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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 53 (1969)

Issue: 2. (February)

First Page: 471

Last Page: 471

Title: Portable Refraction Seismography Survey of Gold Placer Areas Near Nome, Alaska: ABSTRACT

Author(s): H. Gary Greene

Article Type: Meeting abstract


A seismic refraction study of the beach and tundra gold placer areas near Nome, Alaska, was made using a small, light-weight, portable seismograph during the summer of 1967. Geophone configuration and type of energy source were determined during a preliminary experimental survey.

Because the beach study was successful, a short experimental seismic line was completed inland to determine the usefulness of a portable refraction seismograph in permafrost areas near Nome. Basic problems in permafrost areas are the high seismic velocities in the overburden, caused by increase in elastic modules in frozen ground, and the acoustical absorption and Previous HitvariableNext Hit thickness of the overlying tundra. The increase in sediment Previous HitvelocityNext Hit reduces the possibility that there is a marked Previous HitvelocityNext Hit contrast at the bedrock-overburden interface, and the organic material of the tundra absorbs returning seismic energy. These problems were reduced by detonating the explosive energy source on the permafrost surface and by placing the geophones in the thaw zone of silt beneath the spongelike ma ter of the tundra.

The beach survey results indicated that internal stratigraphy of the overburden could be interpreted and seismic velocities assigned to the different units. A very low-Previous HitvelocityNext Hit, dry to damp layer of Holocene sands covering most of the beach has seismic Previous HitvelocityNext Hit values of 0.15-0.73 km/sec. In other low-Previous HitvelocityNext Hit layers included in the overburden, and especially conspicuous near river mouths, Previous HitvelocityNext Hit values range from 0.62 to 1.00 km/sec. A poorly consolidated nearshore or estuarine silt, clay, and sand layer of Sangamon age (late Pleistocene) with Previous HitvelocityNext Hit values of 1.20-1.80 km/sec is below the very low-Previous HitvelocityNext Hit layer. Beneath the estuarine material is a till of Illinoisan age (middle Pleistocene) that has a Previous HitvelocityNext Hit of 2.80-4.00 km/sec. Bedrock was well defined in all seismograms and exhibited Previous HitvelocityTop values from 4.20 to 5.60 km/sec.

A basement contour map of the beach was constructed from depth data obtained along the beach with the refraction seismograph, from offshore seismic-reflection data, and from onshore drillhole information. Several buried channels were identified which may be sites of possible gold placer deposits. Beneath the tundra a bedrock surface dips under Dry Creek from both sides, and a bedrock contour map was drawn from refraction-seismograph data and drillhole information. Results of the Nome tundra survey illustrate the feasibility of the portable seismograph as a placer prospecting tool for use in tundra-permafrost areas.

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Copyright 1997 American Association of Petroleum Geologists