ABSTRACT

3D printing techniques (additive manufacturing) using different materials and structures provide opportunities to understand porous or fractured materials and fluid effects on their elastic properties. We used a 3D printer (Stratasys Dimension SST 768) to print one “solid” cube model and another with penny-shaped inclusions. The 3D printing process builds materials, layer by layer, producing a slight “bedding” plane, somewhat similar to a sedimentary process. We used ultrasonic transducers (500 kHz) to measure the P- and S-wave velocities. The input printing material was thermoplastic with a density of 1.04  g/cc, P-wave velocity of 2167  m/s, and S-wave velocity of 885  m/s. The solid cube had a porosity of approximately 6% and a density of 0.98  g/cc. Its P-wave velocity was 1914  m/s in the bedding direction and 1830  m/s normal to bedding. We observed S-wave splitting with fast and slow velocities of 879 and 835  m/s, respectively. Quality factors for P- and S-waves were estimated using the spectral-ratio method with QP ranging from 15 to 17 and QS from 24 to 27. By introducing penny-shaped inclusions along the bedding direction in a 3D printed cube, we created a more porous volume with density of 0.79  g/cc and porosity of 24%. The inclusions significantly decreased the P-wave velocity to 1706 and 1351  m/s parallel and normal to the bedding plane. The fast and slow S-wave velocities also decreased to 812 and 656  m/s. A fluid substitution experiment, performed with water, increased (20%–46%) P-wave velocities and decreased (9%–10%) S-wave velocities. Theoretical predictions using Schoenberg’s linear-slip theory and Hudson’s penny-shaped theory were calculated, and we found that both theories matched the measurements closely (within 5%). The 3D printed material has interesting and definable properties and is an exciting new material for understanding wave propagation, rock properties, and fluid effects.

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