USE OF EXISTING WELLS IN SEISMOGRAPH WORK 1417 
dome. This shot is recorded on D,; at the surface, also at D, in the well 
on the opposite side of the dome. By assuming the wave front to be the 
same from Shots 1 and 2, it is possible, after determining the time of ar- 
rival of the wave at D;, to calculate the position of the deeper parts of 
the wave front; therefore, to determine definitely the position of a part, 
such as A, at a definite known time. From this, and the recorded time 
of the arrival of the same wave at D,, the interval time between the 
positions A and D, is obtained. With the knowledge of this interval 
time and the velocity of sound in the shales and salt, and with the profile 
previously made of that side of the dome next to the well by the method 
shown in Figure 3, calculation can be made of the position of a point at a 
definite depth on the flank of the dome opposite the well. 
Not only can this operation be completed on a line directly opposite 
the well, but similar lines can be shot, making an angle ranging from 30° 
to 45° on either side of the diametral line. Therefore, by the use of a 
single well, not only can the flank of the dome next to the well be accurate- 
ly profiled, but it is possible to determine definitely the existence or non- 
existence of mushrooming anywhere within a go° angle on the opposite 
side of the dome. 
It is obvious, therefore, that with two, or at the most, three wells, 
properly selected around any dome, the entire periphery of the dome can 
be explored thoroughly for mushroom conditions. 
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