Robert A. Burns

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...

Joint analysis of surface-wave and refraction events from river-bottom sediments

Summary An underwater seismic survey was performed in the Fraser River Delta area near Vancouver, B.C., Canada, using hydrophones. Data were analyzed using multichannel analysis of surface waves (MASW), and refraction tomography inversion provided an accurate P-wave velocity (Vp) profile for riverbottom sediments in the upper 30 m. To optimize the compressional wave velocity function in the 30 to 70 m depth interval, Vp information from a nearby well was used as a priori information. Using shear wave velocity profiles from surface-wave inversion as a priori information allows a fast and reliable inversion of the first arrival events interpreted on an underwater shot gather. The inverted Vp profile, strongly suggesting the presence of gas within water-bottom sediments, corroborates a nearby land well. This example demonstrates the usefulness of joint inversion and its potential for detecting near-surface anomalies.

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.

Imaging permafrost with shallow P‐ and S‐wave reflection

Shallow reflection events observed on coincident Pand Swave data from within the permafrost zone possess sufficient coherency and resolution potential to compensate conventional 2or 3-D data sets for structural artifacts related to lateral travel time variability. Reflections with dominant frequencies above 100 Hz on P-wave data and 40 Hz on S-wave data from within the permafrost have a very broad range of wavelet characteristics, which change vertically and laterally across the 1 km profiles. S-wave reflections appear much more continuous across the section when contrasted with the P-wave reflections. With the exception of the Iperk/MacKenzie reflection P-wave wavelet characteristics change quite dramatically across the almost 1km long shot gathers. Coincident interpretation of the Pand S-wave reflection data can address a variety of both engineering and petroleum exploration imaging problems.

High‐resolution shear‐wave reflection technique for permafrost engineering applications: New results from siberia

A joint Russia-Canada field program in the Yamal Peninsula, Western Siberia, in July 1991, investigated the possibility of using shallow shear wave reflection techniques in a permafrost environment for mapping the subsurface structure and, in particular, the structure of buried bodies of massive ice. Offset VSP and surface reflection techniques were tested, and results indicated that shear wave reflections could be obtained from the top of a massive ice body as shallow as 5 m below ground surface. A short reflection section was recorded and the results compared with a section produced from borehole records. Based on these results, the shear wave reflection technique shows promise as a means of mapping buried ice bodies in this environment.

Measuring Sub‐Seabottom Seismic Velocities — Some Unusual Experiments

During the last 30 years, we have performed a number of unusual experiments to measure Pand Swave velocity structure of unconsolidated sediments below the seafloor at sites to water depths of 700 m. These experiments were designed to aid in specific geotechnical problems related to soil stability along pipeline routes or beneath bottom-founded structures as well as to provide regional information for military purposes. Some of these experiments were done in open-water conditions, but many were performed beneath ice-cover. Early testing was directed towards mapping sub-seafloor ice-bonded permafrost in the Beaufort Sea and elsewhere in the Arctic Archipelago, mainly using dynamite sources. Continuous ice-cover at high latitudes presented specific challenges for refraction methods and various devices and hydrophone arrays were designed for deployment beneath the sea-ice through open leads or drilled holes. Later testing evolved from seabottom-laid hydrophones to towed, near-bottom arrays and non-dynamite sources. As well, mapping of shear wave velocity structure was tested using seabottom coupled interface waves. All of these experiments (successful or otherwise) were learning experiences in one form or other, and it is hoped that documentation of these experiments herein will be of some future research value.

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.