J. A. Hunter

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.

Seismic model studies of the overburden‐bedrock reflection

One aspect of the modification of seismic waves on passage through the Earth is the partitioning of energy at subsurface interfaces as described by the Zoeppritz equations. These equations have been applied to simple two‐ and three‐layer models to determine the variations in the amplitude and phase of a reflection signal at nonnormal angles of incidence. Synthetic seismograms have been produced to illustrate the effect of these variations on a seismic wavelet. It is found that the phase variations can lead to substantial changes in the character of the reflected wavelet as the source‐geophone distance increases. These changes are dependent on the angle of incidence and on the elastic properties of the subsurface layers. In particular, for models approximating an overburden over bedrock situation, the reflected pulse is predicted to “change phase” when the velocity contrast between the two layers is relatively small. This effect has been observed on field records. Geophysicists should be aware of this phen...

Comparison of Site Periods Derived from Different Evaluation Methods

Abstract As a part of our microzonation research activities for the city of Ottawa, the fundamental site period, T 0 , was investigated based on different methods, including (1)xa0the horizontal-to-vertical spectral ratio (HVSR) using microtremor ambient-noise measurements; (2)xa0equivalent single-layer (ESL) modeling, as noted in the current National Building Code of Canada (2005); (3)xa0earthquake weak motion observations; (4)xa0multilayer soil modeling; and (5)xa0finite element modeling for linear and nonlinear soil. The differences between these methods are discussed. We have discovered that T 0 based on the HVSR method systematically deviated from T 0 based on the equivalent single-layer modeling method. The variance was more than 30% for periods longer than 2xa0s, corresponding to impedance boundary depths of more than 75xa0m in the study area. The effect of the shear-wave velocity gradient on T 0 was investigated by applying multilayer soil modeling, which confirms that the actual velocity-depth gradient shifts the evaluated T 0 to shorter periods compared to equivalent single-layer modeling. The effects of soil nonlinearity on T 0 were examined using a finite element method analysis. The dependency of T 0 on the level of shaking is well defined at higher levels of shaking when the peak ground acceleration (PGA) exceeds 80xa0Gal because the soft soil behaves nonlinearly. It has been concluded that, for the case of nonlinear soil response, damping plays an important role in reducing the fundamental frequency. These findings will be used in our future research activities directed at providing fundamental period maps for the city of Ottawa, including fundamental period maps for the strong motion design earthquake.

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.

Empirical Geophysical/Geotechnical Relationships in the Champlain Sea Sediments of Eastern Ontario

Geophysical and geotechnical data are presented from different sites in eastern Ontario where variable geotechnical properties of Champlain Sea sediments (‘Leda Clays’) are found. Sites range from thick “undisturbed” silts and clays, to “disturbed” geologically similar soils (earthquake triggered landslides and other deformed materials). High-resolution seismic profiles provide stratigraphic context for some of the boreholes drilled in the study area. Downhole geophysical logs from 14 boreholes are compared to core sample measurements of porosity, sensitivity, and porewater conductivity to develop useful empirical relationships. According to these relationships, silt and clay sediments can be sensitive or quick when formation conductivity drops below 100 mS/m. Conversely, silts and clays with elevated conductivities (>250 mS/m) are rarely sensitive. Salinity values calculated from porewater conductivity indicate sensitive or quick behaviour may be expected in leached soils when salinity drops below 2 g/l.

Shear Wave Reflection Landstreamer Technology Applied to Soil Response Evaluation of Earthquake Shaking in an Urban Area

Shear wave seismic test locations and boreholes have outlined a buried bedrock valley in the Ottawa, Ontario, suburb of Orleans. This region is in a significant high seismic hazard zone, and the surficial materials are primarily high water-content, poorly compacted Holocene-age Champlain Sea sediments. Since the in-filled sediments exhibit an extremely low average shear wave velocity (~200 m/s) and the bedrock beneath exhibits shear wave velocities on the order of 2500 m/s, it is suspected that the buried valley may give rise to three-dimensional ground motion amplification phenomena in the event of significant earthquakes. In order to better define the buried valley and to prepare for future three-dimensional shake modeling, shear-wave reflection landstreamer lines were acquired, to confirm the depth to bedrock, and to detail its shape. Despite the large broad-frequency band of ambient traffic noise, it was possible to observe reflections from the bedrock as well as additional infra-overburden reflectors. The valley shape and its internal structure, as determined by these surveys, will form a vital contribution to the three-dimensional interpretation for soil response to earthquake shaking in the Ottawa area.