Consider the gravity observations (open circles) collected along a profile acros
ID: 298443 • Letter: C
Question
Consider the gravity observations (open circles) collected along a profile across the topography depicted in the figure below. The Bouguer gravity anomaly effects of the topography were computed using different values of density. From these results which are shown in the upper portion of the figure below,
(a) what is the density of the topographic feature and why?
What do the reduced profiles indicate about the related topographic corrections that are
(b) positively, and
(c) negatively correlated with the topography?
4. How might borehole gravity observations from 3 or more boreholes be used to locate a subsurface body of anomalous density that has not been intersected by any of the boreholes?
• When gravity measurements are recorded in 3 or more boreholes bracketing a massive body.
• A sequence of precise gravity measurements is gathered at discreet intervals by pause and reading the gravimeter at predefined borehole depths. After that corrections applied to these gravity measurements. The correction are such as free air anomaly, bouguer anamoly, gravity drift, latitutde, surface topgraphy etc. Applying these corrections, the residual gravity data provides information about the distribution of rock densitites in both in the vicinity of the hole and away from it.
• Inversion routine and forward modeling and inversion routines (co-rigging) can be used to construct a 3-dimensional representation of the subsurface body from these distribution. Subsurface body of anomalous density can be detected by such 3 D profiles. •
5. Errors which are inherent in geophysical data propagate as various mathematical processes are performed. For a set of 17 observations with the mean value 2.06 and standard deviation 0.32and another set of 39 observations with a mean of 4.54 and standard deviation of 0.26, calculate the
(a) sum, • A+B=6.600v
(b) difference, • A-B= -2.480
(c) product, and • A*B=9.352
(d) quotient of these data, including • A/B=0.458
(e) their combined standard deviation. • new std dev=sqrt(0.36^+0.26^2)=0.44
2.0 J 1.0 gravity profiles reduced for various o-values of topographic density Topographic Feature 500 1000 METERS O 2.0 O 2 O-2.2 O 2.3 o 2 1500 450 440 430 410Explanation / Answer
(a) what is the density of the topographic feature and why?
The density of topographic future in the centre is gentle gradient then in the ends. It seems that formation has more compaction at the ends than in the centre.
What do the reduced profiles indicate about the related topographic corrections that are
(a) positively, and (b)negatively correlated with the topography?
The density contours at the ends shows closer space means more density means high gravity areas. It is correlated positively with the topography.
The contours in the centre are widely spaces shows gentle gradient in density indicates low gravity area means it correlated negatively with topography.
4. How might borehole gravity observations from 3 or more boreholes be used to locate a subsurface body of anomalous density that has not been intersected by any of the boreholes?
Based on borehole gravity survey we can interpret low gravity and high gravity locations at depth. In metallic deposits areas, high gravity shows possibility of mineralised zone. In Coal area the low gravity shows possibility of coal seams at beyond borehole depth. The 3 ore more data gives anomaly zone for mineral deposits. Mineral deposit extents can be arrived based on anomaly zone.
• When gravity measurements are recorded in 3 or more boreholes bracketing a massive body.
In case of gravity high in metallic deposits and in case of gravity low in coal and oil & gas deposits