Seismic-while-drilling by drill-bit source and large-aperture ocean-bottom array

Seismic while drilling (SWD) by drill-bit source has been successfully used in the past decades and is proven using variable configurations in onshore applications. The method creates a reverse vertical seismic profile (RVSP) dataset from surface sensors deployed as arrays in the proximity of the monitored wells. The typical application makes use of rig-pilot reference (pilot) sensors at the top of the drill-string and also downhole. This approach provides while-drilling checkshots as well as multioffset RVSP for 2-D and 3-D imaging around the well and prediction ahead of the bit. For logistical (sensor deployment) and cost (rig time related to technical installation) reasons the conventional drill-bit SWD application is typically much easier onshore than offshore. We present a novel approach that uses a network of passive-monitoring sea bottom nodes pre-deployed for microseismic monitoring to simultaneously and effectively record offshore SWD data. We study the results of a pilot test where we passively monitored the drilling of an appraisal well at the Wisting discovery in the Barents Sea with an ocean-bottom cable deployed temporarily around the drilling rig. The continuous passive recording of vibration signals emitted during the drilling of the well provides the SWD data set, which is treated as a reverse vertical seismic profile. The study is performed without rig-pilot signal. The results are compared with legacy data and demonstrate the effectiveness of the approach and point to future applications for real-time monitoring of the drilling progress, both in terms of geosteering the drill bit and predicting formation properties ahead of the bit by reflection imaging.

The Application of Wave – Equation Datuming to 3D VSP Processing

Abstrak Vertical Seismic Profile (VSP) memperluas aplikasi seismik lubang bor dari hubungan 1D antara waktu-kecepatan hingga citra 2D atau 3D di sekitar lubang bor. Citra seismik yang dihasilkan dari VSP diharapkan memiliki resolusi vertikal yang lebih tinggi dari data seismik permukaan karena gelombang seismik direkam di dalam lubang bor. Namun, pengolahan data VSP 2D dan 3D memiliki tantangan karena sifat asimetri dari penjalaran gelombang membatasi untuk diterapkannya pengolahan data berbasis Common Mid Point (CMP) seperti analisis kecepatan, Normal Moveout (NMO), dan koreksi statik. Penelitian ini mendiskusikan sebuah pendekatan baru untuk mentransformasikan rekaman gelombang P pantul atau upgoing wavefields ke sebuah datum datar di permukaan berbasis persamaan gelombang. Transformasi tersebut menghasilkan gelombang seismik yang seolah-olah direkam pada pseudo-reveiver di permukaan bumi sehingga pendekatan penglahan data berbasis CMP dapat diterapkan. Konsep ini kemudian diterapkan pada sebuah data VSP 3D yang diakuisisi dengan geophone yang ditempatkan sangat dekat dengan permukaan. Hasil pemodelan elastik 2D menunjukkan bahwa gelombang seismik pantul sangat dipengaruhi zona kecepatan rendah di dekat permukaan. Jarak yang jauh antara reflektor target dengan geophone menghasilkan rasio sinyal terhadap bising yang rendah. Kondisi desain akuisisi ini sangat mempengaruhi hasil akhir dari citra VSP 3D. Walaupun begitu, citra VSP 3D yang dihasilkan berdasarkan hasil transformasi gelombang P upgoing ini menunjukkan korelasi yang cukup baik dengan data seismik permukaan di zona reservoir. Kata kunci: 3D VSP, transformasi datum, persamaan gelombang, gelombang pantul, pemodelan

Practical concept of traveltime inversion of simulated P-wave vertical seismic profile data in weak to moderate arbitrary anisotropy

We have developed a practical concept of compressional wave (P-wave) traveltime inversion in weakly to moderately anisotropic media of arbitrary symmetry and orientation. The concept provides sufficient freedom to explain and reproduce observed anisotropic seismic signatures to a high degree of accuracy. The key to this concept is the proposed P-wave anisotropy parameterization (A-parameters) that, together with the use of the weak-anisotropy approximation, leads to a significantly simplified theory. Here, as an example, we use a simple and transparent formula relating P-wave traveltimes to 15 P-wave A-parameters describing anisotropy of arbitrary symmetry. The formula is used in the inversion scheme, which does not require any a priori information about anisotropy symmetry and its orientation, and it is applicable to weak and moderate anisotropy. As the first step, we test applicability of the proposed scheme on a blind inversion of synthetic P-wave traveltimes generated in vertical seismic profile experiments in homogeneous models. Three models of varying anisotropy are used: tilted orthorhombic and triclinic models of moderate anisotropy (approximately 10%) and an orthorhombic model of strong anisotropy (>25%) with a horizontal plane of symmetry. In all cases, the inversion yields the complete set of 15 P-wave A-parameters, which make reconstruction of corresponding phase-velocity surfaces possible with high accuracy. The inversion scheme is robust with respect to noise and the source distribution pattern. Its quality depends on the angular illumination of the medium; we determine how the absence of nearly horizontal propagation directions affects inversion accuracy. The results of the inversion are applicable, for example, in migration or as a starting model for inversion methods, such as full-waveform inversion, if a model refinement is desired. A similar procedure could be designed for the inversion of S-wave traveltimes in anisotropic media of arbitrary symmetry.

Imaging below a complex overburden with borehole-seismic data

We have determined how the measured polarization and traveltime for P- and S-waves can be used directly with vertical seismic profile data for estimating the salt exit points in a salt-proximity survey. As with interferometry, the processes described use only local velocities. For the data analyzed in this paper, our procedures have confirmed the location, inferred from surface-seismic data, of the flank of a steeply dipping salt body near the well. This has provided us more confidence in the estimated reservoir extent moving toward the salt face, which in turn has added critical information for the economic evaluation of a possible new well into the reservoir. We also have found that ray-based vector migration, based on the assumptions of locally plane wavefronts and locally plane formation interfaces, can be used to create 3D reflection images of steeply dipping sediments near the well, again using only local velocities. Our local reflection images have helped confirm the dips of the sediments between the well and the salt flank. Because all parameters used in these processes are local and can be extracted from the data themselves, the processes can be considered to be self-sufficient.