Borehole image compression

A method for obtaining borehole images at a surface installation while a borehole-imaging downhole tool is logging a borehole, the method comprising:

• while the downhole tool is disposed in the borehole:
• obtaining a first borehole image at a first depth of investigation using the borehole-imaging downhole tool;
• compressing the first borehole image using downhole compression electronics to obtain a first compressed borehole image, wherein compressing the first borehole image comprises exploiting a symmetry characteristic of the first borehole image to increase compression efficiency;
• and
• transmitting the first compressed borehole image to the surface installation via a telemetry system,
• wherein compressing the first borehole image comprises determining whether the symmetry characteristic is present in the first borehole image and, if so, exploiting the symmetry characteristic of the first borehole image to increase compression efficiency,
• wherein determining whether the symmetry characteristic is present comprises:
• selecting a set of symmetry line test locations in the first original borehole image;
• for each of the symmetry line test locations:
• extracting a first half image on a first side of the symmetry line test location;
• extracting a second half image on a second side of the symmetry line test location;
• reversing the second half image to obtain a reversed second half image;
• determining a similarity value between the first half image and the reversed second half image;
• identifying one or more minima in a set of similarity values for the symmetry line test locations;
• determining that the symmetry characteristic is present when the one or more minima include only one minimum;
• and
• determining that the symmetry characteristic is not present when the one or more minima include more than one minimum.