Lunar gravity traverse experiment

Purpose: The purpose of the Traverse Gravity Experiment is to measure the value of lunar gravity (relative to the value at the landing site) at selected points along the lunar surface traverse. A secondary purpose is to make an Earth-Moon gravity tie. Instrumental Accuracy and Elevation Corrections: The traverse gravimeter is capable of making measurements to an accuracy of about ±0.6 mgals. The free air correction on the Moon is about 0.2 mgals/m. The uncertainty of elevation accuracies at the gravity stations is expected to be between ±2 m and ±10 m with the smaller number considered more likely. Consequently, the error in the free air anomaly will lie between about ±0.6 mgals and ±2.1 mgals. Lateral Inhomogeneities Resolvable by the Traverse Gravimeter: The gravity difference associated with a layer 500 m thick, of density contrast 0.2 g per cubic centimeter, is 4mgals. Gravity stations 500 m apart will not resolve gravity signals with wavelengths under 1 k. Hence, the traverse gravimeter will only resolve density differences which extend at least a few hundred meters in vertical extent and have wavelengths of at leastl k. At the other extreme the gravimeter can record the effect of very deep and very large masses provided the gravity effect of these masses is at least a few milligals across the traverse. Usefulness of the Traverse Gravity Experiment: The gravity measurement will indicate the presence of lateral inhomogeneities at the kilometer or larger scale. In a general way, they will discriminate between the case where the subsurface contains a jumbled mass of rocks and different densities and the case where the subsurface layers have very small lateral inhomogeneities and, therefore, yield clues regarding the origin of the lunar surface. In a particular way, the lunar traverse gravity measurement will be able to establish the downward extension of rock outcrops on surface - central peaks in craters being an example of such outcrops. In this sense, the traverse gravity measurement provides a very useful extrapolation of surface geological observations. Time Required for Measurements: At the beginning and end of each traverse, the gravimeter will be removed from the LRV and a normal and a bias measurement taken with the gravimeter on the lunar surface. Each measurement takes about 2.5 min. During an LRV stop, it is necessary merely to start a normal measurement by depressing a pushbutton. The gravity measurement still takes 2.5 min but the presence of the astronaut near the gravimeter is not required. After the measurement is taken, the value is stored in the logic circuitry; the astronaut can obtain the value at any later time by depressing a read pushbutton. The gravity value is displayed and is read out by the astronaut. Number of Required Measurements: The number and location of gravity stations is site dependent, and cannot be firmly formulated until the landing site is chosen. As a guideline, one gravity measurement at every science stop would be extremely desirable.     

Non linear inversion of gravity gradients and the GGI gradiometer

All gradiometers currently operating for exploration in the field are based on Lockheed Martin’s GGI gradiometer. The working of this gradiometer is described and a method for robust non linear inversion of gravity gradients is presented. The inversion method involves obtaining the gradient response of a trial body consisting of vertical rectangular prisms. The inversion adjusts the depth to the tops or bases of the prisms. In the trial model all the prisms are not required to have the same area of cross section or the same density (which can also be allowed to vary with depth). The depth to the tops and bottoms of each prism can also be different. This response is compared with the observed values of gradient and through an iterative procedure, the difference is minimized in a least square sense to arrive at a best fitting model by varying the position of the tops or bottoms of the prisms. Each gradient can be individually inverted or one or more gradients can be jointly inverted. The method is extended to invert gravity values individually or jointly with gradient values. The use of Differential Curvature, a quantity which is directly obtained by current gradiometers in use and which is an invariant under a rotation in the horizontal plane, is emphasized. Synthetic examples as well as a field example of inversion are given.     

Pore pressure diffusion and the mechanism of reservoir-induced seismicity

The study of reservoir-induced seismicity offers a controlled setting to understand the physics of the earthquake process. Data from detailed investigations at reservoirs in South Carolina suggested that the mechanism of transmission of stress to hypocentral locations is by a process of diffusion of pore pressure (Pp). These results were compared with available worldwide data. The seismic hydraulic diffusivity, _s , was estimated from various seismological observations, and was found to be a good estimate of the material hydraulic diffusivity, . Application of these results to a dedicated experiment to understand RIS at Monticello Reservoir, S.C., suggested that the diffusing Pp front plays a dual role in the triggering of seismicity. The spatial and temporal pattern of RIS can be explained by the mechanical effect of diffusion of Pp with a characteristic hydraulic diffusivity within an order of magnitude of 5×10⁴ cm²/s, corresponding to permeability values in the millidarcy range. The triggering of seismicity is due to the combined mechanical effect of Pp in reducing the strength and, possibly, the chemical effect in reducing the coefficient of friction between the clays in the pre-existing fractures and the rocks that enclose these fractures.     

Reservoir-induced Seismicity in China

—A review of case histories of reservoir-induced seismicity (RIS) in China shows that it mainly occurs in granitic and karst terranes. Seismicity in granitic terranes is mainly associated with pore pressure diffusion whereas in karst terranes the chemical effect of water appears to play a major role in triggering RIS. In view of the characteristic features of RIS in China, we can expect moderate earthquakes to be induced by the construction of the Three Gorges Project on the Yangtze River.     

Laxmi Ridge – A continental sliver in the Arabian Sea

Identification by Bhattacharya et al. (1994) of seafloor spreading type magnetic anomalies in the basin lying between Laxmi Ridge in the Arabian Sea and the Indian continent necessitates a change in plate tectonic reconstruction. Naini and Talwani (1982) named this basin the Eastern Basin and we will continue to use this term in this paper. Others, in the literature, have called this the Laxmi Basin. Previous reconstructions had assumed that the Eastern Basin is underlain by continental crust. The new reconstruction moves Seychelles' original location closer to India and ameliorates a space problem in the Mascarene Basin. A new rotation pole between anomaly 28 and 34 times avoids skipping of fracture zones resulting from rotation poles described earlier. The negative gravity anomaly over the Eastern Basin is a necessary consequence of a continental sliver lying between oceanic crust on either side. Seismic velocities that are slightly greater than 7 km s^–1 under the Eastern need not be necessarily interpreted as material that underplates continental crust.