THE OCEAN AS AN ACOUSTIC SYSTEM 



By Frank Press and Maurice Ewing 



Columbia University 



Introduction — A cursory examination of the 

 voluminous writing on the subject indicates 

 that there is almost as much disagreement on 

 the observational data of microseisms as there 

 is on the question of their origin. Progress 

 toward a solution of the problem can only be 

 made by first deciding what are the data to be 

 explained. We begin our discussion with a list 

 of what we believe are basic facts derived 

 primarily from data of east coast stations. 

 The list pretends to be neither complete, nor 

 generally applicable to other localities. It is 

 our belief, however, that a successful theory of 

 the origin of microseisms must satisfactorily 

 explain these data (Donn , 1951a, 1951b, 

 1952a, 1952b) in addition to the observations 

 from other localities. We realize that some of 

 the data disagree with other observations re- 

 ported at this meeting. However we are con- 

 vinced that observations on this coast forces 

 one to these conclusions. 



1. Frontal microseisms are generated very 

 soon (often abruptly) after a cold front passes 

 seaward from land, with no obvious correlation 

 to prior wind and sea conditions. 



2. A relatively narrow spectrum of periods 

 appears to be generated by a front, cyclone, or 

 hurricane at a given time when the disturbance 

 is over an area of uniform water depth. Char- 

 acteristic periods of microseisms can be related 

 to generating areas in the ocean. 



3. As a front recedes from shore, the spec- 

 trum gradually shifts to longer periods, and 

 becomes fairly constant after deep water i3 

 reached. 



4. Cold fronts and air masses following 

 them can generate microseisms whereas warm 

 air masses preceding the cold fronts fail to 

 generate microseisms even when strong on- 

 shore winds are present. 



5. In many cases there are no obvious cor- 

 relations between swell and surf conditions and 

 microseisms. 



6. Microseism energy is dissipated by a 

 profound crustal discontinuity at the edge of 

 the continental shelf. Hurricanes crossing the 

 edge suddenly generate larger microseisms. 



It is our opinion that no published theory 



of microseisms satisfactorily explains all of 

 these observed data. The authors' theory 

 (Press and Ewing 1948) advanced some 

 years ago utilizing the Airy phase associated 

 with stationary values of group velocity re- 

 quires long, homogeneous, propagation paths. 

 The work of Donn and others shows that this 

 is not the case for many microseism storms. 

 The work of Longuet-Higgins, and others on 

 stationary gravity waves appears to explain 

 satisfactorily how pressure fluctuations of suffi- 

 cient magnitude to account for microseisms 

 may be communicated to the sea floor. That 

 stationary waves capable of generating micro- 

 seisms occur in the open ocean has not been 

 demonstrated to the satisfaction of many in- 

 vestigators and cannot be reconciled with many 

 of the observations of Donn and others. Many 

 difficulties are found with the theory of surf 

 pounding. The authors have at present no 

 theory which can account for amplitudes and 

 periods of microseisms but feel that the data 

 requires one in which the properties of the 

 ocean-rock acoustic system under the generat- 

 ing area are significant in determining micro- 

 seism periods. A theory should also account 

 for the observation that only certain air mas- 

 ses appear capable of generating microseisms. 



The Ocean as an Acoustic System— Seismic re- 

 fraction measurements and earthquake surface 

 wave studies (Ewing et al, 1950, Ewing and 

 Press 1950, Ewing et al, 1952, Ewing and 

 Press in press, Officer et al, 1952) indicate 

 that the ocean basins are underlain by about 1 

 km of mud with acoustic properties much closer 

 to those of sea water than the underlying crys- 

 talline rock. The mud velocity is about 5500 

 ft/sec with density about 1.5 gm/cm whereas 

 the crystalline rock velocity is about 22,000 

 ft /sec with density 3.0 gm/cm (Donn 1952 b). 

 This is to be compared with a velocity of 5000 

 ft/sec and unit density for sea water. It is 

 seen that a great impedance contrast exists be- 

 tween the water-mud layer and the crystalline 

 floor. A single set of acoustic parameters can 

 be used to specify the unique properties of the 

 ocean-crystalline basement system over a large 

 area of the ocean basin. In many cases, how- 

 ever, microseisms are generated on the conti- 

 nental shelf or near the continental edge and 

 the paths do not cross this excellent acoustic 

 system. 



109 



