Transverse Isotropy In Seuimentary Sequences

Over the last ten years geophysicists have begun to<br>recognize the frequent occurrence and importance of non-isotropic<br>earth sections. In particular, many sedimentary sequences<br>exhibit a transverse isotropy in properties typified by a<br>horizontal preferential oriented fabric which can have a profound<br>effect on the strength of a material and fluid migration through it.<br>A field technique to measure the transverse isotropy of a<br>sedimentary sequence utilizing 3-component seismic shear and<br>compressional wave records from boreholes is described. A<br>maximum shear wave anisotropy of 50% was recorded for a clay<br>sequence with up to 20% compressional wave velocity anisotropy.<br>Laboratory studies including ultrasonic measurements and scanning<br>electron microscope imaging are compared to the field data. The<br>results are explained using a model based on the structural<br>geologists card deck. The ground section at this site can be<br>adequately described as transverse isotropic in velocities with<br>implications to the 2D dynamic moduli characterization.<br>As the engineering industry progresses towards a more<br>comprehensive range of site testing for marginal land<br>construction and with the growing concerns over hazardous waste<br>migration in the ground water the necessity to consider nonisotropic<br>ground sections is becoming more apparent. This work<br>represents one of the many recent attempts to address this<br>complex problem which could add significant information<br>especially in conjunction with other survey techniques, such as<br>borehole seismic tomography.