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NEW ZEALAND ROCKS
Development of Multi-channel Analysis of Surface
Waves (MASW) for characterising the internal structure
of active fault zones as a predictive method to identify
the distribution of ground deformation
……………………………………….……Hastie Award Research
Brendan Duffy
This project is intended to build on the promising results of a class project undertaken in the
vicinity of the Springfield Fault at Dalethorpe, Canterbury. It aims to correlate rock-mechanics
data, obtained at an exposure of the fault and at distal outcrops, with MASW profiles of S-
wave velocity. It further aims to integrate the resultant dataset with a detailed analysis of the
geomorphology of the study area. By adapting and repeating the technique at geologically
diverse sites, it will produce a method of reliably inferring rock-mass parameters from subsurface
shear wave velocities, and of predicting likely damage zones based on geomorphic
evidence. This should allow the proposition of a better, financially viable and scientifically
informed basis for land use planning in proximity to active faulting in New Zealand and
elsewhere.
My research began in late November and one a major focus so far has been the rock
mechanics and spatial variations therein in the vicinity of the Dalethorpe Springfield Ffault
outcrop. This work has evolved into four separate but connected efforts. Firstly, the outcrop
has been analysed using a standard scanline to determine rock-mass parameters such as linear
fracture density, fracture spacing and , Geological Strength Index, etc. Secondly, the crosshole
shear wave velocities of individual tectonized units identified within the outcrop have been
measured directly using horizontal drillholes and a homemade shear wave generator. Thirdly,
I am presently conducting a laboratory experiment to relate reduction in P and S-wave velocity
to increasing fracture density. I am using an ultrasonic seismic analyser to measure velocities
in artificially and incrementally fractured intact Torlesse Greywacke. I will then empirically
derive polynomial regression equations for this relationship and compare these results with
sonic velocities acquired in the crosshole experiment. Finally, I have acquired several MASW
lines; the locations of which have been fixed by surveys. This will allow me to upscale my
outcrop and laboratory-derived velocities and interpretations to seismic survey-scale. By
surveying the locations of the lines relative to other geomorphic features of interest, I have the
necessary controls to integrate the rock mechanics, seismic and geomorphic data collected so
far.
In addition to rock mechanics work, I hope to begin preparing the results of the Dalethorpe
study for publication in May. considerable effort has been spent exploring photogrammetric
methods of outcrop logging with the ultimate intention of producing a 3D metric model of an
extremely sheared outcrop. Standard engineering geological classification of highly fractured
rock is problematic so a practical method of fault zone rock mass characterization is needed.
GSNZ Newsletter 143 (2007) Page 35