Downhole seismic testing

Downhole seismic testing is used to measure the primary wave and shear wave velocity with depth. Based on two parameters and density of soil and rock, one can calculate the elastic modulus, shear modulus, Poisson’s ratio, assessment of fundamental period (To), amplification factor of ground (A) for earthquake resistant design for structures.

sketch-of-downhole-seismic-testing

                     A sketch of downhole seismic testing

Downhole seismic testing utilizes surface source to generate primary wave and shear wave. Source is usually sledge hammer and wooden plank to generate shear wave. One or couple of 3-axis borehole geophone is clamped into sidewall with geophone interval of 1 to 5 m.

Downhole seismic testing need only one borehole. After the drilling is completed, case the boring with 70 to 100 mm inside diameter PVC pipe, between borehole wall and casing is grouted by grout mixture. Grout mixture should be formulated to closely the density of the surrounding in situ material after sodification.

downhole-seismic-testing-at-field

                                                A downhole seismic testing at field

Downhole seismic testing data are displayed by Pickwin to pick the traveltime. The The traveltime of shear wave and primary wave is picked and put into table form to calculate velocity or process by SeisImager/DH software.

Traveltime-curves-longitudinal-and-shear-velocity

                                Traveltime curves, longitudinal and shear velocity models

Fieldwork, processing and interpretation of downhole seismic testing are carried out in accordance with ASTM 7400 – 08: Standard test methods for Downhole Seismic Testing.