9. GRAVITY AT SEAi 



J, Lamar Worzel and J, C. Harrison 

 1. Introduction 



The tcclini(ine of gravity surveying at sea was revolutionized during the 

 International Cileo])hysical Year by the development of gravity meters which 

 can operate on surface ships under reasonably favorable sea conditions. It is 

 now possible to make continuous gravity measurements on ships of 1000 or 

 more tons displacement for a useful proportion of the time in many of the 

 world's oceans and seas. The oceanographic survey ship engaged on bathy- 

 metric surveys should in the future reckon on being able to take conciuTent 

 contiinious gravity and magnetic measurements. These types of survey are 

 conveniently made together as all three methods require the ship to be under- 

 way continuously at constant speed ; they all require precise navigation and 

 the cost of installing and operating the magnetometer and gravity meter is 

 small compared with that of keeping a fair-sized ship at sea. 



The gravitational method is based on measuring the variation in the accelera- 

 tion due to gravity at the sea-level surface. This variation is caused by changes 

 in the distribution of mass beneath the ship. Most imjjortant of these changes 

 is usually that caused by variations in the depth of water, but these can be 

 corrected for ]jrovided that depth soundings are taken during the gravity 

 measurements and that the bathymetry of the surrounding areas is fairly well 

 known. The variations in the corrected gravity anomaly are caused by density 

 variations beneath the sea-bed. 



The method has the advantage that it is not limited with regard to depth. 

 Anomalous masses can be detected however deep they may be, provided that 

 their attraction at the surface is large enough to be measured. The disadvantage 

 is that there is generally no single solution to the problem of finding a mass 

 distribution that will produce the measured gravity field, so that a number of 

 assumptions must be made before a structure can be deduced from a map of 

 gravity anomalies. In these features the method differs somewhat from seismic 

 surveying in which reasonably definite information may be learned about the 

 shallow layers although the depth of 2)enetration is limited by the difficulties of 

 shooting long lines. The two methods thus complement one another, and more is 

 gained by surveying one area in the two ways than the sum of the information 

 from the two surveys taken individually. Now that continuous measurements 

 can be taken from a surface ship underway, gravity survey is an extremely useful 

 method of reconnaissance for locating interesting features which do not show 

 topographically. The detailed analysis of a s\n'vey often requires other kinds of 

 geological and geophysical data before a satisfactory interpretation can be made. 



2. Methods and Theory 



A. Development of Surface-Ship Gravity Meters 

 Gravity measurements at sea need to be accurate to at least 10 mgal to be of 



any use at all, and an accuracy of 5 mgal is often required. An instrumental 

 ^ Lamont Geological Observatoiy Contribution No. 596. 



[MS received August, 1960] 134 



