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