surface waves (MASW). S-wave velocity profiles (S-wave velocity vs. depth) derived from MASW compared favorably to direct borehole measurements at sites in Kansas, British Columbia, and Wyoming. Effects of changing the total number of recording channels, sampling interval, source offset, and receiver spacing on the inverted S-wave velocity were studied at a test site in Lawrence, Kansas. On the average, the difference between MASW calculated V s and borehole measured V s in eight wells along the Fraser River in Vancouver, Canada was less than 15%. One of the eight wells was a blind test well with the calculated overall difference between MASW and borehole measurements less than 9%. No systematic differences were observed in derived V s values from any of the eight test sites. Surface wave analysis performed on surface data from Wyoming provided S-wave velocities in near-surface materials. Velocity profiles from MASW were confirmed by measurements based on suspension log analysis. ⓒ 2002 Elsevier Science Ltd. All rights reserved.

Comparing Shear-Wave Velocity Profiles from MASW with Borehole Measurements in Unconsolidated Sediments, Fraser River Delta, B.C., Canada

Recent field tests illustrate the accuracy and consistency of estimating near-surface shear (S) wave velocities calculated using Multi-channel Analysis of Surface Wave (MASW). To evaluate the technique in a variety of near-surface conditions and through a wide range of velocities, MASW-derived S-wave velocity profiles (S-wave velocity vs. depth) were statistically compared to S-wave velocity profiles measured in seven boreholes in the unconsolidated sediments of the Fraser River delta, near Vancouver, B.C., Canada. An overall difference of approximately 15 percent was observed between these two uniquely determined sets of S-wave velocities from the seven well locations. A blind test of the standalone accuracy of MASW was conducted at an eighth well. For this blind test, S-wave velocity measurements made in and interpreted from the borehole were not made available during MASW data processing. Differences between S-wave velocities using MASW from those measured in the blind test borehole averaged nine perce...

Field comparison of shallow seismic sources near Chino, California

Data from a shallow seismic-source comparison test conducted in an area with a water-table depth in excess of 30 m and near-surface velocities less than 330 m/s were acquired from 13 different sources at a single site near Chino, California. The sources included sledgehammer, explosives, weight drop, projectile impacts, and various buffalo guns. A possible reflecting event can be interpreted at about 70 ms. At this particular test site, the lowly sledgehammer is among the best sources to provide data to see the possible reflection. Our previous work and that of our colleagues suggests that any source could dominate the comparison categories addressed here, given the appropriate set of site characteristics.

Underwater MASW to evaluate stiffness of water-bottom sediments

Stiffness measurements are often necessary for geotechnical characterization of an underwater site. Seismically, these measurements can be made through the dispersion analysis of the Rayleigh-type surface waves. Successful terrestrial application of this method has been reported by many investigators using spectral analysis of surface waves (SASW) and more recently using multichannel analysis of surface waves (MASW). The MASW method was originally developed as a land survey method to investigate the near-surface materials for their elastic properties: for example, the shear-wave velocity (VS), by recording and analyzing Rayleigh-type surface waves using a vertical (impulsive) seismic source and receivers. The acquired data are first analyzed for dispersion characteristics and, from these the shear-wave velocity is estimated using an inversion technique.

Field comparison of shallow P-Wave seismic sources near Houston, Texas

A shallow P‐wave seismic source comparison was conducted at a site near Houston, Texas where the depth to the water table was approximately 7 m, and near‐surface materials consisted of clays, sands, and gravels. Data from twelve different sources during this November 1991 comparison are displayed and analyzed. Reflection events are interpretable at about 40 ms on some 220-Hz analog low‐cut filtered field files, and at 60 ms on most 110‐ and 220-Hz analog low‐cut filtered field files. Calculations and local water well information suggest the 40-ms event is from the top of the water table. Subsurface explosive sources seem to possess the highest dominant frequency, broadest bandwidth, and recorded amplitudes and, therefore, have the greatest resolution potential at this site. Our previous work and that of our colleagues suggests that, given a specific set of site characteristics, any source could dominate the comparison categories addressed here.

Multicomponent high-resolution seismic reflection profiling

Multicomponent seismic reflection methods are a new tool for oil and gas exploration and reservoir monitoring (Miles 1988), but such technologies have not yet been extensively exploited for near-surface exploration related to hydrogeological and/or geotechnical investigations. With the advantage of relatively inexpensive recording systems for near-surface applications, we show that the use of multicomponent high-resolution seismic reflection methods has great potential as a new means of observing and characterizing the physical parameters of the shallow subsurface, and in particular of groundwater reservoirs.

Downhole seismic logging for high‐resolution reflection surveying in unconsolidated overburden

Downhole seismic velocity logging techniques have been developed and applied in support of high‐resolution reflection seismic surveys. For shallow high‐resolution reflection surveying within unconsolidated overburden, velocity‐depth control can sometimes be difficult to achieve; as well, unambiguous correlation of reflections with overburden stratigraphy is often problematic. Data obtained from downhole seismic logging can provide accurate velocity‐depth functions and directly correlate seismic reflections to depth. The methodologies described in this paper are designed for slimhole applications in plastic‐cased boreholes (minimum ID of 50 mm) and with source and detector arrays that yield similar frequency ranges and vertical depth resolutions as the surface reflection surveys. Compressional- (P-) wave logging uses a multichannel hydrophone array with 0.5-m detector spacings in a fluid‐filled borehole and a high‐frequency, in‐hole shotgun source at the surface. Overlapping array positions downhole result...

Sequence stratigraphy of a glaciated basin fill, with a focus on esker sedimentation

A large integrated data set of cores, outcrop data, and seismic transects from the mud-buried Vars-Winchester esker in the Champlain Sea basin, Canada, was studied to gain insight into how muddy glaciated basins fill with sediment, and how esker sedimentary systems contribute to this process. Three stratigraphic units—a till sheet over carbonate bedrock, the Vars-Winchester esker , and overlying Champlain Sea mud—are identified in the data set. The till is massive, mud rich, carbonate rich, and drumlinized. The esker is also carbonate rich, and rests erosively on till or bedrock. It consists of two elements, a narrow gravelly central ridge and a broad sandy carapace. Three units comprise the overlying mud package: gray carbonate-rich rhythmites, massive bioturbated mud, and carbonate-poor, red-and-gray rhythmites. A sequence stratigraphic model is proposed to explain these observations. Emphasis is placed on gradual ice-front translation superimposed by rapid meltwater events. The esker is interpreted to have been derived from the underlying till by water that flowed through a subglacial conduit (R-channel), within which the narrow gravelly central ridge was deposited. Most mud and finer sand bypassed the conduit and was deposited proglacially on the floor of the Champlain Sea, first as sandy outwash and, farther basinward, as muddy carbonate-rich rhythmites. Gradual ice-front retreat superposed distal facies over proximal facies, generating the upward-fining succession that starts with the esker gravel and ends with muddy rhythmites. Most esker sediment appears to have been deposited during rapid, jokulhlaup-like floods that punctuated gradual retreat. Discharges are estimated to have been high, possibly on the order of several hundred to, perhaps more commonly, several thousand cubic meters per second. The chaotic and random-looking appearance of the resultant sedimentological signatures in the esker sensu stricto is sharply contrasted with the regularity of the muddy rhythmites. If the rhythmites are indeed correlative to the esker, which seems reasonable given their geochemistry and the fact that their volume scales to the volume of mud in the till, the flood events that deposited the esker must have been seasonally mediated, and the basin water must have attenuated the flood signal, resulting in a rhythmic “on-off” signature in more distal portions of the system. The regularity of the rhythmites does not betray the chaotic nature of the esker sensu stricto, and vice versa. Studying either one in isolation would lead to a very different “end-member” impression of how eskers form and how esker sedimentary systems operate during the infilling of glaciated basins.

Application of high-resolution seismic techniques in the evaluation of earthquake site response, Ottawa Valley, Canada

Abstract High-resolution seismic surveys, including P- and S-wave studies, have been conducted in an area of the Ottawa River valley located 80 km east of Ottawa (Canada). Based on dating of paleolandslides, the existence of paleoearthquake activity has been postulated in this area. The target zone for the seismic survey is characterized by surface disturbance and sediment deformation. P-wave seismic imaging was used to map the overburden–bedrock interface as well as to indicate reflecting boundaries within the overburden. The area of surface disturbance was found to overlie a buried bedrock basin, 8 km in diameter, infilled with a maximum thickness of 180 m of unconsolidated Quaternary sediments. Preliminary results of core logging show the presence of sand overlain by deformed fine sediments within the disturbed area. Shear-refraction studies reveal differences in the velocity–depth profiles between the disturbed area and the surrounding undisturbed areas. The shear-wave reflection method was used to produce a fundamental resonant period map for the area. Surface sediment disturbance was probably due to a combination of ground-motion amplification due to the basin (thick soft sediments) and the presence of water-saturated sand at depth.

Evaluation of the MASW technique in unconsolidated sediments

Shear (S) wave velocities derived from the MASW (multi-channel analysis of surface wave) technique and borehole measurements at seven well locations in unconsolidated sediments of the Fraser River Delta are compared. The overall difference between these two sets of S-wave velocities is about 15%. S-wave velocities from the MASW technique at an additional location are also obtained and await comparison with borehole measurements.