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Dipole receivers


F ast S-wave


Slow S-wave


R 1 3 x R 1 2 x


R 1 1 x R 1 0 x


R 9 x R 8 x R 7 x R 6 x


Dipole source


Source pulse


R 5 x R 4 x


R 3 x R 2 x


> Shear-w ave splitting in a vertical b orehole in a TIH m edium w ith vertical fractures. No m atter how the dipole source is oriented relative to the fast and slow directions of the m edium , the shear


w ave w ill split into fast and slow com ponents. The fast com ponent aligns parallel to the plane of the fractures, w hile the slow com ponent aligns perpendicular to the plane of the fractures.


R 1 x


R 1 3 y R 1 2 y R 1 1 y


R 1 0 y R 9 y R 8 y


R 7 y R 6 y R 5 y R 4 y


R 3 y R 2 y


R 1 y


U ndisturbed borehole


transmitter pair


Ty Tx


fractures (above). Similarly, in a TIV medium, such as a shale or a finely layered interval, S- waves propagating in a horizontal borehole split, and the fast wave becomes polarized in the bedding plane. The


polarization of S-waves split by


anisotropy cannot be detected by a single monopole receiver. Directional receivers are required. A suitable directional receiver can be created by substituting a single monopole receiver with two or more pairs of monopole receivers. Each pair of monopole receivers acts as a dipole reciever. For adequate recording of flexural waves, at least one dipole receiver is aligned with each dipole transmitter. At each firing of the dipole source, signals are recorded by the dipole receiver oriented “ inline” with that source and also by the dipole receiver oriented “ offline” (above right).1 5


This example shows


recording of flexural waves at 13 receiver stations with eight receivers distributed in a ring at each station.1 6


x θ y x’ Tool axis


F ormation fast shear-wave axis


y’


Tool orientation relative to formation


> Inline and of ine response on azim uthally distrib uted receivers from a b orehole  ex ural w ave in an anisotropic form ation. The  ex ural w ave


w as ex cited b y  ring of the X -dipole transm itter, Tx , show n at the b ottom . In this TIH m edium , the  ex ural w ave splits into fast and slow w aves w ith com ponents of particle m otion on all receivers, not j ust those aligned w ith the tool X -ax is.


Dipole


R eceiver-1 3 ring R eceiver-1 2 ring R eceiver-1 1 ring R eceiver-1 0 ring R eceiver-9 ring R eceiver-8 ring R eceiver-7 ring R eceiver-6 ring R eceiver-5 ring R eceiver-4 ring R eceiver-3 ring R eceiver-2 ring R eceiver-1 ring


L ow-frequency borehole


flexural wave ( exaggerated)


R eceiver array


In isotropic


formations, flexural waves


generated by a dipole source remain polarized in the plane of the source and are detected only on the dipole receiver aligned in that plane. However, in anisotropic formations, the flexural wave splits into fast and slow components aligned with the formation anisotropy. Unless the tool axes are fortuitously aligned with the


15 . Of ine is som etim es referred to as crossline.


16. Pistre V, K inoshita T, Endo T, Schilling K , Pab on J , Sinha B, Plona T, Ik egam i T and J ohnson D: “ A Modular Wireline Sonic Tool for Measurem ents of 3 D ( Azim uthal,


formation’s fast and slow directions, flexural- wave energy will be recorded by the offline as well as the inline receivers.


The directions, or azimuths, of fast and slow shear or flexural waves can be seen in a crossed- dipole log. Creating a crossed-dipole log is a multistep process. The first step is decompo- sition and recombination of the waveforms


Radial, and Ax ial) Form ation Acoustic Properties, ” Transactions of the SPWLA 4 6th Annual Logging Sy m posium , New Orleans, J une 26– 29 , 2005 , paper P.


Spring 2006


41


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