Robert F. Blakely

Random Processes and Lithologic Transitions

Bedding sequences and stratigraphic sections can be profitably considered from the standpoint of their lithologic transitions, that is, the probability of any particular unit being followed by the other units in the sedimentation system. Dependent Markov processes appear to be most appropriate for the study of lithologie transitions. This paper contains an elementary expository account of Markov processes and their relation to sedimentation. A Devonian limestone-dolomite-anhydrite sequence, an Ordovician limestone-shale sequence, and Penn-sylvanian cyclothems show statistical evidence of at least a one-step dependence, or memory. Other geologic evidence also points to the existence of dependent lithologie transitions, especially in transgressive sequences. Markov processes can also be used to synthesize stratigraphie sections.

Geophysical analysis in central Indiana using potential field continuation

Seismic, gravity, and total, vertical, and horizontal magnetic surveys were conducted over a prominent basement anomaly in Hamilton County, Indiana, as part of a statewide effort to understand the nature of the basement complex. Application of Poisson’s relationship between the gravity and horizontal magnetic fields demonstrates that the source is not a single, vertically sided prism, but is a body whose outline (shape) varies with depth. Digital model studies support the suggestion that continuation of the fields observed at the surface to levels below the upper surface of the source can be useful in defining the shape of certain sources. Application of this technique was successful in defining a model whose fields closely match those observed in Hamilton County. Geologically, the final model resembles a vertical pipe associated with a succession of overlying flows, a model that is consistent with our present understanding of basement geology in midwestern United States.

A geophysical study of a basement anomaly in Indiana

Seismic, gravity, aeromagnetic, vertical, and horizontal magnetic surveys were made in Indiana as part of a statewide effort to study the basement complex. These surveys were interpreted collectively in a prescribed sequence that led to information ordinarily not obtained individually from such surveys. Prominent seismic reflections and sharp, coincident magnetic and gravity anomalies in Pulaski County, Indiana, were interpreted to show the depth, geometry, and rock type of an anomalous body within the basement complex. Seismic reflection data show the top of the anomalous body to be at a depth of 1.4 km. Analysis of the aeromagnetic anomaly indicates that the body can be approximated by a thick, vertical prism polarized along the earth’s field. Second derivative analysis of the vertical magnetic anomaly outlines the body as roughly circular and 29km2 in area. Curve‐fitting between the observed vertical magnetic anomaly and an anomaly computed for the prismatic model readily shows that the body has a vert...

Transformation of Resistivity to Pseudovelocity Logs: GEOLOGIC NOTE

Pseudovelocity logs are generated by computer from short-normal resistivity logs by using the relation TT = A + B (R)-1>C. Given the resistivity value R, the transit time TT can be computed if the empirical constants A, B, and C are known. Although two distinct geologic provinces are present in Indiana (the Cincinnati arch and the Illinois basin), the same sets of constants are applicable throughout the state. For wells drilled with a mud range of 2 to 5 ohm-m, visual comparison of the pseudovelocity logs with actual continuous-velocity logs (CVLs) shows close correlation in form. In addition, calculation of the cumulative transit times from a datum to a given depth yields values for pseudovelocity logs and CVLs that compare within 6 percent. The method is most successful if applied to the stratigraphic section below the freshwater--salt water contact. The constants A, B, and C are based on computations from selected depths on resistivity logs and CVLs. Our studies now indicate that R and TT values averaged over regular 10-ft (3 m) intervals also yield reasonable A, B, and C values for computation and eliminate the need for the user to make decisions about what depths are appropriate for selecting R and TT values.

Reply by authors to discussion by Douglas J. Guion

The authors’ model studies depended primarily on downward continuation of the magnetic field (p. 884). Satisfactory results were obtained for magnetic models, but certain discrepancies were observed for gravity (p. 887). Mr. Guion’s comments on some differences between the observed and model gravity anomalies are, therefore, well taken. In his view these differences cannot be attributed to the manner in which the regional gradient is removed or to the choice of density contrast used in constructing the model. Our opinion is that he underestimates the importance of both of these parameters.

Geophysical Analysis in Central Indiana using Potential Field Continuation; discussion and reply

I read with interest the article concerning modeling the Hamilton County, Indiana, gravity and magnetic anomaly. The authors’ method for outlining the igneous body by downward continuation aroused my curiosity to the point that I decided to study the results in detail. My investigation revealed that the calculated gravity effect of the model did not satisfy the observed gravity anomaly. In fact, the amount of mismatch is quite serious.