Conventional Seismogram Research Articles

A resistivity log-derived and gas-corrected pseudo-synthetic seismogram: application to the Fanpokeng Gas Field of northwestern Taiwan

Resistivity logs have been used by seismic interpreters for many years to construct seismograms, particularly in hydrocarbon gas fields where sonic logs are scarce or absent. The correlation between the conventional synthetic seismogram made from density and sonic logs and the pseudo-synthetic seismogram made from density and resistivity logs has not been very satisfactory in the past because many of the pseudo-synthetic seismograms do not include the ‘gas effect’. Our research team has developed a method to correct for the gas effect, and thereby to improve the overall quality of the pseudo-synthetic seismogram. We apply this method to a well and then compare the improved results to a conventional seismogram made from the same well. Our research should be of interest to researchers in New Zealand, Taiwan and other Pacific Rim countries because of the similarities in the tectonic setting of these nations.

On the onset ofPPand other mini-max phases

SUMMARY Theory predicts that pulses that propagate on stationary non-minimum time paths, such as the PP path, undergo phase shifts such that the onset is emergent and the observed pulse is non-causal relative to the arrival time predicted by ray theory. The phase shift isπ/2 radians, at least at high frequencies, so that the observed pulses should approximate to the Hilbert transform of the pulse recorded over a minimum time path. Although onsets of phases that follow non-minimum time paths (usually called mini-max phases) are predicted to be emergent, those observed often seem clear and no more difficult to read than those of arrivals that follow minimum time paths. An example of a PP seismogram which is clearly emergent is shown here. Simulations obtained by Hilbert transforming the impulse responses of conventional long-period and short-period seismographs convolved with attenuation operators. show that the first half-cycle of mini-max phases which should have the emergent onset will usually be of low amplitude relative to the second half-cycle. This suggests that on observed seismograms the first half-cycle of mini-max phases may be obscured by earlier arrivals. The second half-cycle will then be taken as the first motion and could appear to have a well-defined onset. The effects of phase shifts that result in emergent onsets can be corrected for to some extent, and ideally onset times of mini-max phases would be read from such corrected records. However. onset times of PP read directly from conventional seismograms are reported in bulletins and these times have been assumed by some to be reliable enough to be used to determine anomalies in wavespeed in the upper mantle in the vicinity of the reflection point. Perhaps surprisingly the assumption appears to be justified in that the apparent onset time of a mini-max phase on a conventional short-period seismograph may be little different from the onset time read after correction for the π/2 phase shifts.

A seismic event counter for active volcanoes

Abstract Changes in the numerical frequency of microearthquakes have been observed at various active volcanoes. Identification of these periods of change can be made with a short-period, vertical seismometer driving a simple tally register counting circuit. A fully portable prototype model tested at various active volcanoes provided rapid estimations of the occurrence of microearthquakes. On Kilauea Volcano, Hawaii, the numerical results correlate very well with counts made from conventional seismograms. The device is no substitute for continuous seismic records, but provides a very simple method of monitoring changes in local seismicity which can then be evaluated by more sophisticated equipment.

Application of Multichannel Processing to Analysis of Long-Range Refraction Seismograms

Detailed interpretation of seismograms is dependent on the ability to decompose approximately the seismic disturbance into a set of events that have a simple theoretical basis. In the present paper, long-range refraction propagation is considered in terms of geometrical ray theory. The various rays possible in a typical model differ only slightly in frequency content and arrival time, but differ substantially in horizontal (phase) velocity. Individual later-arriving events can only rarely be identified on the conventional seismogram. Use of the frequency-wavenumber description of an event for processing on a multichannel basis allows identification and separation of the desired arrivals, with suppression of other arrivals. Examples are given of the application of such processing to two nuclear events recorded on the cross array at Tonto Forest Observatory (TFO), Arizona. The outputs of the array are combined in such a manner as to enhance successively lower velocities. The resulting “seismograms” may be considered 3-dimensional plots of amplitude versus velocity and arrival time. The PN and PG events are well-defined. There are indications of P* for at least one of the two events. Scattered (low-velocity) energy is negligible immediately after the arrival of PN, but is quite strong after PG. A likely scattering center is the sedimentary ridge north of TFO. The shear wave SG is also identified, and shown to be primarily transverse horizontal (SH). Theoretical computations are presented for the amplitudes of the various events, and the degree of similarity between theory and experiment is taken as a measure of our knowledge of the correct crustal structure.