Publication History: This article was written especially for "Crain's Petrophysical Handbook"  by E. R. Crain, P.Eng. in 2018. This webpage version is the copyrighted intellectual property of the author.

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Resistivity and acoustic i
mage logs display an unwrapped view of a cylindrical image of the borehole wall, scaled in colour to represent the resistivity or acoustic impedance of the rocks. Bedding-planes of planar dipping beds form sine wave shapes across the image, as shown in the two image logs below.


Manual calculation of dip angle and down-dip direction (dip azimuth) can be made by careful observation of the log and a simple calculation:

      1: DIP_ANGLE = ArcTan (H / D)

      2. DIP_AZIM = direction of the deepest point on the sinewave, referred to
                direction scale at top of log

  D = borehole diameter (inches or mm)
  H = peak to peak amplitude of the sine wave (same inits as borehole diameter)


Ultrasonic (acoustic) image log; travel time image (left) indicates borehole diameter, amplitude image (right) shows acoustic impedance, dips calculated from image show both bedding (green) and fracture (blue) dips. Direction scale shows 0 degrees (North) at left edge of image -- some logs have North in the middle of the log track. On this log, the deepest points on many sine waves are near the middle of the track, so dip azimuth is near South (180 degrees).

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