MESURES D'AMPLITUDE DES ONDES ULTRASONIQUES DANS LES SONDAGES*

Since 1955, the continuous velocity measurements in bore holes, made with one, later with more receivers, have become indispensable for the geophysicist, and can be useful to the geologist and the production engineer. From the beginning we have found that the amplitudes of the signals received vary in the course of a single operation. The variations have for a long time been considered as a disturbing phenomenon. It was tried to suppress them by changing the instrument gain.

With increasing precision of the velocity measurements we have sometimes observed anomalies that were hard to explain, discrepancies between the continuous velocity logs and the conventional well velocity survey, as well as between continuous velocity measurements made with different spacings. These phenomena could be observed in cased as well as in uncased holes. The first attempts to explain these phenomena by considering the velocities only, did not lead to an acceptable theory. A further study of these anomalies of the continuous velocity log has shown that in reality they were caused by strong variations of the amplitude which affected the velocity measurements. This result has led us to study the amplitude variations in open as well as in cased holes.

Up to now, the characteristics of the apparatus used in France and in the Sahara have permitted amplitude measurements with a single receiver only. In addition to the “cement bond log” in cased holes, it seems that in open holes correlations can be observed between the measured amplitude and a) the lithology; b) the fluid contained in the pores; c) the continuity of the matrix or, conversely, the permeability. However, it is now certain that numerous factors affect the amplitudes of the signals simultaneously.

In order to study the attenuation of ultrasonic waves in the different formations traversed by a deep well, we have studied the deformations of the signal in the course of its path in the mud and in the formation, restricting ourselves to the P waves. The factors to be taken in consideration are: a) the geometric energy dispersion, inversely proportional to the square of the distance from the source; b) attenuation by absorption and scattering in the mud and in the formation; c) the transmission coefficients at the transition from mud to formation and conversely, as well as at the transition between two formations having different velocities; d) phenomena of signal composition that are due to the fact that the receivers and the transmitters have a non‐infinitesimal length; these last mentioned phenomena have considerable influenc eon the amplitudes of the received signals.

Finally, the practical method for obtaining an attenuation log in an open hole requires the measurement of the ratio of the amplitudes of the first arrivals at the two receivers. Some examples show the importance of amplitude measurements made with two receivers.