ASTM D7400 Downhole Seismic Testing

Scope:

This test ASTM D7400 method is limited to the determination of the interval velocities from arrival times and relative arrival times of compression (P) and vertically (SV) and horizontally (SH) polarized shear (S) seismic waves which are generated near-surface and travel down to an array of vertically installed seismic sensors. The P-wave and S-wave velocities are directly related to the critical geotechnical elastic constants of PPoisson’sratio, shear modulus, bulk modulus, and YYoung’s modulus.

Test Procedure: 

There are three steps involved in this test method ASTM D7400:

• STEP: Boreholeole Preparation)-  The preferred method for preparing for downhole testing incorporates oboreholeole with a grout pipe/pump. A dry test hole is preferred to avoid signal noise caused by waves transmitted through the water column in a water-filled test hole. Drill tboreholeole, with minimum sidewall disturbance, to a diameter not exceeding 175 mm (7.0 in.). After the drilling is completed, case the boring with 50 to 100 mm (2 to 4 in.) inside diameter PVC pipe or aluminum casing, considering the size of the downhole receivers. Before inserting the casing, close the bottom of the pipe with a cap that has a one-way ball-check valve capable of accommodating a 38 mm (1.5 in.) outside-diameter grout pipe. Centre the casing with spacers and insert it into the bottom of the tboreholeole. Grout the casing in place by  inserting a 38 mm (1.5 in.) PVC pipe through the center of the casing, contacting the one-way valve fixed to the end cap by a small diameter grout tube inserted to the bottom of the tboreholeole between the casing and tboreholeole sidewall. Borings may be used either with or without casing; however, if the casing is used, grout should be injected between the casing and the sidewall of the tboreholeole to ensure good contact.

• STEP: II (Seismic hole Penetration Deployment)-  At the start of the cone penetration test, the body of the cone penetrometer should be rotated until the axis of the transducer is parallel to the long axis of the shear beam if a uniaxial configuration is implemented. Suppose a biaxial (X and Y components) or triaxial sensor configuration (X, Y, and Z components) is utilized. In that case, one of the horizontal components should be aligned parallel to the shear beam to maximize the SH-wave amplitude in one component direction. The cone penetrometer is pushed into the ground to the desired downhole test depth, and the CPT instruments and inclination of the cone assembly are monitored during the push. The depth of the receivers should be recorded. Care should be exercised to avoid the sensors’ rotation while adding subsequent rods.

• STEP: III (Downhole Test)- For Step III of ASTM D7400 testing, begin the downhole test by preparing the energy source at its desired location. Place the receivers at the top of the receiver hole. If possible, orient the receiver unit bodies so that the axis of a horizontal transducer is parallel to the long axis of the shear beam. If two separate units are used, lower or push the lower unit at a depth of 1.0 to 1.5 m (3 to 5 ft) below the upper unit. Check the recording equipment and verify timing. Monitor the output of the receivers without activating the energy source to evaluate the ambient seismic noise in the ground. Activate the energy source and display the rreceiver’straces on the recording device. Tests should be conducted separately for better results if both P- and S-wave sources are used. Adjust the amplifier gain and recording time so that the P-wave, S-wave, or both are displayed in their entirety. If the recorder allows the test to be repeated and superimposed on the earlier test, repeat the test 3 to 5 times to improve the signal-to-noise ratio. Record and store the wave train digitally and print a hard copy for all receivers. Perform the following test by lowering the receivers to a depth dictated by known stratification, but typically no greater than 1.5 m (5.0 ft) and no less than 0.5 m (2 ft) from the previous test locations in the borings and repeat the above procedure. Continue with succeeding tests until the maximum hole depth has been reached. 

Sample Size:  

Beam: Metal or Wood and may be encased at the ends and bottom with a steel plate, 

Shear beam length: 2.4m (8 ft.),

Width: 150mm (6 in.),

Hammer: 1 to 15 kg,

Sensor distance from each other: 10cm (4 in.).

Data:

The data stated in SI units and Inch-pound units are considered standard. No other units of measurement are included in this standard.

Conclusion:

This test’s ASTM D7400 method is limited to determining the interval velocities from arrival times and relative arrival times of compression (P) and vertically (SV) and horizontally (SH) polarized shear (S) seismic waves, which are generated near the surface and travel down to an array of vertically installed seismic sensors.