71 



whereA is the range in km at which the bottom refracted wave becomes too weak 

 to give a reliable first arrival, and W is the weight of TNT in lbs. 



Seismic reflection studies have also been made, but have yielded useful 

 results only when made in conjunction with refraction studies. Under favorable 

 circumstances, reflections can be used to detect interfaces masked in refraction 

 studies or for detailed mapping of interfaces identified in concurrent refraction 

 profiles. 



Before getting very far into the details of geophysical measurements it is 

 worthwhile to examine their nature and how they differ, if any, from other types 

 of measurements. I think it is fair to say that most geophysical measurements 

 fall into the class of indirect measurements and by indirect measurements I 

 mean measurements for which there is a large number of possible solutions. 

 As an example of this type of measurement I like to consider the sonic depth 

 finder, partly because it is not generally considered to yield an indirect measure- 

 ment, and partly because it is a very widely used instrument. An echo sounder 

 usually records the echo time from the nearest point on the bottom. In cases 

 where the bottom is flat or very gently sloping this time can be used to determine 

 the bottomi depth beneath the ship provided the sonic velocity from surface to bot- 

 tom is known. In cases where bottom slopes are steep and the topography ir- 

 regular, the position from which the echo is returned is no longer directly be- 

 neath the ship and the depth is frequently indeterminate. For example, the 

 great deeps of the world are found in narrow trenches and it may well be impos- 

 sible to determine their greatest depth by sonic means. 



This is illustrated by Figure 1. The upper part shows a profile of the 

 bottom and the lower part shows the echogram of the apparent depth profile ob- 

 tained by crossing the upper feature with an echo sounder. Some beautiful ex- 

 amples of similar features were de- 

 scribed by Dr. Dietz and his col- 

 leagues. Under ideal circumstances, 

 echoes can be resolved in the overlap- 

 ping region in the center of the valley. 

 Frequently, however, the second echo 

 is obscured by the first one and the 

 formi of the bottom of the channel can- 

 not be determined. 



The degree of indeterminacy 

 will vary considerably with the method 

 of measurement and with the situation. 

 In special cases, seismic refraction 

 travel-time curves can be solved 

 uniquely for seismic velocity as a func- 

 tion of depth. However, any set of 

 surface gravity observations can be 

 solved by an infinite set of possible 

 density distribution within the surface. 

 Most of these can be discarded as being 

 unreasonable but some degree of in- 

 determinacy will always remain. 



Bottom Profile 



rwvvv^vvT 



Echo- Sounder Profile 



Fig. 1. Echogram of V-shaped bot- 

 tom trench. 



The subject of geophysical measurements, if taken literally, is very 

 broad, for geophysics is concerned with the physics of the atmosphere, hydro- 

 sphere, and lithosphere. As the first two of these areas are treated in other 

 topics of this symposium, I think it is proper to limit this subject to what is 

 generally known as solid earth geophysics, or the use of physics to obtain clues 



