TF is broken when using robot_localization package with only IMU sensor and how correctly to use and configurate the package?

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Hi m using this robot_lokalization package to obtain the linear velocity. As only sensor input in my case is IMU. I configure the parameters and set the frames as follow

map_frame: map             
odom_frame: odom         
base_link_frame: /thrbot/base_link 
world_frame: odom

The launch file is the following one

<launch>
  <node pkg="robot_localization" type="ekf_localization_node" name="ekf_se" clear_params="true">
    <rosparam command="load" file="$(find robot_localization)/params/ekf_template.yaml" />
    <param name="odom_used" value="false"/>
  </node>

  <node pkg="tf" type="static_transform_publisher" name="base_link_to_imu" args="0 0 0 0 0 0  /thrbot/base_link /thrbot/imu_link  100" />

</launch>

The robot base_link frame is thrbot/base_link and the IMU one is thrbot/imu_link

Here is the photo of the TF without using the package but when launch the package with the above launch file the TF is not correct anymore as can see here .

To fix the TF i change the configure the parameters in ekf_template.yaml as follow

odom_frame: odom          
base_link_frame: /thrbot/base_footprint  
world_frame: odom

and the launch file to follow

<launch>
  <node pkg="robot_localization" type="ekf_localization_node" name="ekf_se" clear_params="true">
    <rosparam command="load" file="$(find robot_localization)/params/ekf_template.yaml" />
    <param name="frame_id" value="odom"/>
  </node>

  <node pkg="tf" type="static_transform_publisher" name="base_link_to_odom" args="0 0 0 0 0 0 /odom /thrbot/base_footprint 100" />

</launch>

Now the TF looks good as can see Than following parameters are configure in ekf_template.yaml:

imu0: /thrbot/imu
imu0_config: [false, false, false,
              true, true, true,
              false, false, false,
              true,  true,  true,
              true,  true,  true]

imu0_remove_gravitational_acceleration: true
imu0_nodelay: false
imu0_differential: false
imu0_relative: true
imu0_queue_size: 5
imu0_pose_rejection_threshold: 0.8                
imu0_twist_rejection_threshold: 0.8                
imu0_linear_acceleration_rejection_threshold: 0.8

My question is now is this correct configuration for underwater UUV with only IMU sensor input to get linear velocity(regardless of the accuracy and the accumulated error)?