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 (128, 132) associated with the first and second ends (130, 34) of the elongated beam, respectively, each mounting bracket (128, 132) being configured to fasten the elongate beam to an inner surface of the manhole, and wherein each mounting bracket (128, 132) comprises a fixing plate defining one or more apertures configured to receive mechanical fasteners to fasten the respective mounting bracket (128, 132) 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 (128, 132) associated with the first and second ends (130, 34) of the elongated beam, respectively, each mounting bracket (128, 132) being configured to fasten the elongate beam to an inner surface of the manhole, and wherein each mounting bracket (128, 132) comprises a fixing plate defining one or more apertures configured to receive mechanical fasteners to fasten the respective mounting bracket (128, 132) 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.