HANNA: DESIGN OF TRANSMISSION LOSS EXPERI^4ENTS 



A CASE HISTORY: THE CRITIQUE OF A BOTTOM LOSS MEASUREMENT 



Introduction 



The Naval Oceanographic Office (NOO) and the Naval Air Develop- 

 ment Center (NADC) have conducted many measurements over the past 

 several years aimed at determining the bottom reflection loss at 

 low frequencies (less than 1,000 Hz). Although different in detail, 

 the two programs have employed similar experimental techniques. They 

 both tend to use sources and receivers within some hundreds of feet 

 of the ocean surface; in this geometry transmission loss is measured, 

 compared with estimates of the spreading loss through the water, and 

 bottom loss is inferred. A consistent, and surprising, result of 

 most of those measurements is the apparent evidence of negative 

 losses at low frequencies for low grazing angles. This result has 

 serious implications for predicted transmission loss using present 

 models; in the remainder of this discussion the experimental design 

 employed in these measurements will be examined, along with its impact 

 upon the inferred reflectivity. 



An Example 



The case study here assumes a Pacific profile for the water 

 column and a sound velocity gradient in the upper few hundred feet 

 of bottom sediment of 1.0 sec (see Figure 8). This assumed velocity 

 structure for the unconsolidated sediment of the bottom is supported 

 by numerous independent experiments including those being considered 

 here. No discontinuity of the sound velocity into the bottom has 

 been assiomed, although there is evidence that a discontinuity of a 

 few percent often exists. Its absence here is of no material conse- 

 quence for the points to be developed. 



523 



