Sweep frequency response analysis

Hi guys, I am modeling a three layer pavement: asphalt, subbase and subgrade. I am doing a frequency response analysis to the model however with a 20m subgrade thickness, I get 2 peaks of resonant frequency. My concern is only the second peak of the graph, what should I do to remove the first peak? By limiting the frequency range eliminates the first peak however I need the whole frequency sweep data so this approach is not possible. Thank you.