1. A method for performing seismic surveys, comprising: using a seismic vibrator that generates a sweep signal with enhanced low frequency content by generating a combination of a linear and nonlinear sweep, and wherein the nonlinear sweep S nonl (t) as a function of time (t) is calculated by a predetermined algorithm, wherein the predetermined algorithm comprises the formula: s nonl ⁡ ( t ) = a max ⁢ [ ( f disp n + 1 - f 0 n + 1 ) ⁢ t T nonl + f 0 n + 1 ] 2 n + 1 f disp 2 × sin ( 2 ⁢  ⁢ T nonl ⁡ ( ( f disp n + 1 - f 0 n + 1 ) ⁢ t T nonl + f 0 n + 1 ) 1 n + 1 + 1 ( f disp n + 1 - f 0 n + 1 ) ⁢ ( 1 n + 1 + 1 ) ) ⁢ 0 ≤ t ≤ T nonl , wherein: (n≠−2, n≠−1), f disp >f 0 , f 0 is on the order of 1-3 Hz f disp is a displacement frequency that is greater than f 0 , a max is the maximum amplitude of the vibrator, f is frequency, T nonl is the duration of the nonlinear part of the sweep, and the exponent n is between −4 to 4.