1. A method for inverting measured seismic data in frequency domain to infer a model of a physical property representative of a subsurface region, comprising: in iterative inversion of the measured data performed using a computer to estimate the model, after an objective function has been computed measuring difference between model-simulated data u and the measured data, and after a gradient of the objective function with respect to parameters of the model has been computed to establish a search direction for a model update, then performing a line search along the search direction to determine the model update, said line search including, (a) computing a model update by perturbing a current model in the search direction, and then estimating a corresponding perturbation in the model-simulated data u using perturbation theory that includes solving a frequency-domain wave propagation equation written in the form Au=−s, where s is a vector representing frequency-dependent source amplitude and phase, respectively, and A is a matrix that depends on the model, (b) re-computing the objective function using the perturbed model-simulated data, and (c) repeating (a)-(c) a plurality of times with different model perturbations, and selecting the model perturbation that results in a minimum objective function; generating an updated physical property model with the model perturbation that results in the minimum objective function; and prospecting for and producing hydrocarbons in a subsurface region, wherein a hydrocarbon exploration decision is based on the updated physical property model.