Dry Hole

Andesite lava
The dry test hole was located in the Quarternary volcanic area in Japan. The hole was drilled to
the depth of 230 meters at the edge of a lava terrace. Figure 2 shows the lithology and the conventional logging results of this dry hole. The lava terrace was composed of crystalline and hard bedrock such as andesite and basalt lava. The target of FWS logging was fractures of andesite lava between 36 to 114 meters of hole depth. The andesite bedrock lay over the sand and gravel layer such as debris flow at the depth of 114 meters, and was covered with another basaltic lava at the depth of 36 meters. The andesite lava had an auto-brecciated part at the top and bottom. In the middle part of the lava developed fractures such as cooling joints. As the groundwater level was 114 meters below the surface of the land, this test hole was dried out. Fractures such as joints are known to leak water. However we wished to acquire sonic signals at narrow intervals to identify fractures.

The Tube
To fill water in the dry hole for FWS logging, we used a synthetic rubber tube. The tube was set
into the hole, and the joints along the hole were sealed. The tube comprised a long synthetic rubber tube and a clip. The rubber tube was 120 meters long, and 60 millimeters in diameter, made of /Ethylene /Propylene /Diene /Methylene /Linkage (EPDM). EPDM is used for automobile parts such as radiator heater hoses and brake hoses as it is resistant to oil, burning, and weather. Such a tube not only has enough strength to withstand water pressure and handling in the hole, but is also inflatable. The tube inflates and contacts firmly to the hole wall even if the tube diameter is smaller than the hole diameter of 66 millimeters. And a clip was used to seal the bottom part of the tube, and as a weight to pull the tube down. The addition of water into the tube made the tube inflate and contact firmly to the hole wall. The water level reached approximately 70 meters above the groundwater level in the bedrock. Besides FWS in the tube, this hole was also logged using a suite of conventional geophysical logs, caliper, natural gamma spectrum (NGS), temperature, magnetic (Mt), and borehole imaging equipment of a borehole camera in the dry condition.

Figure 2.: Lithology and conventional logging results in the dry test hole. Besides FWS in the tube, this hole was also logged with a suite of conventional geophysical logging, caliper, natural gamma spectrum (NGS), temperature, magnetic (Mt), and borehole camera equipment in the dry condition