Preliminary numerical simulation for the development of a seismic camera

The acoustic camera is an established and highly effective device for localising acoustic sources by the use of a number of simultaneously acquired signals from an array of pressure sensors (microphones). The acoustic camera essentially provides for a highly directional sensor in which the signals arriving from the noise source in the steered main beam of the array are highly amplified relative to the background noise, which arrives at the camera from all directions outside the main beam and is therefore suppressed. The underlying principle of the acoustic camera is the beamforming data processing method which is widely applied in sensor array configurations and acts as a spatial filtering operation. The long term vision of the team is to develop an analogous device, termed here seismic camera, which allow to locate the direction of the noise sources generated from water leaks. This is an array of 3-axis geophones distributed on the ground in the vicinity of the suspected leak to localise and quantify water leaks with significantly greater accuracy and reliability than conventional methods that use just two sensors either side of the leak. The seismic camera differs from the acoustic camera since the array of data is vectorial (three axis geophones provide velocities instead of a scalar pressure field), two or more wave types (compressional, shear and surface waves) propagate simultaneously and the soil properties varies greatly with location, type and condition. In this preliminary feasibility study a time-domain solution calculated from the analytical elasticity equations is considered to generate the numerical data. The wave field is composed by spherical compressional waves radiating directly from the leak which is modelled here as a spherical cavity of radius a. The obtained numerical data is elaborated in order to look at the implementation of the Delay-and-Sum beamforming algorithm for the detection of the leak. Finally, the effects of wave reflection caused by a free surface and the sensor direction of measurement are discussed and it is shown that the beamforming algorithm works better and more precisely in infinity medium models, although the half-space model still presents satisfactory result.