Reading azimuthal gamma-ray images for geosteering

An azimuthal gamma-ray image is one of the most useful pictures in geosteering — but only if you know how it is put together and which way is up.

Sectors around the borehole

As the tool rotates, it splits the gamma measurement into equal angular sectors around the wall of the hole and reports a value for each. With 16 sectors, each covers 22.5°. Sector 1 is centred on the high side of the hole (the top, 0°), and the sector numbers increase clockwise looking downhole — so sector 5 is on the right (90°), sector 9 on the low side (180°) and sector 13 on the left (270°).

The unwrapped image

To show the borehole wall as a flat strip, the cylinder is cut along the high side and unwrapped. The high side then appears at both edges of the track and the low side runs down the centre, with azimuth increasing left to right. Colour maps to gamma value.

The sinusoid tells you the dip

When a bed crosses the hole at an angle, its boundary draws a sinusoid across the unwrapped image. The trough of that sinusoid points to the down-dip azimuth. As you approach a boundary the sinusoid's amplitude and phase change, which is exactly the early warning a geosteerer needs to stay in zone.

Tool reference vs high-side

Two orientation modes are common. Tool reference draws sectors as the tool delivers them — fine when the tool already knows up and down. High-side re-references the image to the true high side, with a clockwise/counter-clockwise toggle and an azimuth offset so sector 1 lands on the real top of the hole regardless of how the tool numbers its sectors.