New Magnetic and Gravity Interpretation Methodologies and Their Innovative Application for Environmental, Exploration, and Planetary-Scale Potential-Field Data II
Presiding: D Ravat, Southern Illinois University at Carbondale; D K Butler, US Army Engineer Research and Development Center
GP23A-01 INVITED 13:30h
Natural Magnetic Fields Preceding the Loma Prieta Earthquake that Damaged San Francisco in October 1989
Available records of the magnetic indices Dst and ap together with USGS standard observatory recordings of the 1-min fields are examined for the time preceding the Loma Prieta earthquake that severely damaged the San Francisco area in October, 1989. The results indicate that a 1990 report claiming the existence of a ULF geomagnetic field precursor signal foretelling a nearby earthquake is very likely false. The observatory study shows that the ULF signal was not local but rather widespread throughout the western United States and therefore expected to be of solar-terrestrial disturbance origin. The two planetary activity indices support this conclusion.
GP23A-02 INVITED 13:50h
Advances in High-Resolution Magnetometry for Mapping Unexploded Ordnance (UXO) and the Challenge of Geologic Noise
We discuss recent advances in the application of high-resolution magnetometry to UXO site characterization, focusing specifically on large-area assessment using helicopter-based magnetic surveys with flight lines only a few meters apart at heights of 1-2 meters above ground level. Motivating this work is the estimated cost of UXO clean-up: roughly 50 billion dollars for military bases in the U.S., involving some 1400 sites and over 10 million acres of land. Excavation is the principal remediation cost because setting detection thresholds low enough to minimize the risk of overlooking UXO generates numerous false positives, each of which must be treated as potentially explosive. The current U.S. Defense Department goal is to reduce the false-positive ratio from 100:1 down to 10:1 using improved geophysical sensors systems coupled with sophisticated discrimination algorithms. Discrimination involves not just sorting the bombs from the scrap metal, but also identification of UXO in the presence of geologic noise. Geologic noise at UXO sites can range from minimal (Badlands Bombing Range, South Dakota) to severe (Pueblo of Isleta bombing targets, New Mexico) and can change dramatically at a given site as a function of the survey scale. We discuss research to characterize geologic noise scaling using a multifractal approach with the eventual goal of simulating the affect of geologic noise on UXO discrimination algorithms.
GP23A-03 14:10h
Use of Absolute Gravity Measurements to Monitor Groundwater in the Española Basin, New Mexico
We present early results of three-year project using absolute gravity instrumentation to monitor groundwater in an arid to semi-arid region in northern New Mexico. Over 100 permanent gravity stations have been established in the groundwater basin. A-10 absolute gravity meters, manufactured by Micro-g Solutions, Inc., have been used to monitor long-term gravity changes in the groundwater basin. Over fifty A-10 sites have been established; other gravity sites have been established by reference to the primary A-10 sites using Scintrex CG-3M relative gravimeters. We have used geodetic-quality GPS surveys to directly measure any possible elevation changes at the gravity sites; thus far, no significant changes in elevation have been observed. For the A-10 gravity sites, we have learned that sites must be constructed rather carefully to minimize noise levels due to certain characteristics of the A-10 measurement system. At good sites, away from regions where we expect changes due to groundwater removal, reproducibility of the A-10 measurements is ±4~ÎœGal. To date, we have data from repeat campaigns over a period of 22 months. We have observed systematic changes in gravity of as much as 14~ÎœGal at certain sites. We have directly incorporated gravity modeling into a detailed 3D groundwater model of the basin. On the basis of groundwater modeling, we believe that such gravity changes are due to increased recharge at some sites, as precipitation began to return to normal amounts after a long, pronounced drought about a year into the study. Somewhat surprisingly, no significant gravity changes have been observed at the Buckman Well Field, a spatially small well field that is heavily pumped as a municipal supply field for Santa Fe, New Mexico. One interpretation of this observation is that pumping at the Buckman Field is accessing nearby surface sources rather than groundwater, despite the fact that pumping is occurring from more than 300~m depth.
GP23A-04 14:25h
Sinkhole Hazard Assessment Using the Gravity and Magnetic Signatures of the Shallow Subsurface Mass Deficiency
Problems associated with sinkhole development are causing serious concerns in land-use planning in the Dead Sea area. We studied the resolving power of gravity and magnetic surveys for the assessment of sinkhole hazard. Microgravity surveying has been used worldwide to detect karst caves in the dense carbonate rocks. However, the Dead Sea sinkholes are developing very rapidly in the young, thick, low-density sediments that fill the Dead Sea graben. The gravity effect of the entire graben reaches -130mGal with a horizontal gravity gradient up to 10mGal/km. To the best of our knowledge a micromagnetic surveying has never been used to delineate subsurface cavities. Assuming that the collapse is caused by large voids in the shallow (a few tens of meters) depth, we defined our survey designs by 3-D gravity and magnetic forward modeling. A density contrast of such a target ranges from 0.9 to 2.0 gr/c3 and numerous magnetic susceptibility measurements suggested an average magnetic contrast of 3*10-4 SI. The modeling showed that high resolution detailed gravity and magnetic surveys can reliably delineate the voids located at the shallow depth. The magnetic dipole anomaly with its positive part located north of the negative part is typical of anomalies of a concealed void. We have been studying the feasibility of methods mapping the sites with sinkholes occurring in alluvial fans and mud flats along the Dead Sea shore since 1999. About 20 thousand gravity stations were measured at eleven sites and micromagnetic measurements were taken at five sites. The results suggest: - The microgravity and micromagnetic data indicate a subsurface mass deficit beneath some of the areas where open sinkholes are observed, suggesting that a mass deficit is presented and additional sinkhole development can be expected. Indeed, new sinkholes opened up at several anomalous locations. - Moreover, the absence of negative residual gravity anomalies and typical magnetic dipole anomalies at the sites with open and/or filled sinkholes, suggests that the subsurface mass deficit was recently compensated and these sinkholes are not presently developing. - The methods can be an effective tool for monitoring the subsurface mass redistribution that may signal impending collapse. - The elongated anomalies observed in several sites indicate that most collapse is related to tectonic lineaments. - The possible wavelength and magnitude of the effects of structural variations could be similar to concealed hollows. This means that the microgravity and micromagnetic surveys should be accompanied by a seismic refraction study.
GP23A-05 14:40h
Magnetic Field Studies of the Indian Sub-Continent.
Available aeromagnetic data over India upto 25 degrees north latitude are utilized to derive the magnetic anomalies. Several data gaps are filled using available / published ground and marine magnetic data. All available data are reduced to a common altitude to prepare a composite magnetic anomaly map of the region. The tectonic blocks, the lithological contacts and structures below thick sedimentary cover, suture zones and depth to the bottom of the magnetic crust, which essentially coincides with the Curie isotherm depth, are derived from this map. The magnetic anomalies in the North Eastern region of India forms a very critical part of the data set for understanding the complexities of the tectonics of the Indian sub-continent. However large data gaps in the Bengal basin, ocean region and other regions where magnetic data is not available severely constrict the interpretation. Under these circumstances the high-resolution satellite geopotential data with its global coverage gives an insight into the long wavelength structure. The oceanic lithosphere surrounding India is constantly under stress due to the spreading activity resulting in intense deformation in the region. The thick sediment deposits in the Bay of Bengal mask the underlying crust and pose severe restrictions in constructing the geodynamical history of the region. The high-resolution satellite geopotential data with its global coverage offer the unique opportunity of addressing some of these issues. The oblique subduction of the Indian Ocean lithosphere at the Andaman island arc trench system deserves special attention. The lithospheric model MF3 derived from the CHAMP data reveal interesting features in and around the Andaman region. Using the CHAMP satellite magnetic anomaly map and the satellite derived magnetization map as base, the tectonic structure in the Bay of Bengal and in particular the subduction at the Andaman island arc become evident. The SEMM map compares fairly well with the isopach map of the region. Further the available gravity and topography maps of the region help constrain the interpretation. Results of such an analysis will be presented.