746 EXPLORATION GEOPHYSICS 



tions). Rays emanating from the shot-point are indicated only at the 

 centers of plotted sections of the interface. Plotted depths are referred to 

 sea level. The thickness of the low velocity layer is indicated. 



The cross section represents the true dip only when the shot-point and 

 spread line lie in the direction perpendicular to the strike of the under- 

 ground strata. However, a section such as illustrated by Figure 454 does 

 depict the dip in the direction of the section, in this case in an east-west 

 direction, which may or may not be the direction of the true dip. 



To draw a contour map of the structural relief corresponding to a 

 horizon at a particular depth, a starting point is chosen on a section and a 

 theoretical surface, often called a phantom or a traverse surface, is drawn 

 from this point paralleling the dips. The phantom is extended into other 

 sections at intersecting or tie-points until the region is covered. After a 

 phantom has been projected parallel to the dips around any complete loop 

 or closed horizontal traverse, it will generally be found that a misclosure in 

 the vertical elevation of the phantom surface exists. Since misclosure 

 may be attributed to several factors, i.e., normal experimental errors 

 involved in the process of measurement, faulting, too large distances 

 between stations, and a possible personal element in actual construction 

 of the section. It is obvious that if vertical breaks in the phantom surface 

 are to be avoided in the drawing of a set of contours on the phantom 

 surface at the points where the closures are attempted, an adjustment must 

 be made for these vertical misclosures. 



Several methods have been developed for making closures of traverses, 

 and anyone attempting this type of work will do well to refer to a good 

 treatise on land surveying. In general it may be said that measurements of 

 distances and angles are never exact quantities. This statement may also 

 be extended to include intervals of time in the case of seismic prospecting. 

 If these observed values are used in computing other quantities, the results 

 are likewise inexact. And since the true value is unknown, the true error 

 is also unknown. It is therefore essential that the geophysical interpreter 

 have a knowledge of the various sources of error, their magnitudes, and 

 the probable error both of the direct field measurements and of computed 

 quantities. In order to make corrections and adjustments, the interpreter 

 must have a knowledge of least squares for a thorough understanding of 

 probable errors. This knowledge should also include weighted observa- 

 tions, weighted averages and the propagation of error. Even this is not 

 enough to qualify a geophysical interpretation, because any interpretation 

 of geophysical data in terms of geologic structure is likely to be in error 

 if due consideration is not given to the known geology of the area under 

 investigation. 



For example, a geophysical interpreter unaware of the presence of a 

 known fault of considerable displacement may erringly project a phantom 

 surface through the faulted zone, make adjustments for vertical misclos- 



