34 



leveling over possibly long distances would favor a more direct indication 

 of any local compaction. It is recommended that a few experimental 

 groundwater table/compaction devices be installed near selected tide gages. 

 These would be located in communities where demand for the local 

 groundwater is high, and compressibility of the underlying strata 

 significant. If these devices prove useful, more should be added until, 

 ideally, every tide gage used in making sea level rise estimates has at 

 least one accompanying compaction device. 



In many regions , tectonic activity may be equal to or greater than 

 compaction. The only means to resolve the apparent rise in sea level at a 

 tide gage into its subsidence, tectonic, and eustatic components with a 

 degree of confidence is to relate tide and compaction gage elevations to an 

 ultra-precise geodetic reference system. Such a program utilizing Very 

 Long Baseline Interferometry (VLBI) and the Global Positioning System (GPS) 

 is described by Carter et al . (1986). The GPS or other satellite system 

 should also be used to monitor the deep ocean as stated in section 2. 



A program is also needed to document compaction rates in those coastal 

 areas currently experiencing high rates of erosion and shoreline retreat to 

 see if compaction is playing a role, and to determine if remedial measures 

 can be implemented. Installing arrays of compaction measuring devices will 

 also permit study of the long-term behavior of the local subsidence as well 

 as its relation to fluctuations and secular depletion of the water table. 

 By sinking nearby devices to different depths, the vertical distribution of 

 compaction can be determined, and by placing arrays farther inland along a 

 transect the spatial behavior of compaction rates can be studied. 



