Lab Experiments (1)

Laboratory Investigations of Cometary Analog Material

	Fig. 1: Laboratory investigation of sound velocities in sintered cometary analog material ('CAM', porous ice). Piezo-electrical accelerometers are attached to the ice for the determination of the arrival time of acoustic waves, which are excited at the rear side. (Performed in the Cold Laboratory at the Institute of Space Simulation, DLR Cologne.)
	Fig. 2: Typical signals on the screen of the analyzer. The arrival time difference together with the known distance of the sensors provide the sound velocities. This shows that sound wave detection in porous media is possible and can lead to the determination of important material parameters (e.g. Young's modulus E).
	Fig. 3: Screen hard copy of an acoustic event in porous ice. The arrival time of the compression (P-)wave can easily be determined; more difficult in this one-axial measurement is the determination of the slower but stronger shear (S-)wave, which is superimposed by the former P-wave. In some cases it reveals as a kind of guess work (see upper curve)!
	Fig. 4: Acoustic signals in sand as an example of a porous media, now measured with multi-axis sensors. Time elapses from left to right, from the bottom to the top the distance between exciter and sensor is increased, the amplitude is represented by colors. In this special setup we see mainly the (longitudinal) P-waves. The slope of the white line represents the velocity of the P-waves.
	Fig. 5: Same measurement as Fig. 4, but now exciter and sensor are arranged in a way that they are most sensitive for (transversal) S-waves. As expected, the S-velocity is lower than the P-velocity. It is obvious that, with multi-axis sensors we can easily distinguish between P and S-waves. That implies, that for a good wave type separation on the Rosetta Lander, multi-axis sensors are required.