FIG. 2. Geometrical configuration of the heater probe and temperature probe of a dual-probe heat-pulse sensor. The arbitrary point (x,y) represents the point at which the Fréchet kernel F(x,y,t) is calculated. The heater and temperature probes are separated by distance a, with the heater probe at (x,y) = (–a/2,0) and the temperature probe at (x,y) = (a/2,0). The scalar quantities r₁ = [(x + a/2)² + y²] and r₂ = [(x – a/2)² + y²] give the distance from the point (x,y) to the heater probe and temperature probe, respectively.