Welcome to my website!

Next to presenting my personal info, the purpose of my website is to distribute some technical stuff and knowledge.
My curriculum vitae and some of my scientific papers about inversion can be found below.
Some computer codes for Graham's algorithm, Cardano's method, Ferrari's method, numerical dispersion analysis, and Cagniard-De Hoop technique are included.
Other technical topics and computer codes will be added later.

Keukenhof, May 2008

Curriculum Vitae

EDUCATION

WORK EXPERIENCE

  1. Geophysical Exploration
    Time-lapse seismic reservoir characterization with the aid of a regularized nonlinear inversion technique.
  2. Medical Imaging
    Acoustic inversion with the aid of the scattering technique. Verification of theoretical results and a comparison in the time and frequency domains.
  3. Regularization
    Stabilization of inversion schemes using the nonlinear multiplicative regularization technique. Applications in seismic signal processing, image processing and medical imaging.
  4. Cagniard-De Hoop technique
    Analytical technique for 3D modelling of impulsive acoustic and electromagnetic waves in stratified media.
  5. Finite Volume Method
    Numerical technique for solving partial differential equations governing the acoustic and electromagnetic waves on unstructured mesh grids.

JOURNAL PAPERS

  1. M. C. H. Lam, "Time-lapse seismic reservoir monitoring with a local waveform inversion technique," Journal of Applied Geophysics, vol. 71, no. 4, pp. 99-108, Aug. 2010.
  2. A. T. de Hoop, C. H. Lam and B. J. Kooij, "Parameterization of acoustic boundary absorption and dispersion properties in time-domain source/receiver reflection measurement," The Journal of the Acoustical Society of America, vol. 118, pp. 654-660, Aug. 2005.
  3. C. H. Lam, B. J. Kooij and A. T. de Hoop, "Impulsive sound reflection by an absorptive and dispersive planar boundary," The Journal of the Acoustical Society of America, vol. 116, pp. 677-685, Aug. 2004. pdf file

CONFERENCE PAPERS

  1. M. C. H. Lam (2010). "On the macro velocity model in full-waveform inversion applied to time-lapse seismic reservoir monitoring,"
    Society of Exploration Geophysicists, SEG/Denver 2010, USA, pp. 2885-2889. pdf file
  2. M. C. H. Lam and M. D. Sacchi (2009). "Seismic signal processing with automatic edge-preserving algorithms,"
    Society of Exploration Geophysicists, SEG/Houston 2009, USA, pp. 2511-2515. pdf file
  3. C. H. Lam. P. M. van den Berg and D. Gisolf (2007). "Nonlinear acoustic inversion of focused seismic data,"
    Society of Exploration Geophysicists, SEG/San Antonio 2007, USA, pp. 1750-1754.
  4. C. H. Lam. P. M. van den Berg and D. Gisolf (2007). "Acoustic time-domain inversion of focused data,"
    European Association of Geoscientists and Engineers, EAGE/London 2007, UK, P278.
  5. C. H. Lam. P. M. van den Berg and D. Gisolf (2006). "On the background model for nonlinear inversion of seismic data,"
    Society of Exploration Geophysicists, SEG/New Orleans 2006, USA, pp. 2012-2016.

TEACHING EXPERIENCE

  1. Lecture assistant Introduction to Electricity and Magnetism (2008,2007).
  2. Lecture assistant Introduction Electromagnetic Waves (2005,2004).
  3. Laboratory course assistant Signal Processing Laboratory Course (2007,2005).
    Algorithm implementation in MATLAB for speech signal processing and wireless communications.

BACKGROUND AND COURSES

National Park, August 2007
Canadian Nature

Scientific Projects

  1. TIME-DOMAIN WAVE REFLECTION
    Title: Transient Wave Reflection against a Planar Impedance Boundary
    Summary: This book presents theoretical time-domain studies of both acoustic and electromagnetic wave reflection against a planar impedance boundary. The interest for investigating analytically the employment of the surface impedance boundary approximation stems from the need of efficient wave simulation tools to tackle large-scale transient wave modelling problems, also called forward problems, in which approximations are desirably invoked for reducing the required computational efforts to a minimum. Presented analytical studies unveil the mathematical structure of the transient acoustic and electromagnetic wave fields after reflection by an impedance plane, about which general purpose wave simulation techniques such as the finite-difference time-domain (FDTD) method and time-domain finite-element method (TD-FEM) can only yield qualitative insights.
    The first part of this book investigates impulsive acoustic wave reflection against porous materials and viscous fluids, which are subjects of interest in sound control and geophysical exploration, respectively. The second part investigates transient electromagnetic wave reflection by imperfectly conductive materials. For these purposes, the elementary two-media configuration is considered, which consists of a planar boundary that separates a simple medium from a complex medium. The reflected wave in the simple medium is of interest and the complex medium is mathematically represented by an approximating impedance wall. At the core of tackling the impedance boundary value problem corresponding to the resulting half-space configuration is the derivation of the space-time reflected-wave Green's function, to which end the extended Cagniard-De Hoop method is invoked for the analytical transformation back to the time domain. The main features of this time-domain method are the preservation of causality by employing the unilateral Laplace transform and uniqueness of the causal wave solution.
    The analytically derived closed-form wave field expressions are fully discussed in the main chapters to reveal the fundamental structure of the space-time reflected wave fields in their dependence of the physical properties of the complex medium. Numerically computed time snaps of the reflected-wave Green's function and the reflected-wave Green's tensors are illustrated to exhibit the spatial distribution of the reflected wave fields in the near, intermediate and far zones. Furthermore, modelled time traces of the reflected wave fields are also included in the main chapters to serve as benchmark results for general purpose wave simulation software.


  2. TIME-LAPSE SEISMIC
    Title: Time-lapse Seismic Reservoir Monitoring with a Local Waveform Inversion Technique
    Summary: The characterization of changes in a producing hydrocarbon reservoir is of importance for both fluid injection strategies and an assessment of the reservoir. In 4D seismic monitoring, a quantitative measure is obtained from changes in the acoustic medium properties, of which the parameter values are extracted from over time acquired seismic data. In this paper, full-waveform inversion is applied to tackling a class of 1.5D time-lapse seismic problems. The local linear inversion concept is at the core of the applied regularized iterative scheme, which aims at locally estimating P-wave velocity parameters from seismic data, and requires a macro velocity model that resembles the low-frequent representation of the true P-wave velocity profile. The comparative inversion results reveal substantial improvement with regard to both kinematics and dynamics after performing three nonlinear iterations. For practical interest, the employment of updated and non-updated background velocity models is addressed.
    Report: pdf file

  3. GEOPHYSICAL INVERSION
    Title: Two-Parameter Acoustic Waveform Inversion in the Local Born Approximation using Inhomogeneous Background Models
    Summary: A local Born inversion based regularized iterative scheme was previously presented, which aims at retrieving only P-wave velocity parameters from seismic data. The additional estimation of mass density parameters is of importance for a number of exploration and reservoir monitoring applications, in which two-parameter full-waveform inversion is desirable for high-resolution imaging of the subsurface. In this abstract, an extension of the previous iterative scheme to performing two-parameter acoustic inversion in the local Born approximation is discussed. Numerical results illustrate the capability of the inversion scheme in simultaneously reconstructing measured borehole profiles for the P-wave velocity and the mass density from noisy synthetic data with SNR = 20 dB. Inhomogeneous background models are employed in this 1.5D study, which serves as a preliminary work to two-parameter inversion in higher dimensions.
    Report: pdf file

  4. MEDICAL IMAGING
    Title: Fourier-Domain Extrapolation with Edge-Preserving Regularization
    Summary: The Fourier-domain extrapolation property of edge-preserving regularization (EPR) is proved in this paper with the aid of the limiting Ewald circle from linear inverse scattering theory. A tomographic imaging strategy based on a sequence of linear inversions is first reviewed. Comparative results obtained from linear inversion with and without EPR are subsequently presented to verify the spectral-domain reconstruction bounds and to illustrate correct amplitude reconstruction exterior to the limiting Ewald disc. The objective is to explain medical imaging results, which have a resolution that is higher than what can be expected based on the source signature's bandwidth if EPR-based linear inversion techniques are employed. Furthermore, super resolution obtained from EPR-based nonlinear inversion methods is discussed in this theoretical study.
    Report: pdf file

  5. INVERSION STABILIZATION
    Title: Edge-Preserving Multiplicative Regularization with Weighted Total Variation
    Summary: An automatic inversion algorithm is presented, which targets at edge restoration in low signal-to-noise (SNR) scenarios and is free from estimating a regularization parameter. The iterative scheme is based on the SNR-independent multiplicative regularization technique and employs a weighted total variation constraint. Compared with standard total variation (TV) regularization, utilizing the weighted strategy is first related to half-quadratic regularization, and then shown to yield improvement in edge preservation for a range of practical SNR values. Furthermore, various deterministic regularization strategies are compared in a deconvolution setting, in which weighted TV regularization is shown to outperform weighted l2-norm regularization.
    Report: pdf file

  6. SCATTERING IN THE TIME AND FREQUENCY DOMAINS
    Title: Review of the Convergence Criterion for the Time-Domain Neumann Series applied to Wave Scattering by Dispersive Metals
    Summary: This article reviews the convergence criterion for the time-domain Neumann series and presents novel numerical experiments. In [De Hoop, J. Opt. Soc. Am. A, vol. 8, pp. 1256-1260, 1991], fundamentally different convergence criteria for the time- and frequency-domain Neumann series were derived and argued to be caused by causality of wave motion, which is only preserved in the time domain. For the class of dispersive metals, it was theoretically shown that only the time-domain Neumann series is unconditionally stable. That result is in this study numerically supported by investigating impulsive electromagnetic wave scattering in a one-dimensional setting. With experiments using different numerical implementations, instability of any FFT-based implementation of the Neumann series is shown.
    Report: pdf file

Some Computer Source Codes

The included computer codes are made available for students, and not for commercial purposes nor scientific research.
Note that though all codes have thorougly been checked, bugs might still be present.
  1. Graham's Algorithm for 2D Convex Hull Construction
    In computational geometry, which is heavily applied to Computer Aided Design tools, a fast generation of the convex hull from a point set in a plane is given by Graham's algorithm. A pseudo code with complexity of O(N log N) is given for example by WIKIPEDIA. Note the sorting algorithm for ordering the angles in the included code. Furthermore, a more efficient implementation using data structures such as linked lists instead of vectors is recommended.
    Graham's Algorithm: doc file


    Procedure in Convex Hull Construction

  2. Cardano's Method for Solving Cubic Equations
    In nonlinear optimization, solving cubic equations with real coefficient is encountered for example if the multiplicative regularization technique is employed. Cardano's method provides a procedure for analytical computation of the three roots of a cubic equation. Many websites are devoted to this Cardano's technique, which outperforms numerical search methods in solving for the roots of a third-order polynomial. The mathematical derivations/procedures can be found in for example WIKIPEDIA.
    Cardano's Method: doc file

  3. Ferrari's Method for Solving Quartic Equations
    In applications such as computer graphics, solving quartic equations with real coefficients must be carried out with the highest efficiency. Ferrari's method provides a procedure for solving for the roots of a fourth-order polynomial by first turning the quartic equation into depressed form and then invoking Cardano's method, e.g. WIKIPEDIA.
    Ferrari's Method: doc file

  4. Numerical Dispersion in Finite-Difference Time-Domain Method
    Numerical dispersion is a well-known phenomenon in grid based time-domain forward modelling techniques such as the FDTD. The included program compares the FDTD 1D technique (without Perfectly Matched Layers) with an analytical solution in a two-media configuration. The source domain is `well-sampled,' whereas the source-free domain is `under-sampled' as a result of a higher mass density in the investigated seismic configuration. This problem arises in general when costly, but necessary, local grid refinement in spatial regions with a higher `index number' is ignored in practical FDTD simulations.
    Numerical Dispersion Analysis: doc file


    Increasing Phase Lagging as Time evolves

  5. Cagniard-De Hoop Method applied to Canonical Fluid/Solid Configuration
    Analyzing transient acoustic wave reflection against a fluid/solid interface is of interest in geophysical exploration, for example in borehole/well log measurements. The included code is based on the Cagniard-De Hoop method, cf. JASA paper by De Hoop and Van der Heijden (1983). Two time traces of the 2D reflected-wave Green's function for the acoustic pressure are depicted below. Note the head-wave arrival due to the P-wave in the solid (first discontinuity) and the Scholte wave (surface wave represented by the last lump). The body-wave arrival and the effect of the S-wave in the solid are found in between these two phenomena, with the order of occurrence being dependent on the consideration of a fast or slow rock formation.
    Cagniard-De Hoop Method (2D Fluid/Solid): doc file


    Acoustic Response of a Fast Rock Formation (left) and Slow Rock Formation (right)

  6. Delaunay Triangulation for 2D Mesh Generation
    Delaunay triangulation method produces a 2D topology and is based on Watson's algorithm. It is heavily used for unstructured mesh grid generation for the purpose of 2D finite-element analysis. A more efficient implementation using data structures such as linked lists instead of vectors is recommended.
    Delaunay Triangulation Algorithm: doc file


    Procedure in 2D Mesh Generation

Some Photos