A vibroseis source system comprising a base plate component, and a reaction mass component which is mechanically isolated from the base plate component via an actuating piston (2010, 2011) internal to the reaction mass component, wherein the actuating piston (2010, 2011) is configured to produce vertical displacement of the reaction mass component during vibroseis operation when the base plate component is coupled against a surface of earth, the vibroseis source system comprising one or more dampers, each damper being a hydraulic damper or a pneumatic damper, each damper having at least one piston rod connection and a cylinder connection, each damper being mechanically connected between the reaction mass component and a frame component of a vehicle on which the vibroseis source system is carried using the at least one piston rod connection and the cylinder connection and providing non-rigid mechanical coupling between the reaction mass component and the vehicle, wherein the one or more dampers are further configured to selectively provide non-rigid coupling between the reaction mass component and the frame component of the vehicle, wherein the vibroseis source system further comprises a control system configured to, during a period of vibration output including at least a low frequency vibration output, selectively engage the one or more dampers, wherein the vibroseis source system further comprises a pair of lift cylinders (1010, 1011) for lifting the base plate component to allow the vehicle to move and for pressing the base plate component against ground during operation, wherein the vibroseis source system further comprises air springs (1090, 1091, 1092, 1093) for isolating vibrations of the base plate component from the frame component, characterized in that the control system is further configured to measure air spring dynamic force of the vibroseis source system, the air spring dynamic force being a dynamic force that passes through the air springs (1090, 1091, 1092, 1093) and adds or removes force applied to the ground, and configured to factor the measured air spring dynamic force into a calculation of a weighted sum to estimate a ground force output on the base plate component. a base plate component, and a reaction mass component which is mechanically isolated from the base plate component via an actuating piston (2010, 2011) internal to the reaction mass component, wherein the actuating piston (2010, 2011) is configured to produce vertical displacement of the reaction mass component during vibroseis operation when the base plate component is coupled against a surface of earth, the vibroseis source system comprising one or more dampers, each damper being a hydraulic damper or a pneumatic damper, each damper having at least one piston rod connection and a cylinder connection, each damper being mechanically connected between the reaction mass component and a frame component of a vehicle on which the vibroseis source system is carried using the at least one piston rod connection and the cylinder connection and providing non-rigid mechanical coupling between the reaction mass component and the vehicle, wherein the one or more dampers are further configured to selectively provide non-rigid coupling between the reaction mass component and the frame component of the vehicle, wherein the vibroseis source system further comprises a control system configured to, during a period of vibration output including at least a low frequency vibration output, selectively engage the one or more dampers, wherein the vibroseis source system further comprises a pair of lift cylinders (1010, 1011) for lifting the base plate component to allow the vehicle to move and for pressing the base plate component against ground during operation, wherein the vibroseis source system further comprises air springs (1090, 1091, 1092, 1093) for isolating vibrations of the base plate component from the frame component, characterized in that the control system is further configured to measure air spring dynamic force of the vibroseis source system, the air spring dynamic force being a dynamic force that passes through the air springs (1090, 1091, 1092, 1093) and adds or removes force applied to the ground, and configured to factor the measured air spring dynamic force into a calculation of a weighted sum to estimate a ground force output on the base plate component.