A scheme to process passive surface waves is briefly described. It transforms wavefields of horizontal plane-wave propgation, measured with receiver spreads along the two orthogonal axes (x and y), into dispersion images. The scheme first transforms the measured wavefields of a particular frequency (ω) into the energy in phase velocity (c)-azimuth (θ) space where multiple sources and modes of surface waves are represented as energy peaks at different azimuths and phase velocities, respectively. The scheme, then, stacks all the energy through the azimuth axis to produce an energy distribution along the phase-velocity (c) axis only. A final image is then created by repeating these steps for different frequencies and displaying the energy in frequency (ω)-phase velocity (c) space. This process greatly alleviates general complications with the passive method such as disturbance of phase velocities due to multiple sources and modes as well as the spatial aliasing.

The Kansas Geological Survey has operated a microearthquake seismograph network since mid-1977. The network now consists of fifteen stations located in the eastern half of Kansas and Nebraska. Locatable microearthquakes with duration magnitudes less than 3.2 occur at the rate of roughly 20 per year in the two-state area, with most of the events ranging from 1.4 to 2.5 in local magnitude. The microearthquake pattern observed over the past ten years is consistent with the pattern of historical earthquakes reported since 1867. Much of the activity occurs along the Nemaha Ridge, a buried Precambrian uplift that runs from roughly Omaha, Nebraska, southward across Kansas to near Oklahoma City. This geological structure has been the site of several earthquakes of MM Intensity VII over the past 125 years. Some seismicity is observed along the northwest flank of the Midcontinent Geophysical Anomaly in Kansas, but little is observed in the Nebraska or Iowa portions of this Precambrian feature. The Central Kansas Uplift, which is a buried anticline similar in age to the Nemaha Ridge, has been the site of several felt earthquakes since 1982. A trend of earthquakes extending northeastward across central Nebraska is not associated with any prominent known geologic structure. All the seismicity in central and eastern Kansas can be roughly correlated to known geologic structures.

No abstract

An improved design of the buffalo gun, incorporating hole drilling and shooting into one operation with almost total blast containment, provided nearly 50 percent more recordable seismic signal than the traditional buffalo gun. The ' auger gun' is designed to optimize field efficiency, source couple, and safety. The auger gun consists of three main parts: power source, auger/screw, and modified buffalo gun. Amplitude spectra indicate the auger gun delivers approximately 20 percent more total energy and 50 percent more reflection energy with no noticeable increase in recorded ground roll at a test site in Lawrence, Kansas. Subtle changes in source environment, such as a 15 cm gap between the detonation point and the base of the hole or inclusion of water as opposed to air at the detonation point, have a significant effect on the quality of the recorded data. The auger gun should increase field efficiency by over 50 percent on most shallow surveys, while improving the signal-to-noise ratio and total energy in comparison to the buffalo gun.