Chap. 9] SEISMIC METHODS 579 



vertical incidence and converted to elevations above sea level. For dip 

 shooting, this form is modified for the calculation of depths and dip from 

 mean times and time gradients (corrected for weathering and to datum). 

 In correlation shooting, stations are arranged on profiles and cross 

 traverses so that an interlocking pattern is formed. Reflection impulses 

 may be entered by appropriate symbols in the reflection log for each sta- 

 tion and may be connected in much the same manner as key beds of ad- 

 jacent well logs (see Fig. 9-92). In dip shooting, dips may be indicated 

 by a short bar at the reflection depths and correlation may be made on a 

 "phantom" horizon or horizons (see Fig. 9-93). Depth determinations 

 made on this basis are not very reliable, but errors are reduced by con- 

 ducting the survey in a closed loop and distributing the errors graphically 

 or by calculation. 



Phantom Horii$o 



Mit. 



IML. 



Fig. 9-93. Dip profile across deep-seated salt dome in southern Louisiana (after 



Goldstone). 



IV. ELEMENTARY THEORY AND DESCRIPTION OF 

 SEISMOGRAPHS 



A. Classification 



Seismographs may be classified according to component recorded or 

 according to application, The first division is customary in station seis- 

 mology. There we speak of horizontal, vertical, and universal seismo- 

 graphs. Fig. 9-94 shows a number of horizontal seismographs in diagram- 

 matic form. The oldest type is probably the simple pendulum (1). The 

 pendulums in 2, 3, and 4 follow the "swinging door" principle. The 

 Wiechert astatic pendulum (5) is used in most seismological observatories 

 of the world. The torsion seismometer (6) has been installed in many 

 U. S. Coast and Geodetic Survey and Carnegie Institution observatories. 

 The construction of Nos. 7 and 8 is applied in vibrographs and prospecting 

 seismographs. 



