Ka Wai Helen Lau
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Position

Post-doc Research Associate

Institution


Department of Oceanography
Dalhousie Seismic Group

Department of Earth Sciences
Dalhousie Imaging Group (DIG)

Supervisors

Prof. Keith E. Louden, Department of Oceanography, Dalhousie University
Prof. Mladen R. Nedimovic, Department of Earth Sciences, Dalhousie University

Research Interests

North Atlantic volcanic and non-volcanic rifted continental margin tectonophysics

I am interested in the mechanism by which the continental crust was extended to facilitate continental breakup followed by the formation of new oceanic crust. My approach is by observing the crustal structures of rifted continental margins on both sides of the North Atlantic Ocean and by comparing them with geodynamic models. Marine seismic imaging methods are being used which involve producing multi-channel seismic (MCS) sections and an acoustic velocity model from wide-angle seismic or ocean-bottom seismic (OBS) data. These results are then used to constrain the crustal origins at different parts of the margin, as well as their geometry.

Regarding magma-poor margins, I started my study on the Newfoundland Basin/Grand Bank margin when I was doing my Ph.D. thesis at Dalhousie University with Dr. Keith Louden. This study was part of the SCREECH project and I worked on Line 3, the southern-most profile. As a result of this project, I become interested in the study of the ocean-continent transition zone which characterizes this margin. I would like to compare my results with those on the conjugated Iberia margin when new results are available to test the conclusions of my thesis and to work out a complete scenario for the rifting process of the whole Newfoundland-Iberia margin. I also have studied Orphan Basin, which is also offshore Newfoundland, in recent years at Dalhousie University through the OBWAVE project. It is an abandoned, rifted continental basin that allows a detailed quantification of the whole crustal thinning as well as its partition in the upper and lower crustal layers. My ambition for future works would be to further investigate the role that exhumed or un-exhumed serpentinized mantle plays and the importance of ductile deformation in the lower crust and/or lithospheric scale detachment in the development of non-volcanic margins.

Regarding magma-rich margins, I studied the structures of the Faroe and Shetland margins in the North Sea after I went to work at the University of Cambridge for the iSIMM project. Unlike non-volcanic margins, this margin is both intruded and extruded by a thick layer of basalt. This makes the imaging of the margin structure both a challenging and yet rewarding task. Again, by using seismic methods, I have been able to define the structures beneath the basalt layer better than had been possible in previous studies. According to the wide-angle data, a step-back in a plot of refracted time versus receiver-shot distance gives a hint for the presence of a low velocity layer, underneath the basalt. I was interested in better constraining the velocity, depth and thickness of this layer which may have interesting implications concerning petroleum exploration.

Bringing all these together, I would like to see if there are connections between the formations of these two kinds of margins. Are they fundamentally the same, except for the presence of melt? If not, how would the present of melt affect the way the lithosphere is being extended? These are examples of the questions that I would like to pursue . Moreover, these questions make it all the more important for research activities on the Nova Scotia rifted continental margin which hosts a transition between magma-rich and magma-poor type margins. My current work at Dalhousie is on the Play Fairway Analysis (PFA) data. I hope that through better characterization of this transition, I will be able to better understand the conditions under which the transition occurred. This in turn may shed light on how volcanism may affect rifting. I think this question is quite fundamental that its solution may have far-reaching impacts in the tectonophysics community.


Co-analysis of multichannel and high density wide-angle seismic data

My three most recent seismic surveys (namely, iSIMM, OBWAVE and PFA) acquired high density/resolution seismic data with constraints from coincident MCS data. In the iSIMM project, I was able to analyse the pre-stack MCS streamer data and the ocean-bottom seismic wide-angle data together, integrating the constraint provided by the two datasets. This approach solves many common problems in literature in which the coincident reflection section and the velocity model are often not consistent with each other. Therefore, the achievements of my results are highly regarded in my field and are benchmark making. I am interested in applying this approach to all of my future studies as much as possible.


Sub-basalt imaging

Although this is the technical side of the tectonophysical problem, sub-basalt imagining is in itself a very interesting topic to me and has wide applications. Due to the high attenuative and reverberative nature of basalt to seismic energy, imaging of the underlying structures using conventional acquisition and processing procedures is usually very difficult. Removal of reverberative noise is normally very challenging. With the use of low-frequency sources in the iSIMM project, the penetration is much improved. My interest is to develop the best practice for this kind of technique and to apply it to more datasets. The same technique can also be adopted to sub-salt imaging which can be relevant to petroleum exploration along part of the Nova Scotia margin.


Water-column seismic

With a background in oceanography, I am also interested in looking at seismic reflections in the water column using existing data. The Faroe-Shetland trough is a very good laboratory for such research purposes due to the presence of different water masses flowing against each other. Seismic survey is also a useful tool for estimating the velocity structure in the water over a large distance (up to hundreds of km). This experience will open collaborations with scientists in oceanography.


Ocean technology

Recently, I have been reading about the effects of air-gun sound on marine mammals and my interest in this area is growing. If opportunity arises, I would commit more time to studies that can contribute to the development of air-guns or other acoustic sources that would be more marine mammal friendly. My first attempt will be the use of marine vibroseis seismic sources.


Current Project

OBWave (Orphan Basin Wide Angle Velocity Experiment)

This project is a colaboration between the Department of Earth Sciences and the Department of Oceanography in the Dalhousie University and ExxonMobil. We acquired high resolution OBS data coincident with an industrial MCS profile. My role is to produce a layered velocity model for input to conjugate margin reconstruction and basin development studies. We are currently concluding this project and a paper have been submitted to Geophysics for review and one in preparation.


OCTOPUS (Ocean-Continent TransitiOn Profile Using Seismic)

This study focuses on characterizing the along strike variation within the OCT of the Northeastern Nova Scotia margin. This project builds upon previous studies including the SMART and the NovaSpan Play Fairway Analysis. Three component data from 20 OBSs were collected over a 240-km long profile. Preliminary model shows high degree of alongstrike variation and seismic anisotropy within the interpreted serpentinized mantle layer (Lau et al. 2011). The margin has been chosen as one of the Discovery Corridor of the GeoPRSIMS program Eastern North Atlantic Margin (ENAM) initiative. The focus is on the transition from magma-rich to magma poor rifting. We hope that our new results together with existing knowledge of the margin will lead to success in any NSF proposals submitted for studies in this margin.


Previous Project

iSIMM (integrated Seismic Imaging and Modelling of Margins)

This project is a joint effort between a number of industrial and academic institutes. This is also a coordinated effort between seismic imaging groups and theoretical earth modelling groups to tackle the then inability to image beneath basalt layers and to model properly the stretching, thermal and subsidence history of the NW European margin. Four seismic profiles have been acquired near the Faroe Islands and Hatton Bank where both MCS and OBS data were collected. With these challenges in mind, the aquisition was done using state-of-the-art Q-marine streamer from Schlumberger and an innovative low-frequency air-gun source (peak amplitude at ~10Hz).

The iSIMM group in the University of Cambridge has participated in the acquisition and post-cruise data processing and analysis by a number of Ph.D. students and post-docs. Due to their outstanding progress in constraining the margin structures through generating good results in p- and s-wave velocity modelling of OBS data and pre-stack and post-stack time migration of MCS data, we are now entering the final phase of the project in which more advanced approaches, such as pre-stack depth migration of MCS and OBS data, are being used to further constrain previous results.

My contribution to this project was to investigate the significance of the low-frequency seismic source to sub-basalt imaging (Lau et al., 2007); and to constrain the seismic velocity structures and thicknesses of the proposed low velocity layer underlying a thick sequence of flood basalt beneath the Faroe-Shetland Trough and the Fugloy Ridge (Lau et al., 2010). This layer has been interpreted as being sedimentary rock. I have also been looking at the reflectivity in the water column within the Faroe-Shetland Trough which reveals some very interesting reflections.


Mariprobe/SCREECH (Studies of Continental Rifting and Extension on the Eastern Canadian sHelf)

This seismic survey is an international collaboration between several academic institutes for a structural study on the Newfoundland Basin and Grand Bank margin. It filled the scientific gap in a complete margin study since the Iberia margin has been intensively studied and a conjugate study on the Canadian side was long awaited.

My contribution was mainly the processing of MCS data for the seaward half of Line 3 and p-wave velocity modelling of the OBS data for the whole profile. Basement fault blocks were observed underneath a high amplitude, reverberative reflection after careful demultipling work using fk filtering and predictive deconvolution. Continental crustal is therefore found to extend more seaward than was previously speculated by observing magnetic anomaly. Other key observations include: lower-than-normal mantle velocities suggesting serpentinized mantle; the absence of lower crustal crustal velocity above without detachment, suggesting ductile deformation in the lower crust. Structures along other profiles farther north are somewhat different, suggesting considerable alongstrike variations.