Effects of an off-centered tool on dipole and quadrupole logging

Geophysics ◽  
2006 ◽  
Vol 71 (4) ◽  
pp. F91-F100 ◽  
Author(s):  
Joongmoo Byun ◽  
M. Nafi Toksöz
Keyword(s):  
Shear Velocity ◽  
Dipole Source ◽  
Flexural Wave ◽  
Dipole Mode ◽  
Quadrupole Mode ◽  
Receiver Array ◽  
Zero Response ◽  
Receiver Arrays ◽  
Logging Tool ◽  
Discrete Wavenumber Method

An acoustic logging tool in inclined or horizontal boreholes may be placed apart from the center and produce additional complicated wavefields. We investigate the effects of an off-centered tool on monopole, dipole, and quadrupole logs due to off-centering of the tool. In recent logging tools, monopole, dipole, and quadrupole logs can be obtained by adding or subtracting responses at four monopole (pressure) receiver arrays at right angles. We examine the responses of the four monopole receiver array system for three directions of eccentricity: inline direction of the dipole source (Off [Formula: see text] case), crossline direction (Off [Formula: see text] case), and [Formula: see text] to the dipole source (Off [Formula: see text] case). To simulate responses at each receiver array, we use the discrete wavenumber method. In a dipole tool positioned on the borehole axis, the inline receiver arrays have only the flexural wave while the cross receiver arrays have zero response. However, an off-centered dipole source produces nondipole modes as well as the dipole mode. For all cases with a dipole source, the dipole mode was enhanced by subtracting responses at one of the inline receiver arrays from those of the other receiver array, and we could extract the formation shear velocity. Similar to the dipole source, an off-centered quadrupole source introduces additional nonquadrupole modes. The quadrupole mode was enhanced by subtracting the sum of responses at receiver arrays below negative poles of a quadrupole source from the sum of responses at receiver arrays below positive poles for all cases. In addition, the formation shear velocity was obtained from the time semblance calculation.

Finite‐difference modeling of elastic wave propagation: A nonsplitting perfectly matched layer approach

Geophysics ◽  
2003 ◽  
Vol 68 (5) ◽  
pp. 1749-1755 ◽  
Author(s):  
Tsili Wang ◽  
Xiaoming Tang
Keyword(s):  
Wave Propagation ◽  
Finite Difference ◽  
Elastic Wave ◽  
Shear Velocity ◽  
Perfectly Matched Layer ◽  
Elastic Wave Propagation ◽  
Dipole Source ◽  
Flexural Wave ◽  
Field Approach ◽  
Split Field

In this paper, we present a nonsplitting perfectly matched layer (NPML) method for the finite‐difference simulation of elastic wave propagation. Compared to the conventional split‐field approach, the new formulation solves the same set of equations for the boundary and interior regions. The nonsplitting formulation simplifies the perfectly matched layer (PML) algorithm without sacrificing the accuracy of the PML. In addition, the NPML requires nearly the same amount of computer storage as does the split‐field approach. Using the NPML, we calculate dipole and quadrupole waveforms in a logging‐while‐drilling environment. We show that a dipole source produces a strong pipe flexural wave that distorts the formation arrivals of interest. A quadrupole source, however, produces clean formation arrivals. This result indicates that a quadrupole source is more advantageous over a dipole source for shear velocity measurement while drilling.

scholarly journals Origin of the hemispheric asymmetry of solar activity

2018 ◽  
Vol 618 ◽  
pp. A89 ◽  
Author(s):  
M. Schüssler ◽  
R. H. Cameron
Keyword(s):  
Solar Activity ◽  
Hemispheric Asymmetry ◽  
Source Term ◽  
Random Excitation ◽  
Dynamo Model ◽  
Dipole Mode ◽  
Poloidal Field ◽  
Quadrupole Mode ◽  
Stochastic Fluctuations ◽  
Magnetic Period

The frequency spectrum of the hemispheric asymmetry of solar activity shows enhanced power for the period ranges around 8.5 years and between 30 and 50 years. This can be understood as the sum and beat periods of the superposition of two dynamo modes: a dipolar mode with a (magnetic) period of about 22 years and a quadrupolar mode with a period between 13 and 15 years. An updated Babcock–Leighton-type dynamo model with weak driving as indicated by stellar observations shows an excited dipole mode and a damped quadrupole mode in the correct range of periods. Random excitation of the quadrupole by stochastic fluctuations of the source term for the poloidal field leads to a time evolution of activity and asymmetry that is consistent with the observational results.

Statistically perturbed geophone array responses

Geophysics ◽  
1989 ◽  
Vol 54 (10) ◽  
pp. 1306-1317 ◽  
Author(s):  
David F. Aldridge
Keyword(s):  
Vertical Plane ◽  
P Wave ◽  
Angle Of Incidence ◽  
Physical Treatment ◽  
Seismic Receiver ◽  
Receiver Array ◽  
System Input ◽  
Response Variation ◽  
The Time Domain ◽  
Receiver Arrays

Seismic‐receiver arrays implemented under typical field conditions are subject to a variety of perturbing influences. The array responses that are actually achieved differ, perhaps substantially, from the nominal response associated with ideal conditions (precise positioning, vertical plants, identical geophones, perfect ground coupling, etc.). Variations in receiver array response may degrade the effectiveness of multichannel processing and analysis schemes that rely upon channel‐to‐channel waveform constancy. In effect, array‐response variation is a form of noise added to recorded waveforms and is thus potentially harmful. A rigorous physical treatment of the response of a geophone array to incident plane‐wave elastic radiation forms the point of departure for assessing the importance of response perturbations. The hard‐wired multiple seismometer group, long transmission line, and recording‐system input impedance are considered an electromechanical system. An individual geophone may have arbitrarily specified position and axial orientation and is modeled as a ground‐motion transducer that incorporates, to first order, the effect of compliant coupling to the earth. Elastic waves (of either vibratory mode) can be incident from any direction. This generality built into the mathematical description of receiver‐array response allows numerous array types (including those designed to record shear waves) to be analyzed. All parameters that determine the response value are then subjected to controlled random perturbations in order to evaluate the statistical variability of the complex valued array‐response function. Transformation of the perturbed responses to the time domain indicates the extent of waveform variability induced by geophone‐array diversity. Computational studies indicate that, for vertical or near‐vertical plane P‐wave incidence, reasonable variations in the controlling parameters do not reduce waveform coherence by any major amount. Peak times of reflection signal recorded on well planted geophone arrays typically vary by up to 4 ms. As the angle of incidence increases or the quality of the field‐array implementation degrades, the wavelets exhibit increasing amplitude loss, wave‐shape alteration, and incoherence that may affect an interpretation.

Beamform processing for sonic imaging using monopole and dipole sources

Geophysics ◽  
2020 ◽  
Vol 86 (1) ◽  
pp. D1-D14
Author(s):  
Nobuyasu Hirabayashi
Keyword(s):  
Signal To Noise Ratio ◽  
Random Noise ◽  
Dipole Source ◽  
P Waves ◽  
Arrival Times ◽  
Acoustic Reflection ◽  
Back Azimuth ◽  
Sonic Logging ◽  
Logging Tool ◽  
Dipole Sources

New processing techniques are presented that enhance event signals for sonic imaging using monopole and dipole sources. The techniques use the azimuthally spaced receivers of a sonic logging tool. Sonic imaging, which is also known as borehole acoustic reflection surveys, uses a sonic logging tool in a fluid-filled borehole to image geologic structures. Signals from monopole and dipole sources are reflected from geologic interfaces and recorded by arrays of receivers of the same tool. Because the amplitudes of the event signals are very weak compared with the direct waves, borehole modes, and noise, the event signals are often difficult to extract. To enhance the weak event signals, beamforming techniques were developed to stack the waveforms from azimuthally spaced receivers of the tool for given azimuthal directions. For the incident P-waves from the monopole source, phase arrival times for the azimuthal receivers are time shifted for stacking using properties of wave propagation in the borehole. For the incident SH-waves from the dipole source, the signs of waveforms for the receivers are changed for specified azimuths. When the waveforms are stacked for the back azimuth of the event signals, the signal-to-noise ratio of the event signals is significantly improved because the event signals are enhanced whereas the direct waves are relatively smeared, and random noise is canceled. Therefore, the stacked waveforms also provide accurate back azimuths of the incident waves.

Gaussian beam imaging of fractures near the wellbore using sonic logging tools after removing dispersive borehole waves

Geophysics ◽  
2020 ◽  
Vol 85 (4) ◽  
pp. D133-D143
Author(s):  
David Li ◽  
Xiao Tian ◽  
Hao Hu ◽  
Xiao-Ming Tang ◽  
Xinding Fang ◽  
...  
Keyword(s):  
Gaussian Beam ◽  
Field Data ◽  
Gas Production ◽  
Dipole Source ◽  
Data Sets ◽  
Data Set ◽  
Receiver Array ◽  
Gaussian Beam Migration ◽  
Sonic Logging ◽  
Borehole Waves

The ability to image near-wellbore fractures is critical for wellbore integrity monitoring as well as for energy production and waste disposal. Single-well imaging uses a sonic logging instrument consisting of a source and a receiver array to image geologic structures around a wellbore. We use cross-dipole sources because they can excite waves that can be used to image structures farther away from the wellbore than traditional monopole sources. However, the cross-dipole source also will excite large-amplitude, slowly propagating dispersive waves along the surface of the borehole. These waves will interfere with the formation reflection events. We have adopted a new fracture imaging procedure using sonic data. We first remove the strong amplitude borehole waves using a new nonlinear signal comparison method. We then apply Gaussian beam migration to obtain high-resolution images of the fractures. To verify our method, we first test our method on synthetic data sets modeled using a finite-difference approach. We then validate our method on a field data set collected from a fractured natural gas production well. We are able to obtain high-quality images of the fractures using Gaussian beam migration compared with Kirchhoff migration for the synthetic and field data sets. We also found that a low-frequency source (around 1 kHz) is needed to obtain a sharp image of the fracture because high-frequency wavefields can interact strongly with the fluid-filled borehole.

Parametric estimation of phase and group slownesses from sonic logging waveforms

Geophysics ◽  
1992 ◽  
Vol 57 (8) ◽  
pp. 978-985 ◽  
Author(s):  
Kai Hsu ◽  
Cengiz Esmersoy
Keyword(s):  
Parametric Method ◽  
Estimation Method ◽  
Parametric Estimation ◽  
Flexural Wave ◽  
Smooth Functions ◽  
Dispersive Waves ◽  
Prony Method ◽  
Receiver Array ◽  
Frequency Independent ◽  
Sonic Logging

Sonic logging waveforms consist of a mixture of nondispersive waves, such as the P‐ and S‐headwaves, and dispersive waves, such as the Stoneley and pseudo‐Rayleigh waves in monopole logging and the flexural wave in dipole logging. Conventionally, slowness dispersion curves of various waves are estimated at each frequency, independent of data at other frequencies. This approach does not account for the fact that slowness dispersion functions in sonic logging are continuous and, in most cases, smooth functions of frequency. We describe a parametric slowness estimation method that uses this property by locally approximating the wavenumber of each wave as a linear function of frequency. This provides a parametric model for the phase and group slownesses of the waves propagating across the receiver array. The estimation of phase and group slownesses is then carried out by minimizing the squared difference between the predicted and observed waveforms. The minimization problem is nonlinear and is solved by an iterative algorithm. Examples using synthetic and field data are shown and the results are compared with those obtained by the conventional Prony method. Based on the comparison, we conclude that the parametric method is better than the conventional Prony method in providing robust and stable slowness estimates.

Simultaneous inversion of formation shear‐wave anisotropy parameters from cross‐dipole acoustic‐array waveform data

Geophysics ◽  
1999 ◽  
Vol 64 (5) ◽  
pp. 1502-1511 ◽  
Author(s):  
Xiaoming Tang ◽  
Raghu K. Chunduru
Keyword(s):  
Objective Function ◽  
Shear Wave ◽  
Waveform Inversion ◽  
New Technique ◽  
Dipole Source ◽  
Waveform Data ◽  
Simultaneous Inversion ◽  
Receiver Array ◽  
Acoustic Array ◽  
Anisotropy Parameters

This study presents an effective technique for obtaining formation azimuthal shear‐wave anisotropy parameters from four‐component dipole acoustic array waveform data. The proposed technique utilizes the splitting of fast and slow principal flexural waves in an anisotropic formation. First, the principal waves are computed from the four‐component data using the dipole source orientation with respect to the fast shear‐wave polarization azimuth. Then, the fast and slow principal waves are compared for all possible receiver combinations in the receiver array to suppress noise effects. This constructs an objective function to invert the waveform data for anisotropy estimates. Finally, the anisotropy and the fast shear azimuth are simultaneously determined by finding the global minimum of the objective function. The waveform inversion procedure provides a reliable and robust method for obtaining formation anisotropy from four‐component dipole acoustic logging. Field data examples are used to demonstrate the application and features of the proposed technique. A comparison study using the new and conventional techniques shows that the new technique not only reduces the ambiguity in the fast azimuth determination but also improves the accuracy of the anisotropy estimate. Some basic quality indicators of the new technique, along with the anisotropy analysis results, are presented to demonstrate the practical application of the inversion technique.

The two-photon laser beam as a breathing mode of the electromagnetic field

1978 ◽  
Vol 56 (4) ◽  
pp. 442-446 ◽  
Author(s):  
D. J. Rowe
Keyword(s):  
Electromagnetic Field ◽  
Laser Beam ◽  
Single Photon ◽  
Coherent Beam ◽  
Dipole Mode ◽  
Linear Processes ◽  
Breathing Mode ◽  
Quadrupole Mode ◽  
Two Photon ◽  
Coherent Mode

It is predicted that the beam from a two-photon laser, if it can be made to operate in a coherent mode, will have properties quite different from the classical properties of the standard single-photon laser. Whereas the coherent beam of the single-photon laser is a dipole mode of the electromagnetic field, that of the two-photon laser is expected to be a monopole or quadrupole mode. It is predicted that the coherence properties of the two-photon laser will show up rather dramatically in non-linear processes.

Simulation and interpretation of sonic waveforms in high-angle and horizontal wells

Geophysics ◽  
2017 ◽  
Vol 82 (4) ◽  
pp. D251-D263 ◽  
Author(s):  
Ruijia Wang ◽  
Wilberth Herrera ◽  
Carlos Torres-Verdín
Keyword(s):  
Guided Waves ◽  
Frequency Dispersion ◽  
Dispersion Curves ◽  
Dipole Source ◽  
Flexural Waves ◽  
High Angle ◽  
S Waves ◽  
Sonic Logging ◽  
Logging Tool ◽  
Dipole Sources

We have developed a new method to process and interpret sonic waveforms acquired in high-angle (HA) and horizontal (HZ) wells in the vicinity of layer boundaries. Numerical simulations are examined for HA/HZ fluid-filled borehole models, in which sonic tools with monopole and dipole sources operate across a horizontal bed boundary in hard and soft formations. Simulations are performed with a 3D time-domain finite-difference method assuming a sonic logging tool, in which each receiver station consists of eight azimuthal receivers. Instead of conventional processing for monopole and dipole waveforms, which takes the average and difference, respectively, of waveforms acquired with azimuthal receivers, we analyze the waveforms acquired individually by each azimuthal receiver. For dipole sources, we refer to the dispersive signals recorded by individual receivers as pseudo-flexural waves, and we first process them with a weighted spectral semblance method to obtain the frequency dispersion curves. We then apply a correction to the dispersion curves of receivers with specific azimuths to estimate the formation shear slowness. Close to a horizontal bed boundary, we select two azimuths: One of them is the azimuth with the corresponding receiver positioned within the top layer and having minimal sensitivity to the bottom formation. The second azimuth is such that the corresponding receiver is positioned within the bottom formation and has minimal sensitivity to the top formation. Simulations show that the presence of a bed boundary significantly alters the propagation of P-, S-, and flexural waves. Rather than by borehole guided waves, in HA wells, receiver signals generated with a monopole source are mostly influenced by converted P-P and S-S waves induced by the bed boundary. Apparent slownesses of these converted waves are determined by their direction of propagation and the corresponding true formation slowness. Also, values of apparent slownesses are consistently lower than the true formation slownesses. In soft formations, borehole guided pseudo-flexural waves generated by a vertical dipole source show good azimuthal resolution for the HA and HZ wells. Processing the pseudo-flexural waveforms separately for the top and bottom receivers yields unbiased shear slownesses for formations above and below the well, respectively. This procedure enables the accurate evaluation of formation heterogeneity due to the bed-boundary effects.

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