ACOUSTIC AND OPTICAL IMAGING TOOLS FOR FRACTURED-BEDROCK AQUIFER INVESTIGATIONS

The combined interpretation of acoustic- and optical-borehole images provides detailed information on the location, orientation, and characteristics of fractures and other structural features, and on the lithology of fractured-bedrock aquifers. This information is useful for collection and interpretation of flowmeter profiles, geophysical logs, and core samples, and it can provide insight for conceptual models of fractures, ground-water flow, and contaminant transport.

Optical-imaging tools (OTV) use a charge-coupled device (CCD) camera and a conical or hyperbolic mirror that reflects an image of the borehole wall into the camera lens to produce a magnetically oriented image. The CCD camera digitizes the intensity of the color spectrum in red, green, and blue. The high-resolution images can be used to characterize fracture orientation, lithology, infilling, alteration and oxidation. OTV images can be collected above or below the water surface in the borehole, provided the water is sufficiently clear for viewing the borehole wall. Drilling sediments in the borehole fluid, oxidation products, and biological activity can limit the quality of the images.

Acoustic-imaging tools (ATV) measure the reflection of an ultrasonic source, and record the transit time and amplitude of the reflected acoustic signal. Fixed-transducer systems focus the acoustic beam onto a rotating convex reflector that reflects and receives multiple pulses per revolution. Because the acoustic beam is highly focused, these tools can be used in wide-diameter boreholes, reportedly up to 20 inches. A new generation of ATV tools combines fixed-transducer technology with the ability to collect multiple reflections along a single reflection trace. These ATV tools can resolve reflections off polyvinyl chloride (PVC) casing and off bedrock behind the casing. They may prove useful in assessing grout placement or identifying fractures behind PVC casing in the absence of grout. ATV images can be collected in water- or light mud-filled boreholes.

Because the OTV and ATV tools measure different properties, not all features are seen by the individual imaging tools. However, these tools can complement each other if used in conjunction, which allows for integrated interpretation of data collected.