Crosshole Seismic Methods Per ASTM Standard

Acknowledgements: Reprinted in part from US Army Corps of Engineers, Manual EM 1110-1-1802 titled Geophysical Methods for Engineering and Environmental Investigations.

Crosshole: General Procedures
The primary purpose of obtaining crosshole data is to obtain a detailed in situ seismic-wave velocity profile for site-specific investigations and material characterization. Crosshole velocity data are valuable for assessing man-made materials, soil deposits, or rock formations.

The crosshole seismic technique determines the Compressional (P-) and/or Shear (S-) wave velocity of materials at depths of engineering and environmental concern. The data can be used in problems related to soil mechanics, rock mechanics, foundation studies, and earthquake engineering. Crosshole geophysical testing is generally conducted in the near surface (upper one hundred meters) for site-specific engineering applications. All of a material's dynamic elastic moduli can be determined from knowledge of the in situ density, p-, and S-wave velocity. Because procedures to determine material densities are standardized, acquiring detailed seismic velocities yields the required information to analytically assess material properties. Low-strain material damping and inelastic attenuation values can also be obtained from crosshole surveys. However, the most robust application of crosshole testing is the ability to define in situ shear-wave velocity profiles for engineering investigations.

The objective of acquiring crosshole data can be multipurpose; that is, the seismic velocity results obtained may be used for evaluation of lateral and vertical material continuity, liquefaction analyses, deformation studies, or investigations concerning amplification or attenuation of strong ground motion. 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. During phase two field exploration the information gathered is more critical to the site-specific design.

Crosshole techniques are most useful when phase one site explorations indicate horizontal and particularly vertical variability of material properties. When layers of alternating density or stiffness are either known to exist or are encountered during phase one field investigations, then crosshole seismic tests are recommended to define the in situ velocities within each layer. Acquiring crosshole seismic data resolves hidden layer velocity anomalies which cannot be detected with conventional surface methods, allows both final interpretation of other surface geophysical data (seismic or electrical) and permits both empirical and theoretical correlation with other geotechnical measurements. In order to have quantitative and quality assured results, crosshole tests performed for either engineering or environmental problems should be conducted in accordance with procedures established by the American society for Testing and Materials (ASTM). Crosshole seismic test procedures are outlined in ASTM test designation D4428/D4428-91 M-84 (May,1991). The ASTM procedures provide specific guidelines for borehole preparation, data acquisition, and data reduction/interpretation. Since the inception of the ASTM standard in 1984 crosshole geophysical surveys have become more widely used and accepted for engineering as well as environmental applications. The detailed site information obtained from the crosshole tests leads to the overall acceptance of the validity of the velocity data. Results of the crosshole tests which conform to the ASTM standards are used to derive liquefaction parameters and site-specific seismic response. Foundation design to conform to the International Building Code (IBC) requirements is often based on crosshole seismic testing.

Crosshole: Theory and Equipment
Crosshole testing takes advantage of generating and recording (seismic) body waves, both the P- and S-waves, at selected depth intervals where the source and receiver(s) are maintained at equal elevations for each measurement.



The figure illustrates a general field setup for the crosshole seismic test method. Using source-receiver systems with preferential orientations in tandem (i.e., axial orientations which compliment the generated and received wave type/signal) allows maximum efficiency for measurement of in situ P- or S-wave velocity depending on the axial orientation. Due to the different particle motions along the seismic ray path it is crucial to use optimal source-receiver systems in order to best record crosshole P- or S- waves (Hoar, 1982). Stoke (1980) demonstrated that particle motions generated with different seismic source types used during crosshole testing are three directional. Therefore, three-component geophones with orthogonal orientations yield optimal results when acquiring crosshole P- and/or S-wave seismic signals. With 3-component geophones there is one vertically oriented geophone and two horizontal geophones. For crosshole tests one horizontal geophone remains oriented parallel to the axis between the boreholes (radial orientation) ; and, the other one remains oriented perpendicular to the borehole axis (transverse orientation) .In this case, the two horizontal axis geophones must remain oriented, radically and transversely, throughout the survey.

For either surface or crosshole seismic testing in unconsolidated materials, P-wave velocity measurements are greatly affected by the moisture content or percent saturation (Allen et al.1980). In crosshole testing the seismic measurements encroach closer to the water surface with each successive depth interval. As the 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 (e.g., Poisson's ratio increases to 0.48- 0.49 in 100% saturated soils). Hence, below the water surface the P-wave is commonly termed the fluid-wave, because its propagation velocity is governed by the pore fluid(s) not the formation density. Fluid-wave velocities in fresh water range from 1,400 to 1,700 m/s, depending upon water temperature and salt content. S-waves generated in crosshole testing may be split into two wave types, each with different particle motions; SV- and SH- waves, vertical or horizontal particle motions, respectively. Shear-waves have the unique capability of polarization, which means that impacting the material to be tested in two directions (up or down, left or right) yields S-wave signals which are 180ø out of phase. A seismic source with reversible impact directions is the key factor for quality crosshole S-wave data acquisition and interpretation.



The second figure shows a series of crosshole SV-waves with reversed polarity (note the low amplitude of the P-wave energy compared to the S-wave energy) received at both receiver boreholes.

Typically, the S-wave generated in most crosshole testing is the SV-wave which is a vertically-polarized horizontally propagating Shear wave. That is, the ray-path is horizontal but the (shear) particle motion along the ray path is in the vertical plane. These SV-waves are generated with commercially available borehole impact hammers which have reversible impact directions (up or down); and, they are also the easiest to record because only one vertically oriented geophone is required in each receiver borehole. Alternatively, SH-waves can be generated and recorded in crosshole testing. SH-waves also propagate horizontally, but their (shear) particle motion is in the horizontal plane (i.e., horizontally-polarized horizontally propagating S-waves) .Therefore, in order to generate and record SH-wave signals horizontal impacts and geophones are required; also, the orientation of the source and receiver must be parallel while their respective orientation remains perpendicular to the axis of the boreholes (transverse orientation).

Theoretically, there is no difference in the body wave velocity for SV- and SH-waves, which justifies use of the uncomplicated vertical source for generation for SV-waves, and vertically oriented geophones for signal detection. There are studies, however, which indicate significant velocity dependence of the SV- and SH-waves due to anisotropic states of stress in either the horizontal or vertical stress field (particularly in soil deposits; Redpath et al. 1982) or fractured rock formations (White 1983). The requirement for multiple drill holes in crosshole testing means that care must be taken when completing each borehole with casing and grout. The ASTM procedures call for PVC casing and a grout mix that closely matches the formation density. Basically, borehole preparation and completion procedures are the success or failure of crosshole seismic testing. Poor coupling between the casing and the formation yield delayed arrival times and attenuated signal amplitudes, particularly for (higher frequency) P-waves. Matching the formation density with a grout mix is not too difficult, but in open coarse-grained soils problems arise during grout completion with losses into the formation. Even small grout takes begin to affect the velocity measured between two closely spaced drill holes. Several techniques to plug the porosity of the surrounding formation are commercially available (e.g., cotton-seed hulls, crushed walnut shells, or increased bentonite concentration in the grout mix). It should be recognized that increasing the ratio of bentonite/cement within the grout mix does effect the density, but as long as the mix sets and hardens between the casing and the in situ formation then quality crosshole seismic signals will be obtained.

Another critical element of crosshole testing, which is often ignored, is the requirement for borehole directional surveys. There are several very good directional surveys tools available which yield detailed deviation logs of each borehole used at a crosshole site. Borehole verticality and direction (azimuth) measurements should be performed at every depth interval that seismic data are acquired. with the deviation logs corrected crosshole distances between each borehole may be computed and used in the velocity analysis. Because seismic wave travel times should be measured to the nearest tenth of a millisecond, then the relative borehole positions should be known to within a tenth of a foot. Assuming that the boreholes are vertical and plumb leads to computational inaccuracies and ultimately to data which cannot be quality assured.

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. Crosshole measurements rely considerably on the premise that the trigger time is precisely known. The recorded trigger signal from zero-time geophones or accelerometers mounted on the downhole impact hammer allow accurate timing for the first arrival at each drill hole. The trigger signal becomes uniquely critical when only two drill holes are used (i.e., source and one receiver) because there is no capability of using interval travel times; in this case, the velocity is simply determined through distance traveled divided by direct travel-time. 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, the digital data can be filtered, smoothed, and time-shifted during analysis. Also, digital signal processing may be directly performed for coherence, frequency-dependent attenuation, and spectral analysis. Numerous studies have shown that the effects on crosshole measurements by the choice of geophone is not critical to the results (e.g., Hoar 1982) .There are only two requirements for the receivers: the receiver (velocity transducer) must have a flat or uniform output response over the frequency range of crosshole seismic waves (25 to 300 Hz) ; and, a clamping device must force the receiver against the borehole wall such that it is not free- hanging. The clamping device should not affect the mechanical response of the geophone (i.e., resonance), nor should the uphole signal wire. If an SH-wave source is selected then horizontal geophones must be used, and oriented as previously described, to detect the SH-wave arrivals. It is paramount that the polarity of each geophone be known prior to data acquisition because the direct arrivals of S-waves with reversed polarity can be easily misinterpreted. Hoar (1982) provides an excellent description of picking p- and S-wave arrivals from recorded crosshole signals. Hoar's dissertation shows that with proper borehole completion, digital recording equipment, and a preferential source-receiver system clean reversed polarized and interpretable S-wave signals are relatively easy to acquire.

Crosshole: Advantages/Disadvantages
Crosshole seismic testing has the unique advantage of sampling a limited volume of material at each test depth. Thus, the final result is a detailed and accurate in situ velocity profile. Crosshole tests are not unique in the use of preferential source/receiver configurations, however, there is the distinct opportunity to preferentially excite particle motion in a single directions with respect to the vertical borehole wall. Because of the specific and known particle-motion direction, the crosshole test permits exact interpretation of mode of propogation of the direct arrivals. The in situ material-property information yields the best input data for the engineering application (liquefaction, deformation, or strong motion characterization). Crosshole seismic testing has the definitive advantage of assessing a complex layered velocity structure with alternating high and low relative velocities. Other surface techniques such as spectral analysis of surface waves can theoretically evaluate the high/ low layered velocity structure, but due to a number of inherent assumptions associated with surface geophysical methods several non-unique velocity profiles may be derived (from inverse modeling) without specific information about the subsurface layering at the site. Because 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.) will augment site-specific design. Sites of particular concern are those with liquefaction potential where the subsurface contains considerable unconsolidated coarse-grained materials and standard geotechnical test procedures (blow counts and material sampling) can not effectively evaluate in situ properties. For successful engineering analysis of coarse-grained materials, crosshole testing is one of the most acceptable geophysical techniques available.

The primary detriments or obstacles encountered during crosshole testing are typically related to the placement and completion of the drill holes. Sites where non-invasive techniques are required due to hazardous subsurface conditions, crosshole seismic tests are not applicable because of tight regulatory procedures regarding drilling, sampling, and decontamination. However, sites where detailed in situ P- and S- wave velocities are required drill hole completion must follow ASTM procedures, and when unusual conditions exist (e.g., open-work gravels) specialized techniques for borehole completion should be employed. The Bureau of Reclamation has encountered numerous sites in the western US where loose, liquefiable sand and gravel deposits needed to be investigated. Crosshole testing effectively evaluated the in situ materials with P- and S- wave velocity measurements but considerable care and caution was used for completion of each borehole.

The seismic data for crosshole testing needs considerably more waveform interpretation because refraction events from high velocity layers either above or below a low velocity layer must be identified and the first-arrival velocity corrected. Direct-wave arrivals are easily recognized as long as the previously described field equipment is utilized for preferential excitation of the signals.

Downhole: Advantages/Disadvantages
Downhole methods use a single hole and surface sources. Shear and compressional waves are independently generated and are recorded by a three-component downhole receiver. The advantage of this variant on downhole seismic measurements is in the drilling economics. However, the raypath averages the velocities along its length and, in areas with lots of lateral and vertical variation, the resolution of the method is inferior to that of the crosshole methods. The classic engineering tradeoff of precision and resolution versus economics must be made. For non critical structures in areas of low geologic variability, downhole measurements may suffice.

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