194 BULLABD AND MASON [CHAP. 10 



since they give no indication of the lateral extent or direction of anomalous 

 magnetic trends, and, therefore, provide no basis for quantitative geological 

 intei-pretation. For example, isolated lines across the area covered by the mag- 

 netic map of the north-east Pacific (Fig. 11) would give no information about 

 the magnetic trends ; the general impression of the magnetic character of the 

 area that would be gained from a study of such lines would depend entirely on 

 their orientation. If the geological pattern is to be revealed by a magnetic 

 survey, the lines along which measurements are made must be close enough 

 together to allow a contoured map of the field to be drawn. 



Experience shows that if the sharpest anomalies, arising from sources close 

 to the ocean floor, are to be adequately resolved, the separation between 

 adjacent lines must not exceed about half the depth of the water. Satisfactory 

 contouring requires a relative positional accuracy between adjacent lines of 

 about 20% of the spacing, i.e. about 10% of the depth, or about 500 m over 

 the deep ocean. Over a small area buoys can be moored and the survey con- 

 ducted relative to them. In surveys covering a larger area, elaborate radio- 

 navigation systems are necessary. Even then, the operational range is restricted 

 to a few hundred kilometres from land, though systems are available wliich 

 give ocean- wide coverage to an accuracy of within a few kilometres. 



The survey conducted by the Scripps Institution of Oceanography, in 

 conjunction with the U.S. Coast and Geodetic Survey, off the west coast of the 

 United States (Mason, 1958; Mason and Raff, 1961; Raff and Mason, 1961) 

 represents the first attempt to make a detailed magnetic map of an extensive 

 area of the oceans. The results are of exceptional interest in that they reveal 

 major structural trends of which there is little or no indication in the topo- 

 graphy, and they provide evidence for unsuspected horizontal displacements 

 along some of the faults of the north-east Pacific greater than any that have so 

 far been observed over the continents. 



The area covered by the survey lies off the foot of the continental slope along 

 the Pacific coast of the United States ; it is 400-500 km in width and extends 

 off-shore from the north Mexican border in the south to the southern end of 

 Queen Charlotte Island in the north. It was completely covered with a regular 

 grid of lines, mostly about 8 km apart, which were positioned with a probable 

 error of 100 m by means of a radio-navigation system. The survey itself was 

 made with a highly stabilized fluxgate magnetometer which, by frequent 

 comparison with a proton precession magnetometer, provided a measure of 

 the total field with a standard deviation from the absolute value of about 15 y ; 

 over periods of a few hours the stability was better than 1 y. 



The magnetic anomaly map, Fig. 11, is a plot of the observed field from 

 which the regional field has been subtracted. The latter was derived from the 

 map of the observed field by a smoothing process. No attempt was made to 

 remove the effect of diurnal and other time variations of the earth's magnetic 

 field as these were thought to have no significant effect on the final plot. 



The general features of the bathymetry have been described by Menard 

 (1955). The area is crossed by two of the four great, almost east-west, fault 



