کاربرد برش‌نگاری شکل موج داده‌‌های لرزه‌ای در یک بررسی لرزه بین‌چاهی مقیاس مهندسی

نویسندگان

1 گروه فیزیک زمین، موسسه ژئوفیزیک دانشگاه تهران

2 دانشکده مهندسی نفت، دانشگاه صنعتی امیرکبیر، تهران

چکیده

برش‌نگاری شکل موج داده‌های لرزه‌ای به منزله ابزاری کارا در تصویرسازی داده‌های لرزه‌ای قادر است تصاویر با قدرت تفکیک زیاد از ساختارهای زمین‌شناسی عرضه کند و در سال‌های اخیر مورد توجه محققان قرار گرفته است. از آنجا که در این نوع برش‌نگاری علاوه بر داده‌های زمان سیر از دامنه و شکل موج لرزه‌نگاشت‌‌ها نیز استفاده می‌شود در مقایسه با برش‌نگاری زمان سیر قدرت تفکیک بیشتری دارد، اما به سبب پیچیدگی‌های محاسباتی کمتر مورد استفاده عملی قرار گرفته است. در این مقاله نتایج اِعمال برش‌نگاری شکل موج روی داده‌‌های لرزه‌ای حاصل از آزمایش بین‌چاهی عرضه می‌شود. آزمایش لرزه بین‌چاهی روشی متداول در بررسی ساختارها و بی‌هنجاری‌‌های زمین حد فاصل دو گمانه است که با توجه به هندسه برداشت عبوری انعطاف خوبی در پوشش یکنواخت زمین مورد بررسی فراهم می‌کند. در ادامه، کاربرد روش روی داده‌‌های مصنوعی مورد ارزیابی قرار می‌گیرد. در انتها نیز داده‌‌های یک آزمایش بین‌چاهی مهندسی با استفاده از این روش وارون‌سازی می‌شود که طبق نتایج آن تصویری با جزئیات بیشتری از ساختار مورد بررسی حاصل می‌‌شود.

کلیدواژه‌ها


عنوان مقاله [English]

Application of seismic waveform tomography in an engineering seismic cross-hole study

نویسندگان [English]

  • N. Amini 1
  • A. Javaherian 2
1
2
چکیده [English]

Seismic tomography is an imaging technique which creates maps of subsurface elastic properties such as P/S wave velocity, density and attenuation, based on observed seismograms and use of sophisticated inversion algorithms. Amongst different acquisition geometries, seismic cross-hole tomography has a special position in geophysical surveys with many applications in hydrocarbons, coal and other minerals exploration and engineering purposes investigations related to constructions. Main goal of these studies is obtaining precise information about the earth structure (layers structure, impedance of layers, faults and fractures) or anomalies (objects, pipes, voids).
  Traveltime tomography is a conventional approach to convert special phase of waveform travletimes (such as P or S wave arrivals) to corresponding parameters. Low computational effort is needed to perform traveltime tomography, but the results suffer from the lack of high resolution. Seismic waveform tomography is an efficient tool for high resolution imaging of complex geological structures and has been widely used by researchers in the field of exploration seismology. As waveform tomography exploits waveforms, in addition to traveltimes, it has superior resolution comparing to traveltime tomography but its computational complexities have limited its everyday use in real world applications.
In this study we focus on application of waveform tomography in an engineering purpose seismic cross-hole study. Our approach relies on solution of acoustic wave equation in frequency domain and minimizing residual of calculated wavefield and observed seismograms. Frequency domain approach lets simultaneous sources modeling and implementing frequency dependent absorption mechanisms. This approach leads to a large system of equations. To solve the large system of equations sparse direct solvers can be used. The mixed-grid finite-difference used to discretize continuous second order hyperbolic acoustic wave equation. Although elastic modeling is more the realistic and near to observed data, most researchers prefer to use acoustic wave equation instead of elastic one due to lower computational costs. Instead, we pre-process the observed data to increase comparability of observations and modeling. These pre-processing include suppressing phases cannot be explained by acoustic modeling such as S waves or Rayleigh waves or scaling seismograms to take into account amplitude vs. offset effects in acoustic and elastic cases. Waveform tomography is very a nonlinear problem with a very rugged cost function. To overcome this nonlinearity, we solve the problem using hierarchical approaches. We start inversion from low frequency components, where the cost function is smoother, and then proceed to higher components. Lower frequency inversion results have been used as initial velocity model for higher frequency inversion.
A synthetic example has been used to test the performance of the algorithm in the absence and presence of noise. As the results show the performance of current waveform tomography algorithm decreases in case of noisy data, which implies the importance of denoising before inversion and/or employing regularization. Another strategy which helps to control noise issue is simultaneous inversion of frequency components in different groups, as showed in real data example. Lastly a real cross-hole dataset acquired for engineering purposes has been studied. The traveltime tomography result is used as starting model for waveform tomography. The results of waveform tomography are in agreement with downhole measurements.

کلیدواژه‌ها [English]

  • Seismic waveform tomography
  • Cross-hole seismic
  • Wave Equation
  • Frequency domain