Measurements of Surface Stability of Engineered Geodetic Control Points

Roger Bilham

CIRES Dept. of Geological Sciences, University of Colorado-Boulder

Athough tectonic deformation is driven by stresses deep within the Earth it can be measured only close to the Earth's surface, where signals are contaminated by displacements generated by variations in groundwater hydrology, atmospheric temperature and pressure, and solar radiation. By suitable control-point design it is possible to suppress some of these surface-signals, however, rarely are measurements undertaken to assess the stability of an improved control point. The horizontal stability of surface monuments can be measured with inclinometers and tiltmeters, and their vertical stability with simple extensometers. Although it is generally assumed that monument noise falls to insignificant levels below 10 m depth few experimental data are available to quantify this assumption. Data from several experimental monuments in clays are described that yield measured stabilities of ±0.5 mm/year despite significant variations in soil moisture content.

INTRODUCTION

The Earth's surface offers us a two dimensional view of subsurface tectonics, upon which are superimposed signals of non-tectonic origin. Non-tectonic signals are generated by temperature, pressure and moisture changes at the Earth's surface, and because they are usually not of interest and reduce the accuracy of observational geodesy, they are classed as noise. Recent improvements in GPS survey accuracy (Bevis et al., 1997) provide survey accuracies approaching 3 mm and further improvements are anticipated. Hence it is considered desirable to suppress surface noise to sub mm levels at all frequencies of interest.

FIGURE 1 Elastic discontinuities (topography, fissures or variations in elastic modulus) on the Earth's surface result in local amplification of applied strain-fields.

A second form of noise arises from the distortion of the tectonic strain-fields by elastic heterogeneity of the Earth's surface. Mountains, open fissures, cliffs, or changes in rock strength will either interact with tectonic signals to distort their surface expression, or they will generate local signals with wavelengths similar to the scale of the feature. Examples are shown schematically in Figure 1, Figure 2 and Figure 3. Horizontal strain fields applied to a vertical fissure or a cliff, for example, are manifest as displacements of the edge of the fissure. This signal will add to, or subtract from, the observed deformation field depending on the placement of the monument. A common example of fissure enhancement is the concentration of tectonic strain across faults in the form of fault creep.



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