Chap. 9] 



SEISMIC METHODS 



609 



/ \v\\v\v\\v\ '^ ^r^ 



tion shooting is the Schweydar-Askania two-component seismograph. 

 Descriptions and diagrams of it are given by Edge and Laby. ^ The 

 construction of the vertical seismograph is similar in regard to suspension, 

 mass, and lever, to the Mintrop seismograph. The mass is suspended 

 from a horizontal spring, whereas the horizontal seismograph mass is sus- 

 pended on a vertical spring. The combined instrument is so set up that 

 the plane of this spring is at right angles to the firing line. In the earlier 

 models a bow-string attachment transferred the movement of the end of 

 the lever to a mirror spindle. The illustrations in Edge and Laby give 

 the details of this arrangement. In later models, a string was tied to the 

 end of the lever, wrapped around the mirror spindle, and kept taut by a 

 spring. 



The natural frequency of both seismometers is about 15 cycles. Damp- 

 ing is accomplished by a vane attached near the end of the lever and im- 

 mersed in an oil chamber. In the models developed subsequently, the 

 masses are cylindrical and the magnifying lever is somewhat shorter than 

 in the first model. Provision is made for attaching a mirror device to 

 the head of the instrument so that the shot instant may be recorded on 

 the same strip as the two components. 



Several other mechanical prospecting 

 seismographs were constructed, in the 

 period from 1924 to 1929, by Ricker, 

 Truman, Taylor, and others. 



2. Electrical seismographs, (a) Electro- 

 magnetic seismographs. A modification 

 of the Schweydar mechanical seismograph 

 was used extensively at one time as an 

 inductive seismograph. A coil was at- 

 tached to the end of the magnifying lever. 

 This was free to move in the field of an 

 electromagnet supplied with current from 

 a storage battery. The period of this 

 seismograph was about 0.03 second. It 

 was used without intermediate amplifier 

 with a Zeiss loop galvanometer. Fig. 

 9-1 13a shows a dual-coil variant of the 

 original instrument. In the Cambridge 



Instrument Company's electromagnetic refraction seismograph, the magni- 

 fying lever makes an angle with the horizontal. A string is fastened to its 

 end. This in turn is wound around the shaft of a galvanometer acting as 

 a generator. The seismograph is used in connection with a Cambridge 

 string galvanometer. 



/e amplifiN' 



^\\\\\\\\\\\\\r^ 



Fig. 9-113o. Dual-coil induction 

 seismograph. 



61 OV- cit., figs. 158-160 and 255-257. 



