Richard H. Godson

MAGNETIC ANOMALY MAP OF NORTH AMERICA

Data presented on the magnetic anomaly map in the continental areas have largely been derived from published map sources. However, these maps for the most part have been digitized and subjected to various processing steps in preparation for compositing. The marine areas of the map consist mostly of digital data track, except in the Gulf of Mexico, Caribbean Sea and the continental shelf areas of Alaska where the data were obtained from digitized maps.

GRAVITY ANOMALY MAP OF NORTH AMERICA

The Gravity Anomaly Map of North America is the product of a 12-year international effort to compile, critically edit and merge gravity anomaly data on a continental and global scale. This color‐pixel map, printed on four quadrant sheets at a scale of 1:5 000 000 and including a fifth sheet showing a color index map with data references, is the first such map at this large scale to include several hundreds of thousands of precise surface data of the United States, Canada, Mexico and Central America as well as other high‐quality surface data from neighboring continental and oceanic areas. The map, which shows Bouguer gravity anomalies on land and free‐air gravity anomalies over oceans, is remarkable for its detail. Sixty‐six colors or shadings have been used in a carefully conceived nonlinear scheme to show anomalies at a 5 or 10 mGal interval over a dynamic range from about −300 mGal to +130 mGal.

DEPENDENCE OF IN-SITU COMPRESSIONAL-WAVE VELOCITY ON POROSITY IN UNSATURATED ROCKS?

The relation of in‐situ compressional‐wave velocities to porosities, determined by seismic refraction for unsaturated near‐surface rocks from different areas in Arizona, New Mexico, and California, is grossly similar to relations determined by other investigators for water‐saturated rock and unconsolidated sediments. The principal difference is that in the porosity range 0.0–0.2, compressional waves travel somewhat more slowly in unsaturated rocks than in water‐saturated rocks, and much more slowly, in the porosity range 0.2–0.8. The function, ϕ=−0.175 ln (α)+1.56, where ϕ is the fractional porosity and α is the compressional‐wave velocity, was obtained as a least squares fit to the experimental data. Bulk densities are reported for all samples; moisture contents are reported in some instances.

USGS potential-field geophysical software for PC and compatible microcomputers

Potential‐field software currently in use by the USGS Branch of Geophysics has recently been modified for use on PC/MS-Dos based microcomputers and is available at nominal cost from the USGS Open‐File Report Services. Version 2.0 represents a substantial upgrade of version 1.0 of the PC software (US Geological Survey Open‐File Report 89–197 A-F) released in 1989. It includes many new programs and program repairs, fuller documentation, and on‐line help. Both executable and Fortran source‐code are included.

Attenuation measurements in the field

The amplitudes of the P head wave were measured on an aa lava flow, on unconsolidated cinders, and on compact limestone. The data are satisfied by the equation A=Nd-2exp(-πdQ-1ανα-1), where A is the amplitude, d the distance, Qα the attenuation coefficient for P waves, ν the frequency, and α the P wave velocity. By assuming a complex shear modulus μ* but a real λ one finds Qα/Qβ=0.5α2/β2, where β is the shear wave velocity. This formula is in reasonable agreement with published data.