A computer implemented method for determining impulse response [u s (t)] and reflection response [r(t)] of a plane acoustic reflector at zero offset, due to a point acoustic source by varying parameters consisting of reference wave speed about which speed perturbation takes place (c), change in wave speed across a reflector (Δc), plane-wave-reflection coefficient of a reflector for normal incidence, depth to the reflector from the source/receiver point (h) and source time function s(t), employing Fourier method, wherein perturbation parameter is substituted in terms of reference wave speed and change in wave speed at an infinite plane interface across which the wave speed changes from the reference wave speed to a sum of the reference wave speed and the change in wave speed, followed by removing the restrictions of Born approximation, the method being useful for seismological applications, the method comprising the steps of: i) inputting distance of the reflector (h), reference wave speed (c), change in wave speed (Δc), source time function s(t) and truncation time of the impulse response of a reflector in equation A; u s t = R 8 πh δ t − τ − 2 τ 2 H t − τ t 3 ii) obtaining impulse response u s ( t) of a plane acoustic reflector at zero offset due to a point acoustic source according to u s t = R 8 πh δ t − τ − 2 τ 2 H t − τ t 3 wherein: R = Δc/ (2c+Δc); t = observation time that begins at the onset of the source; δ(t) = Dirac delta function; c = Reference wave speed; Δc = change in wave speed; h = shortest distance between the source and the reflector; τ = 2 h / c; H= Heaviside function, denoted by H (t - τ) and is defined as H t − τ = 1, t > τ = 0, t < T iii) obtaining reflection response [r(t)] of a plane acoustic reflector at zero offset due to a point acoustic source described by source time function s(t), from the expression as obtained in Equation B: r t = R 8 πh s t − 2 τ 2. s t * H t − τ t 3 wherein: R = Δc / 2 c + Δc; τ = 2h/c; and * denotes convolution; H= Heaviside function; t = observation time that begins at the onset of the source; s(t) = source time function; c = Reference wave speed; Δc = change in wave speed; and h = shortest distance between the source and the reflector. i) inputting distance of the reflector (h), reference wave speed (c), change in wave speed (Δc), source time function s(t) and truncation time of the impulse response of a reflector in equation A; u s t = R 8 πh δ t − τ − 2 τ 2 H t − τ t 3 ii) obtaining impulse response u s ( t) of a plane acoustic reflector at zero offset due to a point acoustic source according to u s t = R 8 πh δ t − τ − 2 τ 2 H t − τ t 3 wherein: R = Δc/ (2c+Δc); t = observation time that begins at the onset of the source; δ(t) = Dirac delta function; c = Reference wave speed; Δc = change in wave speed; h = shortest distance between the source and the reflector; τ = 2 h / c; H= Heaviside function, denoted by H (t - τ) and is defined as H t − τ = 1, t > τ = 0, t < T R = Δc/ (2c+Δc); t = observation time that begins at the onset of the source; δ(t) = Dirac delta function; c = Reference wave speed; Δc = change in wave speed; h = shortest distance between the source and the reflector; τ = 2 h / c; H= Heaviside function, denoted by H (t - τ) and is defined as H t − τ = 1, t > τ = 0, t < T iii) obtaining reflection response [r(t)] of a plane acoustic reflector at zero offset due to a point acoustic source described by source time function s(t), from the expression as obtained in Equation B: r t = R 8 πh s t − 2 τ 2. s t * H t − τ t 3 wherein: R = Δc / 2 c + Δc; τ = 2h/c; and * denotes convolution; H= Heaviside function; t = observation time that begins at the onset of the source; s(t) = source time function; c = Reference wave speed; Δc = change in wave speed; and h = shortest distance between the source and the reflector. τ = 2h/c; and * denotes convolution; H= Heaviside function; t = observation time that begins at the onset of the source; s(t) = source time function; c = Reference wave speed; Δc = change in wave speed; and h = shortest distance between the source and the reflector.