def vsh_gr(gr, sand_line, shale_line):
    """
    Generate Vshale values from gamma ray values and sand / shale cutoffs
    """
    
    shale_line = float(shale_line)
    sand_line = float(sand_line)
    
    vsh_grindex = (gr - sand_line)/(shale_line - sand_line)
    
    # clip values to range 0 - 1
    vsh_grindex[vsh_grindex > 1] = 1
    vsh_grindex[vsh_grindex < 0] = 0
        
    return vsh_grindex