A sensor unit support structure for supporting a sensor unit in a manhole , the support structure comprising: an elongate beam adjustable along its length to span a width of the manhole , and support the sensor unit within the manhole , the elongate beam having a first end and a second end opposing the first end ; first and second mounting brackets associated with the first and second ends of the elongated beam , respectively, each mounting bracket being configured to fasten the elongate beam to an inner surface of the manhole , and wherein each mounting bracket comprises a fixing plate defining one or more apertures configured to receive mechanical fasteners to fasten the respective mounting bracket to the inner surface of the manhole ; and a hanger to couple the sensor unit to the elongate beam , the hanger comprising a first coupling portion at a first end of the hanger configured to couple the hanger to the elongate beam , and a second coupling portion at a second end of the hanger configured to couple the hanger to the sensor unit , wherein the first coupling portion comprises an adjustable clamp configured to allow the hanger to be slidable along the elongate beam , and configured to selectively clamp the elongate beam to fix the hanger at a selected position relative to the elongate beam , characterized in that the clamp comprises a hook having two arms configured to extend over and around part of the elongate beam , a clamping member , and a tightening mechanism configured to clamp the elongate beam between the clamping member and the hook , and wherein the second coupling portion comprises an adjustable swivel-joint configured to allow the sensor unit to be rotated with respect to the hanger , and configured to be selectively tightened to fix the sensor unit at a selected angle relative to the hanger . an elongate beam adjustable along its length to span a width of the manhole , and support the sensor unit within the manhole , the elongate beam having a first end and a second end opposing the first end ; first and second mounting brackets associated with the first and second ends of the elongated beam , respectively, each mounting bracket being configured to fasten the elongate beam to an inner surface of the manhole , and wherein each mounting bracket comprises a fixing plate defining one or more apertures configured to receive mechanical fasteners to fasten the respective mounting bracket to the inner surface of the manhole ; and a hanger to couple the sensor unit to the elongate beam , the hanger comprising a first coupling portion at a first end of the hanger configured to couple the hanger to the elongate beam , and a second coupling portion at a second end of the hanger configured to couple the hanger to the sensor unit , wherein the first coupling portion comprises an adjustable clamp configured to allow the hanger to be slidable along the elongate beam , and configured to selectively clamp the elongate beam to fix the hanger at a selected position relative to the elongate beam , characterized in that the clamp comprises a hook having two arms configured to extend over and around part of the elongate beam , a clamping member , and a tightening mechanism configured to clamp the elongate beam between the clamping member and the hook , and wherein the second coupling portion comprises an adjustable swivel-joint configured to allow the sensor unit to be rotated with respect to the hanger , and configured to be selectively tightened to fix the sensor unit at a selected angle relative to the hanger . <EOS>