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The principle of Acoustic televiewer tools is based on the analysis of the reflected signal of an ultrasonic beam from the borehole wall (Zemanek, 1970). The resulting data are the travel time (proportional to the borehole radius) and the amplitude (proportional to the rock impedance). The analysis provides a high resolution caliper of the borehole as well as the rock impedance (rock density, rock p-wave velocity). Travel time data and amplitude values are converted into colorized images of the borehole wall. Figure 2a gives a brief overview of the data flow of the acoustic Televiewer from the borehole wall to the resulting image. The advantage of the acoustical image is to receive additional physical properties of the borehole surrounding rock.
Due to the testing principle it is necessary to have water filled borehole.
Tool design philosophy & applications
Acoustic Borehole scanner tools generates an image of the borehole wall by transmitting ultrasound pulses from a rotating sensor and recording the amplitude and travel time of the signals reflected at the interface between mud and formation (borehole wall).
Most common applications are
Fracture detection and evaluation
Detection of thin beds
Determination of bedding dip
Lithological interpretation
Breakout analysis
Monitoring of earth stress field
Casing inspection
High resolution caliper
The latest generation of ALT acoustic televiewer is a multi-echo system. This is achieved by digital recording of the reflected wave train. On line analysis of the acoustic data is made by a DSP (Digital Signal Processor). Sophisticated algorithms allow the system to detect the reflection from the acoustic window and to separate/classify all subsequent echoes. Minimum input of the operator is needed to enable:
Automatic optimization of measurement window under all borehole conditions
Automatic adaptation to variable borehole conditions
Improvement of dynamic range of signal detection
Very high travel time resolution
Implementation of different operating mode
Main features
Direct linear 14bit, 10mhz A/D conversion (automatic gain settings)
Dynamic range of amplitude measurement is 84 db
Calliper resolution is better than 0,1mm
Orientation sensor which does not require field calibration
Increased telemetry bandwidth (data transmission rate up to 500kbits depending on wireline), 166kbps typical on 300m four-conductors.
Full wave form recording and real time display for quality control
Multiecho mode to realize acoustic measurements through plastic pipes (require the plastic pipe to be centralized inside the borehole)
Pipe-inspection mode to detect inner corrosion, outer corrosion, and wall thickness
Automatic measurement window definition to gain optimum performance and variable borehole conditions
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