I am Ka Wai Helen Lau

I work as a

Marine Seismologist and Geophysicist


Department of Earth Sciences
Dalhousie Imaging Group (DIG)

Research Interests

My research interest has been evolving throughout the years as new discoveries and hypotheses are generated and new data acquisitional and analytical technologies are available. In my earlier years as a Ph.D. student, I was trained using only sparse datasets and relatively labour intensive analytical methods although it was already a state-of-the-art project that resulted in two publications still widely cited today (e.g. Lau et al., 2006). The world is very different now and my current project uses data that are of much higher resolution even for regional scale profiling (500+ km) and cheaper computation allows tomographic methods that can deal with large datasets through automations. Along-strike variation in continental rifted margin structures is also receiving more attention as along-dip conjugate studies become more mature (e.g. Lau et al, 2019). I predict that future focus will be more 3-D oriented as it better describes the physical world and higher degree of automations will then be needed for the increase amount of data and complexity. This is also the direction that I am happy to steer towards.

Previous Research

I have always wondered about the mechanism by which the continental crust was extended to facilitate continental breakup followed by the formation of new oceanic crust. While geodynamic numerical modelling, which is used to simulate tectonic events, has greatly improved our understanding of the physics behind tectonic processes such as the rifting of tectonic plates that eventually formed the oceans, I am bothered by the lack of real observations capable of discriminating good models from bad ones. My approach to solve this problem 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 acoustic velocity models from wide-angle seismic or ocean-bottom seismograph (OBS) data. These results are then used to constrain the crustal origins at different parts of the margin, as well as their geometry.

My Ph.D. thesis was my first attempt in such a quest through which I observed the Newfoundland Basin/Grand Bank margin under the supervision of Keith Louden at Dalhousie University. It is a magma-poor margin which was believed to be the best site for observing structural fabrics related directly to the rifting processes. This study (the SCREECH project) was part of an international collaboration in which I worked on Line 3, the southern-most profile (Lau et al., 2006). As a result of this project, the continent-ocean transition (COT) zone which was observed on this margin had become my new research focus. It has also been a benchmark study to show the importance of exhumed serpentinized mantle in asymmetrical rifting and had since been incorporated in later geodynamic models (e.g. Huismans & Beaumont, 2014). In terms of methodogy, there were only 24 OBSs deployed at 10–40 km spacing along a 550-km long wide-angle data profile and a 6-km-long streamer was used for MCS data acquisition. I used the Rayinvr forward modelling method ( Zelt & Smith, 1992) which is relatively labour intensive as the picking of wide-angle phases could only be done manually. Regarding the coincident MCS profile, I performed post-stack time migration which was then converted to depth using the final wide-angle velocity model. Through this, I was able to image the top basement which was obscured by reverberations along previous MCS profiles of the area.

To add to my portfolio of magma-poor margins, I also have studied the Orphan Basin, offshore Newfoundland, through the OBWAVE project with K. Louden and Mladen Nedimović. 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 (Lau et al., 2015). Contrary to the SCREECH result, we did not observe serpentinized mantle even under crust hyperextended to beta factor that should have facilitated its formation. This supported a search for new criteria for ductile deformation in the lower crust which had already been considered in geodynamic modelling. Technology had advanced much in this project where 89 OBSs were deployed and retrieved at 3–5 km spacings in 2 legs. Such a high data resolution had allowed my colleague to use a first-arrival tomographic method that depends less on modeller’s subjective input which could be a source of errors ( Watremez et al., 2015). Based on this result, I was able to produce a high resolution Rayinvr layered model in a more objective manner.

While magma-rich margins were believed to be less optimal for observing extensional fabrics, their petroleum potentials are no less than that of magma-poor margins. To learn more about magma-rich margins, I studied the structures of the Faroe and Shetland margins during my time at the University of Cambridge and the Schlumberger Cambridge Research for the iSIMM project under Bob White and Phil Christie (Lau et al., 2010). Unlike magma-poor 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. Although it was my second project, it was already technologically a big leap forward from my first one. The high-resolution wide-angle data were acquired using 100 OBSs at 2–6 km spacing and MCS data were collected using a 12-km long streamer. This allowed me to use a creative approach that jointly modeled the OBS and streamer data for the wide-angle refractions and reflections from and through the thick basaltic layer and to perform pre-stack depth migration. The resulting MCS section and velocity model were consistent with each other. I was then 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. This low velocity layer was interpreted to be sub-basalt sediment and may have interesting implications concerning petroleum exploration.

My exposure to both magma-poor and magma-rich margins had inspired me to look for any 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 questions were actually very relevant for both tectonic and exploration interests on the Nova Scotia rifted continental margin which hosts a transition between magma-rich and magma-poor type margins but was relatively underexplored. Since 2015, my work at Dalhousie, with funding from the Offshore Energy Research Association (OERA), has been on Nova Scotia using both the Play Fairway Analysis (PFA) data and data previous collected by Dalhousie (OCTOPUS). Coincident long-streamer GXT profiles were available as constraints. High resolution layered velocity models were once again produced using Rayinvr but this time at both dip and along-strike orientations covering the northeastern Nova Scotian margin (Lau et al., 2018, 2019). These recent results imaged in detail the crustal structures of the magma-poor COT which was interpreted to be a mix of continental fragments, serpentinized mantle and embryonic oceanic crust. The high degree of along-strike variation was also previously unknown. This demonstrated the importance of 3-D rift models.

Current Research

I continue to believe that an extensive study of both the Nova Scotia margin and its conjugate Moroccan margin will shed light on how volcanism may affect rifting. Solutions of this fundamental question will have far-reaching impacts to the tectonophysics community. However, my velocity model on the Nova Scotian margin shows architecture that is very different from that of its conjugate, the Moroccan margin, although both sides used the Rayinvr method. To solve this mystery, I am remodelling the MIRROR-1 profile using first-arrival tomographic inversion, followed by full waveform seismic inversion to minimize any potential bias that may have been the source of differences with the layered model approach previously used. The OBS dataset is, however, of moderate resolution (<10 km receiver spacing) compared with my previous projects. However, the VMTomo algorithm that I am using has shown records of success with even sparser datasets. I am currently finalizing the VMTomo model and a manuscript for this new result. The result of full waveform inversion is, however, disappointing due to the high demand of constraints for this method and so new strategies are to be developed to hopefully circumvent this problem.

Reference (cited above)

Biari, Y., Klingelhoefer, F., Sahabi, M., Aslanian, D., Schnurle, P., Berglar, K., et al. (2015). Deep crustal structure of the North-West African margin from combined wide-angle and reflection seismic data (MIRROR seismic survey). Tectonophysics, 656, 154–174. https://doi.org/10.1016/j.tecto.2015.06.019

Huismans, R. S., & Beaumont, C. (2014). Rifted continental margins: The case for depth-dependent extension. Earth and Planetary Science Letters, 407, 148–162. https://doi.org/10.1016/j.epsl.2014.09.032

Lau, K. W. H., Louden, K. E., Funck, T., Tucholke, B. E., Holbrook, W. S., Hopper, J. R., & Christian Larsen, H. (2006). Crustal structure across the Grand Banks–Newfoundland Basin Continental Margin – I. Results from a seismic refraction profile. Geophysical Journal International, 167(1), 127–156. https://doi.org/10.1111/j.1365-246X.2006.02988.x

Lau, K. W. H., Nedimović, M. R., & Louden, K. E. (2018). Continent-ocean transition across the northeastern Nova Scotian margin from a dense wide-angle seismic profile. Journal of Geophysical Research: Solid Earth, 0. https://doi.org/10.1029/2017JB015282

Lau, K. W. H., Nedimović, M.R. & Louden, K.E., 2019. Along-strike variations in structure of the continent-ocean transition at the northeastern Nova Scotia margin from wide-angle seismic observations. Journal of Geophysical Research: Solid Earth, v.124. https://doi.org/10.1029/2018JB016894

Lau, K. W. H., Watremez, L., Louden, K. E., & Nedimovíć, M. R. (2015). Structure of thinned continental crust across the Orphan Basin from a dense wide-angle seismic profile and gravity data. Geophysical Journal International, 202(3), 1969–1992. https://doi.org/10.1093/gji/ggv261

Lau, K. W. H., White, R. S., & Christie, P. A. F. (2010). Integrating streamer and ocean-bottom seismic data for sub-basalt imaging on the Atlantic Margin. Petroleum Geoscience, 16(March 2011), 349–366. https://doi.org/10.1144/1354-0793/10-023

Watremez, L., Lau, K.W.H., Nedimović, M.R. & Louden, K.E., 2015. Traveltime tomography of a dense wide-angle profile across Orphan Basin, Geophysics, v. 80(3), B69-B82, doi: 10.1190/geo2014-0377.1.

Zelt, C. A., & Smith, R. B. (1992). Seismic traveltime inversion for 2–D crustal velocity structure. Geophysical Journal International, 108, 16–34. https://doi.org/10.1111/j.1365-246X.1992.tb00836.x