Method and apparatus for minimizing cavity effects in acoustic logging

Claim 1

APPARATUS FOR PRODUCING A LOG OF THE ACOUSTIC PROPERTIES OF EARTH FORMATIONS ADJACENT A BOREHOLE COMPRISING FIRST AND SECOND TRANSMITTERS FOR SUPPLYING IMPULSES AT DIFFERENT TIMES TO THE EARTH FORMATIONS AND FIRST AND SECOND RECEIVERS, SAID TRANSMITTERS AND RECEIVERS BEING ARRANGED IN LINE AND ADAPTED TO BE MOVED THROUGH THE BOREHOLE, SAID TRANSMITTERS BEING SO SPACED FROM SAID RECEIVERS THAT THE DIFFERENCE BETWEEN THE FIRST TRANSMITTER-FIRST RECEIVER SPACING AND THE FIRST TRANSMITTER-SECOND RECEIVER SPACING IS EQUAL TO THE DIFFERENCE BETWEEN THE SECOND TRANSMITTERSECOND RECEIVER SPACING AND THE SECOND TRANSMITTER-FIRST RECEIVER SPACING, A SINGLE RECORDER CHANNEL INCLUDING AN INTEGRATING ELEMENT, A FIRST CIRCUITRY INTERCONNECTING SAID RECORDER CHANNEL, SAID RECEIVERS AND A FIRST OF SAID TRANSMITTERS FOR DEVELOPING FIRST ELECTRICAL DIFFERENCE SIGNALS PROPORTIONAL TO THE DIFFERENCE IN TRAVEL TIMES OF THE IMPULSES ARRIVING AT SAID FIRST AND SECOND RECEIVERS FROM SAID FIRST TRANSMITTER, A SECOND CIRCUITRY INTERCONNECTING SAID RECORDER CHANNEL, SAID RECEIVERS AND A SECOND OF SAID