USGS Groundwater Information: Branch of Geophysics
What is the multi-echo acoustic televiewer (ATV)?
The multi-echo acoustic televiewer (ATV) is a borehole geophysical probe that uses the acoustic method. The multi-echo ATV tool works similarly to regular ATV tools and is used to generate high-resolution, magnetically oriented acoustic reflectivity images of the borehole wall. The multi-echo tool shows multiple arrivals along a single trace.
Why collect multi-echo ATV data?
Multi-echo acoustic televiewer data can be used to:
How does the multi-echo ATV work?
Acoustic televiewers (ATV) emit an acoustic pulse. ATV tools use an ultrasonic pulse-echo signal source. The multi-echo ATV tool uses a fixed source that is reflected off a rotating mirror, which focuses the acoustic beam on the borehole wall.
As the acoustic wave travels outward from the source that emitted it, part of the wave can be reflected at an interface when there is a change in acoustic impedance across the interface. Changes in lithology or structure can result in changes in acoustic impedance large enough to reflect part of the acoustic signal. The acoustic pressure waves of this reflected signal are detected by the tool. Travel time and amplitude of the reflected acoustic wave are recorded by the ATV tool. The travel-time data can be used to generate high-resolution acoustic images of the borehole wall and caliper logs with caliper resolution better than 0.1 millimeter. The multi-echo ATV records multiple arrivals (reflected signals sensed by the tool) along individual traces. The analog signal is digitized downhole by the tool, and the digital signal is sent uphole along a wireline for display and analysis on a computer.
ATV images can be collected in water- or light mud-filled intervals of boreholes. Changes in borehole diameter related to structures such as fractures, foliation, and bedding planes scatter energy from the acoustic beam, reduce the signal, and produce recognizable features on the digital images. Acoustic impedance contrast between the borehole fluid and the borehole wall causes the signal to be reflected, creating an acoustic image of the borehole wall. Lithologic changes, foliation, bedding, and sealed fractures may be detected even when there is no change in borehole diameter if there is sufficient acoustic contrast. Because the ATV data are oriented to magnetic north, the orientation (strike and dip) of features can be determined.
The tool also collects data on the deviation of the borehole from vertical so that its three-dimensional location can be determined. In addition, the orientation of planar features determined in the images can be corrected for deviation.
Effective imaging of the borehole wall with an ATV tool requires that the tool be centered within the borehole and that the borehole casing be centered in the borehole.
Sample Multi-Echo ATV Data
Multi-echo ATV data can be visually represented in different ways. A radial plot (fig. 1) of an example data set shows the arrival times of the reflected signal plotted azimuthally. The first (innermost) reflector, indicated in black, is the reflection from the edge of the tool. The second reflector (red) is the reflection off of the plastic casing in the borehole. The third reflection (blue) is off of the borehole wall behind the plastic casing.
In figure 2 a single trace line plot is shown from the same data set as figure 1. The single trace of the sample multi-echo ATV data shows three arrivals or reflected signals, with two-way travel time measured in microseconds: the first high-amplitude peak (black arrow) is the reflection from the edge of the tool; the second high-amplitude peak (red arrow) is the reflection from the plastic casing; and the third-high amplitude peak (blue arrow) is the reflection from the borehole wall bedrock behind the plastic casing.
Oriented, 360-degree views of multi-echo ATV data can also be shown in flattened, projected images with the image split along north. Figure 3 shows the second and third arrivals (reflectors) recorded over a section of borehole. The arrival times for the second reflector show the smooth plastic casing. The arrival times of the third reflector indicate the presence of (1) a casing centralizer and (2) a fracture behind the casing. If the magnetic orientation of the data and the deviation of the borehole are known, the strike and dip of individual features can be estimated.
For More Information...
USGS offices and cooperators can contact Carole Johnson (OGW BG) at cjohnson or (860) 487-7402 x17 to learn more about using the multi-echo acoustic televiewer tool or to discuss related USGS project or training needs.
For more in-depth information on the acoustic televiewer logging method, see:
Johnson, C.D., 2002, Acoustic and optical imaging tools for fractured-rock aquifer investigations [abs.], in Geological Society of America 2002 Annual Meeting Abstracts with Programs: Denver, Colo., Geological Society of America, v. 34, no. 6, p. 228.
Keys, W. Scott, 1990, Borehole Geophysics Applied to Ground-Water Investigations: Techniques of Water-Resources Investigations of the United States Geological Survey, Book 2, Chapter E2, 150 p.
Williams, J.H., and Johnson, C.D., 2000, Borehole-wall imaging with acoustic and optical televiewers for fractured-bedrock aquifer investigations: in Seventh International Symposium on Borehole Geophysics for Minerals, Geotechnical, and Groundwater Applications, October 24-26, 2000, Proceedings: Houston, Texas, Minerals and Geotechnical Logging Society, p. 43-53.
Data images courtesy Jim LoCoCo, Mount Sopris Industries.
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