Wave Gradiometry Using a Three-dimensional Array of Experimental MEMS- based Accelerometers

Donny T. Dangkua, Charles A. Langston, and Chris H. Cramer
Center for Earthquake Research & Information, University of Memphis

We use 3-D wave gradiometry in an array field experiment composed of experimental three-component micro-electro-mechanical-system (MEMS)-based accelerometers. 15 accelerometers are configured into an irregular cubical 3-D gradiometer array installed between 0.5-1.5 m depth in several 2-in boreholes with a spacing interval of the boreholes between 0.3-0.6 m relative to a reference accelerometer located at the center of the cube. We record a vertical hammer source at various distances away from the reference station. Using the recorded seismograms and the position of the accelerometers, we calculate the displacement gradient tensor using a linear inversion method. Monte Carlo simulation is used to calculate 95% confidence limits in order to estimate the associated uncertainties in each displacement gradient. The displacement gradient tensor gives aerial and shear strains, and rigid body rotation in all three Cartesian directions, that are caused by waves that propagate through the array. The two wave gradiometry coefficients for all three directions in the Cartesian coordinate system are also calculated using the linear inversion method. The first coefficient gives the change in geometrical spreading and the second coefficient the slowness of the wave propagating along each axis of the coordinate system. We resolve the azimuths of incoming P and Rayleigh waves which are comparable to the true azimuth. This field method will be used to determine the shear modulus reduction curve of soils in observing nonlinear soil behavior due to increasing amplitude of strong ground motions.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013