R. L. Good

Shallow seismic reflection mapping of the overburden‐bedrock interface with the engineering seismograph—Some simple techniques

Where unconsolidated overburden exceeds 20 m in thickness, the reflection method may be efficiently used with a 12‐channel engineering seismograph to map topography on the overburden‐bedrock interface as well as possible structure within the overburden. The two techniques we suggest are the simplest forms of reflection profiling which can be applied with a minimum amount of equipment which we suggest is a 12‐channel enhancement seismograph, a 12‐geophone array, and a hammer source. These techniques require good transmission characteristics of the overburden as well as a sharp velocity discontinuity at the overburden‐bedrock interface. For data processing and display a microcomputer is essential.

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.

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...

Near-surface geophysical techniques for geohazards investigations: Some Canadian examples

Over the last 40 years, there has been an expansion of activity in applications of near-surface geophysical techniques for various types of geohazards investigations in Canada; numerous national and international research groups have been working with the Near Surface Geophysics Section of the Geological Survey of Canada to develop techniques for specific Canadian engineering and environmental geohazards problems. A few of the more interesting examples from widespread parts of the country are discussed in this paper.

Multichannel Analysis of Underwater Surface Waves Near Vancouver, B.C., Canada

Surface (Scholte) waves acquired during underwater seismic surveys with hydrophone arrays are analyzed using the multichannel analysis of surface waves (MASW) method to construct shear-wave velocity (Vs) profiles for the upper 40-m of water-bottom sediments in the Fraser River delta area, near Vancouver, British Columbia, Canada. Shear wave profiles are obtained using the Rayleigh-wave inversion method (based on the multimodal dispersion curves) since theory suggests the difference in phase velocities between the two types of surface waves (i.e., the Scholte vs. the Rayleigh waves) is minor and usually falls below the uncertainty of the measurement. Vs profiles calculated from dispersion analysis are compared with measured Vs profiles available from nearby land boreholes (within a few hundred meters of the underwater sites). The comparison shows MASW values are in good agreement with the overall trend of borehole values, but lower in general by about ten percent. This shift seems to be attributable to the water-bottom sediments being softer and their density being greater (at depths < 5 m) than sediments on land. Simple multichannel processing (surgical mute) seems critical to suppress the influence of the strong broad-band channel waves trapped in the water layer before extracting the dispersion curve.

Near-surface shear wave reflection surveys in the Fraser River delta, B.C., Canada

Summary Shallow shear wave reflection surveys using high frequency vibroseis techniques provides information about the Tertiary bedrock surface and Pleistocene sediments on the Fraser River delta, in B.C., Canada. Besides the clear advantage of shear wave profiling in this shallow gas environment, the added resolution potential and ability to measure the shear wave velocity field enhances the fusion of this surface seismic data into earthquake site response estimations. Surface materials ranging from undisturbed, native delta sediments to clay/ rubble fill used in dike construction seemed amenable to the generation and recording of shear waves when using a small (6,000 kg) vibrator and engineering recording systems. The thickness of Pleistocene sediments and depth to Tertiary bedrock were mapped at three locations within the delta where gaps existed in the geologic record. These three sites provided cultural and near-surface settings that uniquely tested this shallow imaging technique.

Shallow shear wave seismic reflection profiling in the Fraser River delta, British Columbia

Shallow shear wave seismic reflection methods were used to identify the top of’ Pleistocene sediments beneath the Fraser River delta in southwestern British Columbia. Three profiles were recorded using a hammer and mass energy source in the southern part of the delta were previous compressional wave surveys had encountered some difficulties due to near-surface gas accumulations. Based on correlations with nearby borehole logs, the Holocene/Pleistocene surface was interpreted to range in depth from approximately 35 m to 80 m and exhibit significant local variations (e.g., dip). The survey was also successful in imaging the top of Pleistocene even when it was composed of different materials (i.e., stiff glacial till or sand/silt deposits). A structure map of the top of Pleistocene sediments beneath the Fraser River delta would provide valuable input for accurate earthquake hazard modeling in the area. This study suggests that shear wave reflection techniques, combined with borehole and existing compressional wave reflection data, might effectively be used to produce such a map.

Application of Shear Wave Seismic Techniques to Earthquake Hazard Mapping In the Fraser River Delta, British Columbia

Using various shear-wave seismic methods, the shearwave velocity-depth structure of thick unconsolidated sediments in the Fraser River Delta, British Columbia, is currently being mapped on a reconnaissance scale. The data base provides input parameters for computer modelling of earthquake ground motion amplification response; as well, shallow shear-wave velocity structure can be indicative of regional variations in liquefaction potential during earthquake loading.

Shallow shear wave reflection survey in the Canadian Arctic

Shear wave seismic reflection was used to delineate cyclic deposits of massive ice and glacial till in the upper 30 m at a site near Tuktoyaktuk, Northwest Territories, Canada. Shear wave reflections with dominant frequencies in excess of 350 Hz were recorded with a steel shaft and 1 kg hammer source and single 50 Hz horizontal geophones with 30 cm spikes. A least four reflecting interfaces between 10 and 30 m depth can be correlated to borehole data. Previous regional borehole information suggested the presence of a single massive layer of segregated ice beneath glacial till. The seismic data were interpreted to indicate at least three distinct layers of structurally deformed ice that were later confirmed by drilling and geophysical logs.