This page contains a Flash digital edition of a book.
Compressional wave


1 3 1 2


1 1 1 0 9 8 7 6 5 4 3 2 1 0 1 ,0 0 0 Time, µs


> Ty pical w aveform s from a m onopole transm itter in a fast form ation, show ing com pressional, shear and Stoneley w aves. The pink dashed lines are arrival tim es. A sonic-logging tool receiver array is show n at left.


3 ,0 0 0 5 ,0 0 0


Shear wave


Stoneley wave


lengths comparable to the borehole diameter— the Stoneley amplitude decays very little with distance from the borehole wall. At sufficiently low frequencies, the amplitude is nearly constant from one side of the borehole to the other, creating what is known as a tube wave. An example of a tube wave is the water-hammer effect that can sometimes be heard in plumbing pipes when flow is suddenly disrupted. The low-frequency Stoneley wave is sensitive to formation permeability. When the wave encounters permeable fractures or formations, the fluid vibrates relative to the solid, causing viscous dissipation in these zones, which attenuates the wave and slows it down (previous page, right). The reductions in Stoneley-wave energy level and velocity vary with wave frequency. Stoneley-wave dispersion data over a wide bandwidth of frequencies can be inverted to estimate formation permeability.8


Open fractures


can also cause Stoneley waves to reflect back toward the transmitter. The ratio of reflected to incident energy correlates with fracture aperture, or openness. This technique for the detection of permeable fractures works well in hard formations.9


Spring 2006


All of the above waves propagate symmetri- cally up and down the borehole, and can be detected by monopole receivers— typically hydrophones. Hydrophones are sensitive to pressure changes in the borehole fluid, and have omnidirectional response, meaning that they respond equally to pressure changes from any direction. Waveforms recorded at a given depth are initially displayed as a time series from the array of receivers (above). In some recordings, the


4 . Strutt J W, 3 rd Baron Ray leigh: “ On Waves Propagated Along the Plane Surface of an Elastic Solid, ” Proceedings of the London Mathem atical Society 17 ( 1885 ) : 4 .


Ray leigh w aves on the Earth’ s surface have vertical and horizontal com ponents of m otion. Other surface w aves discovered b y A. E. H. Love have tw o horizontal m otion com ponents.


5 . Stoneley R: “ Elastic Waves at the Surface of Separation of Tw o Solids, ” Proceedings of the Roy al Society , Series A 106 ( 19 24 ) : 4 16– 4 28.


6. Scholte J G: “ On the Large Displacem ents Com m only Regarded as Caused b y Love Waves and Sim ilar Dispersive Surface Waves, ” Proceedings of the K onink lijk e Nederlanse Ak adem ie van Wetenschappen 5 1 ( 19 4 8) : 5 3 3 – 5 4 3 .


P-, S- and Stoneley-wave arrival times can be seen clearly, but often, data-processing techniques are used to pick times accurately. The difference in arrival times divided by the distance between receivers yields the slowness for each mode. However, in many recordings, high noise levels, bad hole conditions or other factors can cause these arrivals to be indistinct or mixed with each other. In such cases, visual or automated picking of arrival times fails to yield true slownesses.


7 . Bohlen T, K ugler S, K lein G and Theilen F: “ Case History 1. 5 D Inversion of Lateral Variation of Scholte-Wave Dispersion, ” Geophy sics69 , no. 2 ( March– April 2004 ) : 3 3 0– 3 4 4 .


8. Wink ler K W, Liu HL and J ohnson DJ : “ Perm eab ility and Borehole Stoneley Waves: Com parison Betw een Ex perim ent and Theory , ” Geophy sics5 4 , no. 1 ( J anuary 19 89 ) : 66– 7 5 .


9 . Hornb y BE, J ohnson DL, Wink ler K W and Plum b RA:


“ Fracture Evaluation Using Re ected Stoneley Wave Arrivals, ” Geophy sics5 4 , no. 10 ( Octob er 19 89 ) : 127 4 – 1288.


37


R eceiver number


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68