These test methods are limited to the determination of the velocity of two types of horizontally travelling seismic waves in soil materials. GEOPHYSICAL SYSTEMS. NDE ™» One Platform – Multiple NDE Tests geo- Crosshole/Downhole Seismic» ASTM D/DM/D(DS). Crosshole seismic test procedures are outlined in ASTM test designation D M (). The ASTM procedures provide specific.
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D44228 of Reclamation, Since considerable confidence can be placed on engineering scale crosshole seismic data, computation of in situ low-strain elastic constants Shear and Young’s modulus, Poisson’s ratio, etc. The clamping device should not affect the mechanical response of the geophone i.
During phase two field exploration, the information gathered is more critical to the analytical site-specific Characterization. The recorded trigger signal from zero-time geophones or accelerometers mounted on the downhole impact hammer allows accurate timing for the first arrival at each drill hole.
It is beyond the scope of these test methods to consider significant digits used in analysis methods for engineering design. There are several very good directional survey tools available that yield detailed deviation logs of each borehole used at a crosshole site. P-waves are generated with a sparker or small explosive device one that will not damage the PVC casing such that along the assumed straight-ray propagation path the seismic impulse compresses and rarefies the materials radially toward the receiver borehole s.
Because of this, the crosshole test permits much easier interpretation of direct arrivals in the recorded waveforms. Based on this discussion, to ensure that true in situ velocities are presented, crosshole measurements should be performed a minimum of four measurement intervals below the zone of concern to adequately define the velocity profile. Crosshole SV-wave paired borehole records at five depths.
Standard Test Methods for Crosshole Seismic Testing
Numerous studies have shown that the effects on crosshole measurements by the choice of geophone are not critical to the results e. Also, digital signal processing may be directly performed for coherence, frequency-dependent attenuation, and spectral analysis.
Crosshole measurements rely considerably on the premise that the trigger time is precisely known as well as recorded. The procedures used do not consider material variation, purpose for obtaining the data, special purpose studies, or any considerations for the users objectives; and it is common practice to increase or reduce significant digits of reported data to be commensurate with these considerations. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other.
Poor coupling between the casing and the formation yields delayed arrival times and attenuated signal amplitudes, particularly for higher frequency P-waves.
Experience has proven that for optimal measurement of the P-wave signal, a hydrophone has the greatest pressure-pulse sensitivity for compressional-wave energy. Active view current version of standard. Hoar provides an excellent description of picking P- and S-wave arrivals off recorded crosshole signals. For engineering applications, there is not much advantage in determining via modeling the ray coverage or residual velocity resolution because crosshole testing at the engineering scale utilizes a simple horizontal, adtm path geometry to determine average velocity.
V 2 across the seismic interface determine if the refracted wave will arrive before the direct wave. Also, lithologic information such as stratigraphy and material type are determined from the drilling and sampling program prior to seismic data acquisition; this allows reliable constraints, or boundary conditions, to be placed on the field data along the boundaries of the material between the boreholes.
In this system, the pound lbf represents a unit of force weightwhile the unit for mass is slugs. In this example, refractions occur in a situation similar to that depicted in figure 3; that is, d428 occur from high-velocity materials either above or below the low-velocity layer.
Several techniques to plug the porosity of the surrounding formation are commercially available e. Compute hypotenuse distance H i: As d442 vadose zone and water surface are encountered, P-wave velocities become dependent upon the percent saturation, and the Poisson’s ratio is no longer a valid representation of the formation characteristics.
Assuming that the boreholes are vertical and plumb leads to computational inaccuracies and ultimately to data that axtm be quality assured. When approaching seismic interfaces, refracted-wave arrivals begin to be timed as the first arrival, which could easily be misinterpreted as direct-wave arrival.
Data Acquisition Recording instruments used in crosshole testing vary considerably, but there are no standard requirements other than exact synchronization of the source pulse and instrument trigger for each recording. Utilizing digital recording equipment affords the operator the ability to store the data on magnetic media for analysis at a later date; but more importantly, digital data can be filtered, smoothed, and time-shifted during analysis.
Therefore, a systematic comparison of measured travel times, computed direct velocities, and interval velocities at each recording depth enables interpretation of true in situ velocity at all measurement depths. Compute travel times through both materials: In crosshole testing, the seismic measurements encroach closer d4428 the water surface with each successive depth interval. C4428, borehole preparation asm completion procedures are the success or failure of crosshole seismic testing.
General Crosshole Procedures | Environmental Geophysics | US EPA
Typically, crosshole surveys are a geophysical tool for performing explorations during what are considered phase two field investigations where phase one field investigations include surface geophysical surveys, follow-up drilling, trenching, and sampling of the in situ materials. Combining values from the two systems may result in non-conformance with the standard.
Crosshole velocity data are valuable for assessing man-made materials, soil deposits, or rock formations.