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on the tool.3


A monopole transmitter sits on


each end of the receiver array, and another monopole transmitter and two orthogonally oriented dipole transmitters are located farther down the tool (below).


Each of the three Sonic Scanner monopole transmitters produces a stronger pressure pulse than transmitters in previous sonic tools. With a sharp “ click,” they generate clear P- and S-waves, the low-frequency Stoneley mode and the high- frequency energy needed for cement evaluation. Each of the two dipole transmitters is a shaking device consisting of an electromagnetic motor mounted in a cylinder suspended in the


tool. This mechanism generates a high-pressure dipole signal without inducing vibration in the tool housing. The shaking source can be driven in two modes: the traditional dipole source in pulse mode produces a deep “ click” ; the new source also produces a “ chirp” with a frequency sweep (bottom left). The chirp mode sustains each frequency for a longer duration than narrow- band dipole sources, providing more dipole energy to the formation.


As in earlier sonic tools, such as the DSI monopole E lectronics R 1 3 1 0 ft R eceiver section R 1


X and Y dipole


U pper monopole I solator L ower monopole F ar transmitter section F ar


> The Sonic Scanner tool, w ith 13 ax ial stations in a 6-ft receiver array . Each station has eight azim uthally distrib uted receivers, giving the tool 104 sensors. Three m onopole transm itters allow acq uisition of long-spaced and short-spaced data for b orehole com pensation at vary ing depths of investigation. Tw o orthogonal dipole transm itters generate


 ex ural w aves for characterization of shear-w ave slow ness in slow and anisotropic form ations.


The freq uency sw eep of the Sonic Scanner dipole transm itter. The strong chirp creates a


w ide-b and response ( inset) that is  at from ab out 3 00 Hz to 8 k Hz.


4 0 0 2 0 0 0 0 – 2 0 0 – 4 0 0 0 2 4 6 8 1 0 Time, ms 1 2 1 4 1 6 1 8 2 0 1 0 0 8 0 6 0 4 0 2 0 F requency, H z 1 0 3 1 0 4


Dipole Shear Sonic Imager, the two dipole sources are oriented orthogonally. One vibrates in line with the tool reference axis, and the other at 90° to the axis. These devices generate strong flexural modes— waves that gently shake the entire borehole the way a person might shake a tree from its trunk. Flexural modes propagate up and down the borehole and also into the formation to different depths that depend on their frequencies. The frequency content— 300 Hz to 8 kHz— of the new chirp dipole source excites flexural modes in all borehole and formation conditions, including slow formations, and ensures maximum signal-to-noise ratio. The new sonic tool delivers P, S, Stoneley and flexural-mode waveforms with unprecedented quality. An example from a typical fast formation offshore Norway shows waveforms acquired from the monopole and dipole transmitters (next page, top). At high frequencies, the monopole source generates clear P-, S- and Stoneley waves, while at low frequencies, it generates predominantly Stoneley waves. The X- and Y- dipole transmitters generate flexural waves. The dispersion curves show slowness versus frequency for the nondispersive shear, slightly dispersive Stoneley and highly dispersive flexural arrivals. The low-frequency limit of the flexural-wave dispersion curve is in line with the slowness of the shear head wave and the true shear slowness of the formation. The two flexural curves match, indicating absence of azimuthal anisotropy. Waveforms from the same sources in a slow


formation in the USA display evident differences compared with fast-formation results (next page, bottom). The high-frequency monopole source


3 . 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, 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.


Pistre V, Plona T, Sinha B, K inoshita T, Tashiro H, Ik egam i T, Pab on J , Z eroug S, Shenoy R, Hab ashy T, Sugiy am a H, Saito A, Chang C, J ohnson D, Valero H-P, Hsu CJ , Bose S, Hori H, Wang C, Endo T, Y am am oto H and Schilling K : “ A New Modular Sonic Tool Provides Com plete Acoustic Form ation Characterization, ” Ex panded Ab stracts, 7 5 th SEG Annual Meeting, Houston ( Novem b er 6– 11, 2005 ) : 3 68– 3 7 1.


16


Oilfield Review


P ressure, P a


M agnitude, dB


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