Chap. 9] SEISMIC METHODS 503 



3. Fan-shooting equipment. Equipment and instruments used in fan 

 shooting depend to some extent on the purpose of the survey. Fan-shoot- 

 ing equipment for oil exploration is more elaborate than that employed in 

 mining and engineering applications, since the distances between shot 

 point and receiving points and the distances between individual fan sta- 

 *tions are much greater. For this reason, the shooting technique also 

 differs. In oil exploration, where large charges are required for fans three 

 to seven miles in length, a hole is drilled first to a depth of 20 to 25 feet; 

 a large cavity is blown out at the bottom of the hole, and the main charge 

 is placed in it. A surface charge is hooked in with the main charge to 

 transmit the sound of the explosion. The two charges are then fired 

 simultaneously from a shooting truck which is set up at a safe distance 

 and is equipped with a radio transmitter to relay the instant of firing to 

 the receiving points. The latter are individually equipped with either 

 photographically recording mechanical seismographs or with electrical 

 detectors connected to a three- to four-stage amplifier and oscillograph. 



The recording camera often contains another oscillograph element for 

 the recording of the soimd of the explosion, which is received by an elec- 

 trical microphone (blastophone). A third element may be used to record 

 the shot-instant signal picked up by the radio receiver, or the shot instant 

 may be recorded on the seismic detector trace. The sound record serves 

 to calculate the distance between shot point and receiving point. Since 

 wind direction, velocity, air temperature, and barometric pressure must be 

 known for an accurate evaluation of the record, the receiving units are 

 usually equipped with meteorological apparatus. Mechanical seismo- 

 graphs are set up in small tents, and electrical detectors are buried in auger 

 holes several feet deep. The radio receivers are equipped with antennas 

 strung out on bamboo poles, one on the recording truck and the other set 

 up some distance from it. The receiving apparatuses with seismic, mete- 

 orological, and radio accessories are carried in recording trucks, of which 

 there are usually three to six to each fan party. These operate simul- 

 taneously in fan arrangement for one shot and are then moved to the next 

 portion of the fan, the shot being repeated at, or very close to, the original 

 shot location. At the present time the fan-shooting technique in oil ex- 

 ploration is more or less past history in this country, but it is still being 

 applied in foreign oil exploration. 



In mining and in engineering applications the dimensions of the fans 

 are small, and therefore multi-channel electrical equipment with a com- 

 mon recording element is much preferred. Four to twelve electrical de- 

 tectors are connected to a recording truck, recording trailer, or recording 

 tent, which is set up approximately in the middle of the arc of the fan. 

 At this central recording point are the amplifiers and the recording camera 



