Eos, Vol. 82, No. 30, July 24,2001
EOS
VOLUME 82
NUMBER 30
JULY 24, 2001 PAGES 325-332
EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION
New Digital Data Base Helps to Map North America PAGES 3 2 5 , 3 2 7 , 3 2 9 - 3 3 0 A new effort is underway to compile an upgraded digital magnetic anomaly data base and map for the North American continent by 2002.This program is a joint effort by the Geo logical Survey of Canada (GSC),the U S . Geo logical Survey (USGS),and Consejo de Recursos Minerales de Mexico (CRM). An integrated, readily accessible, modern digital data base of magnetic anomaly data spanning North America will b e a powerful tool for evaluating the structure, geologic processes, and tectonic evolution of the continent, and may also b e used to help resolve societal and scientific issues that span national boundaries. Maps derived from the digital data base will provide a view of continentalscale trends not available in individual data sets, help link widely separated areas of out crop, and unify disparate geologic studies.
America's (GSA) Decade of North American Geology (DNAG) program.The Canadian com ponent of the DNAG map was based on a 2-km grid [Dods et al., 1987] and consists of data cov ering 70% of the land mass of Canada.The
largest data gaps are over western Canada and the Arctic Islands. No data over Mexico were published in the DNAG map. Although this first magnetic map was a pioneering effort when it was constructed in analog form, it is often inadequate for addressing current socioeconomic problems that require modern digital analysis. Moreover, the analog techniques used to assemble the data did not properly reconcile the disparate flight specifica tions of individual surveys, and this resulted in
Magnetic Anomaly Data Magnetic data allow us to s e e through geo logic as well as anthropogenic cover, such as vegetation, soil, desert sands, glacial till, manmade features, and water to reveal lithologic variations and structural features such as faults, folds, and dikes. In general, they reflect variations in the distribution and type of mag netic minerals—primarily magnetite—in the Earth's crust. Magnetic rocks can b e mapped from the surface to great depths, depending on their dimensions, shape, magnetic proper ties, and the character of the local geothermal gradient. In many cases, examination of mag netic anomalies provides the most expedi tious and cost-effective way to accurately map geologic features in the third dimension— depth—at a range of scales. Airborne and marine magnetic data have been collected in North America primarily by the governments of Canada, the United States, and Mexico. In the early 1980s, the first magnetic map was produced for the United States [Zietz, 1982] .A digitized version of this analog map constitutes most of the data for the contermin ous United States in the present North American magnetic map compilation [Committee for the Magnetic-Anomaly Map of North America, 1987], constructed as part of the Geological Society of
Fig. 1. Map showing line spacing of surveys that will be included in the North American grid and map. Contiguous surveys with similar line spacings have been combined. Marine magnetic data with variable line spacings will be used in the offshore areas. The existing Arctic compilation will be used for Greenland and adjacent marine areas [Macnab et al, 1995]. Central American data will be included where publicly available. Original color image appears at the back of this volume.
Eos,Vol. 82, No. 30, July 24,2001
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Fig. 2. Examples of merged trans-national magnetic compilations, (a) Magnetic data over British Columbia, Canada; Washington, Idaho, Montana, and portions of Oregon, United States. Original color image appears at the back of this volume. substantial inconsistencies that b e c a m e more obvious after the data had been digitized [Com mittee for the Magnetic-Anomaly Map of North America, 1987; National Research Council, 1993]. As a result of these past compilation problems in the United States and recent major improve ments in data coverage, primarily in Canada and Mexico, an effort to compile a new digital data base covering North America is clearly warranted and was o n e of the recommenda tions of the U.S. Magnetic-Anomaly Data Set Task Group [1994]. Canadian Data. Canada has conducted a systematic aeromagnetic mapping program since 1947 [Teskey et al., 1993], often on a costsharing basis between federal and provincial governments. Surveys are contracted out by the GSC and are often jointly funded with industry partners from both the petroleum and mining sectors. Most surveys (70% of land coverage) were flown at a line spacing of 0.8 km and flight height of 0.3 km. More detailed mapping at line spacings down to 0.2 km and regional surveys
over sedimentary basins at up to 3 km have also been carried out (Figure 1).Approximately 20% of the survey data were digitally acquired; the rest were digitized from 1:50,000 contour maps. Data sources also include ship-borne surveys off the Pacific and Atlantic coasts, donated gridded data sets from industry and U.S. Navy airborne surveys in the Arctic Ocean. Data are maintained in a national aeromagnetic data base containing over 12,000,000 km of flight line data and are available in several formats (see http://dcinfo. agg.nrcan.gc.ca/toc.html).In the period 1988-1999 after the DNAG compilation, the GSC has flown 85 surveys and acquired 1,750,000 line-km of magnetic data. Coverage of western Canada and the Arctic has improved significantly U.S. Data.The USGS pioneered the first air borne geophysical survey for geologic purposes in 1945. Since then, the USGS has acquired aeromagnetic data for essentially all of the United States in a piecemeal fashion. Surveys were designed for many purposes, so they varied widely in both size of coverage and anomaly
resolution (Figure l).Data collection spanned changes in acquisition techniques, from analogbased before the 1970s, to modern digital systems using global positioning system (GPS) navigation. After more than 50 years, the USGS digital and analog archives of aeromag netic data comprise more than 1000 surveys consisting of millions of line-km of data that would cost hundreds of millions of dollars to re-fly today For the national compilation that will ultimately form part of the North American data base, the USGS is constructing a data base con taining the original aeromagnetic survey data in digital form from surveys, maps, and grids from existing public domain and, where available, pro prietary magnetic data. All survey data will be stored in a consistent ASCII format with archival information available to the public. Mexican Data. High-resolution aeromagnetic surveys in Mexico have been flown by CRM in support of mineral exploration since 1962. Prior to 1995, the surveys were flown along north- and northeast-trending flight lines spaced 500-1000 m apart and flight heights of 120 m,305 m,and 450 m above the terrain. Since 1995, the CRM has been systematically flying surveys over the coun try's mining exploration areas to provide new coverage over most of the land surface of Mexi c o by 2002.These surveys are being flown with Cesium magnetometers positioned by differen tial GPS along north-south-trending flight lines spaced 1000 m apart.The flight height varies from 305 m to 450 m above the terrain.When processing is complete, the CRM will merge the magnetic data and produce a grid with a spac ing of 200 m for most of Mexico. As part of a long-term exploration program, Mexico's national oil company Petroleos de Mexico (Pemex),has completed aeromagnetic surveys covering much of Mexico and part of the Gulf of Mexico with line spacings ranging from 2 km to 6 km and flight heights from 450 m to 3050 m. Older aeromagnetic data collected over small-scale prospect areas throughout the past decades have been reprocessed to take into account the secular variation field and the international geomagnetic reference field (IGRF); and the data have continued upwards to a uniform level of 3500 m. Different smooth ing and contouring techniques were then applied to the older data sets, which were then merged with more recent data to generate a national aeromagnetic anomaly map [UrrutiaFucugauchi and Ornelas-Valdes, 2000]. Offshore and Proprietary Data. Publicly avail able marine magnetic data will b e incorporat ed into the compilation to a distance of 200 km from the edge of the continent (Figure 1). Since the DNAG compilation, the offshore data coverage has significantly improved (http://www.ngdc.noaa.gov/mgg/geodas/). In addition to the publicly available airborne and marine data, the GSC, USGS, and CRM will seek the cooperation of private industry to integrate proprietary magnetic data, s o m e of which is in jeopardy of being lost. If possi ble, proprietary data will b e purchased and included in the U.S. grid.Those holding data sets that are not yet included in the data base are encouraged to contact the author representing the relevant country
Eos,Vol. 82, No. 30, July 24, 2001
Fig. 2. Examples of merged trans-national magnetic compilations, (b) Magnetic data over parts of Arizona and New Mexico, United States; Sonora and Chihuahua, Mexico. Original color image appears at the back of this volume.
The North A m e r i c a n Magnetic Anomaly Grid a n d Map The leveling and merging of aeromagnetic surveys in Canada to produce a country-wide compilation was started in the late 1970s. A second phase was started in the late 1980s and has continued, resulting in a regional compilation for the majority of the Canadian land mass and offshore area [Teskey et. al, 1993] .As part of the new North American map, the remaining data—which mostly cov ers the Arctic Islands—will b e merged with the existing data base to produce the most upto-date coverage of Canada. Over the last d e c a d e , the USGS has engaged in an effort to digitize, process, and merge the existing survey data into regional compila tions at U.S. state scales. Many states have b e e n digitally compiled over the last d e c a d e (for progress on state compilations, s e e http://crustal.cr.usgs.gov/crustal/projects/NAM AD/index.html).These intermediate compila tions will b e merged to form a consistent grid of magnetic data for the United States with a spacing of 1 km. For the Mexico section of the North Ameri can compilation, early analog surveys that have b e e n digitized by the CRM will b e con tinued to a standard level of 305 m above the terrain and merged with similarly continued grids of the modern digital data. Holes in the coverage will b e filled in with Pemex data, which will also b e continued to a level of 305 m above terrain. The North American Magnetic Anomaly data base will include the individual grids from Canada, the United States, Mexico, the Arctic [Macnab et al., 1995], and marine data (Figure l ) . E a c h grid will b e re-projected to the DNAG projection [Committee for the
Magnetic-Anomaly Map of North America, 1987], regridded to a spacing of 1 km, continued to an elevation of 305 m above terrain, and merged to form a c o m m o n North American magnetic grid that will b e publicly available and free of charge.The new North American magnetic anomaly map will first b e presented at the GSA annual meeting in 2002. Around the s a m e time, the continental grid, national grids, and digital survey boundary files will b e available from the GSC (http://gdcinfo.agg.nrcan.gc.ca/toc.html), USGS (www.usgs.gov), and CRM (http://www. coremisgm.gob.mx/inicio.html) Web sites. In addition, the consistent ASCII data base for publicly available aeromagnetic surveys for the United States will also b e available online. Examples of existing merged data sets at different scales from southwestern Canada and the northwestern United States (Figure 2 a ) and the southwestern United States and northwestern Mexico (Figure 2 b ) demonstrate the continuity of magnetic trends across the borders. Future Updates Low-altitude Data. The GSC is currently undertaking aeromagnetic surveys in several parts of Canada. S o m e of this data acquisition fills in gaps in the national coverage, while other detailed surveys will provide higher resolution data over areas previously flown. Even though the present coverage of Canada appears nearly complete, many areas comprise either surveys flown with widely s p a c e d flight lines of up to 6 km or data donated as grids for which data quality is difficult to assess. The planned upgraded data base for the United States will still contain major problems due to poor data resolution related to wide
flight-line spacings and high flight altitudes as highlighted by the U.S. Magnetic-Anomaly Data Set Task Group [1994]. Many existing aeromagnetic surveys in the United States are not detailed enough to resolve important geo logic features or merge with higher-resolution data from Canada and Mexico. Data c o l l e c t e d over a large part of the United States do not adequately define three-dimensional sources shallower than several kilometers, primarily because of large distances between flight lines. Approximately 90,000 line-km of new data are being collected annually by the USGS. Continued collection of new low-altitude data will b e required to produce a more relevant U.S. magnetic anomaly map. For Mexico, o n c e the processing of the national 1-km-spaced data are complete, the CRM intends to fly higher-resolution surveys in areas of interest for resource exploration, as well as for hazard and environmental assessments. High-altitude Data Collection. Even though the new North American compilation will represent a significant upgrade, the existing patchwork of surveys is inherently unable to accurately represent anomalies with wave lengths greater than 1 0 0 - 3 0 0 km [U.S.Magnet ic-Anomaly Data Set Task Group, 1994] .The lack of information about long wavelength anomalies is related to datum shifts between merged surveys.These shifts are caused by data acquisition at widely different times and by differences in data processing procedures. The artifacts produced by the shifts have wavelengths the size of o n e or more survey areas and c a n easily e x c e e d 100 nT. Reliable measurement of long wavelength anomalies is limited by individual survey sizes, regardless of data quality. For example, it is not possible to definitively measure anomaly wavelengths greater than 100 km when the dimension of the survey area is less than 100 km. The remedy is to adequately sample anom alies of larger widths and use these measure ments to tie together survey data sets for North America [National Research Council, 1993]. For wavelengths greater than roughly 500 km, satellite data c a n b e used to correct the long wavelength field. New, higher-resolu tion satellite data and advancements in data processing techniques have resulted in acceptable long wavelength information at low altitudes by downward continuation of the satellite data. For Canada and the United States, the optimum satellite data, c o m b i n e d with existing high-altitude aeromagnetic sur veys, will likely lead to a viable reference sur face to correct most of the long wavelength problems. No such correction is expected for the Mexico compilation b e c a u s e of the short period of time over which the data are being collected.
Authors Carol A. Finn, Mark Pilkington, Alejandro Cuevas, and Jaime Urrutia For additional information, contact Carol A. Finn, U.S. Geological Survey, Denver, Colo. USA; E-mail:
[email protected].
Eos, Vol. 82, No. 30, July 24, 2001 References Committee for the Magnetic-Anomaly Map of North America, Magnetic-Anomaly Map of North America, Geol. Soc. Am. Continent-Scale Map-003, scale 1:5,000,000, Boulder, Colo., 1987. Dods, S. D., D. J.Teskey, and PJ. Hood, Magnetic Anomaly Map of Canada, fifth edition, Geol. Surv. Can. Map 1255A,scale 1:5,000,000, Ottawa, 1987. Macnab, R., J.Verhoef,W. Roest, and J. Arkani-Hamed, New data base documents the magnetic character
of the Arctic and North Atlantic, Eos, Trans. AGU, 76,449,1995. National Research Council, The National Geomag netic Initiative Report, 246 pp., National Academy Press,Washington D.C., 1993. Teskey, D. J., PJ. Hood, L. W Morley R. A. Gibb, PSawatzky M. Bower, and E. E. Ready,The aeromagnetic survey program of the Geological Survey of Canada: Contribution to regional mapping and mineral exploration, Can. J. Earth Set, 30,243-260,1993. U. S. Magnetic-Anomaly Data Set Task Group, Ratio nale and operational plan to upgrade the U.S.
Deep Sea Vibracoring System Improves ROV Sampling Capability PAGES 3 2 5 - 3 2 6 A vibracoring system developed at the Monterey Bay Aquarium Research Institute (MBARI) for use off remotely-operated vehicles (ROVs) is capable of obtaining 7.7-cm-diameter sediment cores that are up to 2.5 m in length. The ability to collect long core samples, even in coarse-grained materials, from precisely-located sites vastly expands the geologic sampling capa bilities of ROVs. Obtaining sea floor samples from topographi cally complicated deep-sea environments where sediment is coarse-grained has been challeng ing. Sub-sea floor sediment sampling traditionally depends on wire-line gravity or piston coring sys tems. However, wire-line samplers are ineffective at penetrating into sand or gravel, do not provide visualization of the sea floor sample site, and are difficult to precisely locate. Deep submergence vehicles provide access to sea floor environments and enable visual inspec tion of the local environmental variability How ever, their geologic sampling capabilties are frustratingly limited, especially in coarse-grained sediment. Because most deep-sea vehicles are nearly neutrally buoyant, the downward force that can be exerted against the bottom for core insertion is limited by the vehicle's downward thrust. Although longer cores are commonly desired, this limitation has resulted in the typical manipulator-obtained cores being ±30 cm long. Vibracorers are the system of choice for collecting modest length cores in water-saturat ed, coarse-grained sediment.These corers work by inducing high-frequency vibrations in the core liner that in turn liquefy the sediment immediately around the core cutter, greatly reducing the sediment resistance.Vibracoring systems are typically used in shallow water because they require sea floor power sources. MBARI operates the ROV Ventana out of Moss Landing, California, which is directly at the head of Monterey Canyon. Like most submarine canyons, Monterey Canyon contains sand and gravel, although only limited samples have been collected. A vibracoring system was fabricated that combines the visualization capabilities and hydraulic system of the ROV with the sample collection abilities of vibrating core insertion systems. The MBARI vibracoring system consists of a 3-m-high rigid rectangular aluminum frame attached to Ventanas bow (Figure l).The frame
serves as a guide to control core insertion and as a winch mount to extract the core from the bot tom. A central slide bar moves freely up and down the frame as the cores are collected. In the center of the slide bar is an aluminum block that was machined to accept a standard aluminum irrigation tube that serves as the core liner. A met al core catcher and cutter are fitted at the lower end of the core liner.Two hydraulically-driven motors attached on top of the slide bar generate strong vibrations.The motors provide most of the ~68 kg total weight on the core cutter that helps the system penetrate into the sediments when the vibrator motors are running.When the motors are working, the winch line is slowly slacked to allow core advancement as the sedi ments along the core liner are liquefied. Core
magnetic-anomaly data base, 25 pp., National Academy Press, Washington D.C., 1994. Urrutia-Fucugauchi, J., and R. Ornelas-Valdes, Aeromagnetic anomaly map of Mexico— Implications for crustal structure, magmatism and tectonics (abstract), £bs,Trans.AGU, 81, F355,2000. Zietz, I., Composite magnetic anomaly map of the United States; Part A, Conterminous United States, U.S. Geol. Surv. Investigations Map GP-954-A,59 pp., 2 sheets,scale 1:2,500,000, Washington, D.C., 1982.
recovery is assisted by a piston that is fixed to the rigid frame top during penetration, but stays at the top of the core barrel during core extrac tion. A hydraulic guillotine at the frame's base is closed after the core is extracted from the bot tom to prevent leakage of sand through the catcher during core recovery Similar systems can be easily fabricated for use on most other ROVs. To date, we have collected over 68 cores in water depths of up to 1156 m. Up to 7 cores have b e e n collected in a day. Many of the cores from the axis of Monterey Canyon con tain 1-2 m of sand and gravel and could not have b e e n obtained by any other existing technique.These samples are being collected to characterize material transport in subma rine canyons. The demonstrated improvement in ROV sampling capability is applicable in a number of other settings. For example, we are now tar geting sea floor fluid seepage sites associated
Fig. 1. Photograph showing the launch of ROV Ventana with the vibracoring Original color image appears at the back of this volume.
system
attached.
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Fig. and data tion can
1. Map showing line spacing of surveys that will be included in the North American grid map. Contiguous surveys with similar line spacings have been combined. Marine magnetic with variable line spacings will be used in the offshore areas. The existing Arctic compila will be used for Greenland and adjacent marine areas [Macnab et al, 1995]. Central Ameri data will be included where publicly available.
Eos,Vol. 82, No. 30, July 24, 2001
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Fig. 2. Examples of merged trans-national magnetic compilations, (b) Magnetic Arizona and New Mexico, United States; Sonora and Chihuahua, Mexico.
data over parts of
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Fig. 1. Photograph
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