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oa What Elastic Parameters Control Direct P-wave Amplitude in DAS Measurements?
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 4th EAGE Workshop on Fiber Optic Sensing for Energy Applications, Aug 2024, Volume 2024, p.1 - 5
Abstract
The amplitude of a seismic wave measured by a sensor depends on the properties of the medium around the sensor: the softer the medium, the larger the amplitude. For instance, if the sensors are distributed along a borehole, this dependency can be used to characterise the medium around the borehole and to detect changes of the medium properties over time. The specific parameter that controls the seismic amplitude depends on the quantity measured by the sensor as well as the type and direction of wave motion. Continuity of the stress traction implies that distributed acoustic sensor (DAS) amplitude of a low-frequency plane P-wave propagating along the fibre should scale with the inverse of the plane-strain (or P-wave) modulus M=(c∧2 ρ) of the formation (ρ is formation density and c is the P-wave velocity). Conversely, energy-flux conservation implies that DAS amplitude is proportional to (M∧3 ρ∧(-1) )∧(-1/4)=(c∧3 ρ)∧(-1/2). To understand the applicability of these two relationships, we perform theoretical analysis and numerical simulations, which reveals that the DAS amplitude scales with 1/M for thick layers and with 1/√(ρc∧3 ) for thin layers (relative to the wavelength).
