The quantitative data presented by Mitchell confirm that sidescan sonars operating at frequencies as low as 4.5 kHz should be able to detect fields of iron-manganese (Fe-Mn) nodules, and are clear support for our original qualitative interpretation. We stress that our original paper (Scanlon and Masson, 1992) was entirely qualitative. No claims were made for the efficiency of GLORIA in assessing nodule abundance within a particular nodule field; indeed, a specific conclusion was that higher resolution sonar, photography and sampling might all be required to fully assess a field's economic value. We disagree with the statement that GLORIA is "potentially unreliable for mapping Fe-Mn nodule fields" as suggested by Mitchell. At all frequencies analysed by Weydert (1990), all nodule abundances between 20 and 80% show a backscatter level significantly above that of the background sediments (see Mitchell"s figure 1). This would generate a recognisable signal on a sonar record, and thus the sonar would be capable of mapping the field. We accept that, at low frequency, it may be difficult to relate backscatter character to nodule abundance within a field, but this will not prevent mapping of the limits of the field.

To allow comparison of our data with that of Mitchell, we have calculated the acoustic contrast between the interpreted nodule field and the surrounding area of sediment covered seafloor north of the Puerto Rico Trench. Relative backscatter was calculated, from the unprocessed sonar data, along sections parallel to the sonar vehicle track, to eliminate range dependent backscatter variation and system generated artifacts. Appropriate corrections were made for the non-linear signal compression in the recording signal. The increase in acoustic backscatter over our nodule field, relative to sedimented seafloor, is +11 dB in the near and medium range of the sonar field at the GLORIA frequency of 6.5 kHz (Fig. 1). An even greater contrast occurs in the far range, because, at very low grazing angles, the backscatter from sediments decreases much more dramatically than that from nodules.

Estimates of nodule coverage from seabed photographs (5-25%) exist for only a few sites within the area we interpret as a Fe-Mn nodule field. If the estimate is typical of the whole field, then our measured value of +11 dB for the backscatter contrast between sediments and nodules is considerably higher than the value of about +5 dB predicted by Mitchell's calculations for this degree of nodule coverage. The observed contrast could be explained, as a problem of our interpretation, if some geological factor other than Fe-Mn nodules were contributing to the overall backscatter signal within the nodule field. This was discussed in our original paper, and was considered unlikely. Areas of higher nodule coverage could also exist, but the remarkably uniform backscatter of the whole nodule field argues against this.

An alternative explanation is that the real seabed geology may not precisely fit the theory. Both Weydert (1990) and Mitchell note the anomalous backscatter data obtained at 9 kHz in Weydert's original experiment. Such anomalous backscatter could also occur at 6.5 kHz. In addition, Mitchell assumes that the nodules can be considered as individual spheres, building on Weydert (1990) who noted that, in his study area, nodule distribution on the seabed was not random and that nodules were usually separated from one another by at least one nodule diameter. In contrast, the seabed photograph in our original paper shows groups of closely packed or even touching nodules, separated by areas of relatively clear seafloor (Scanlon and Masson, 1992, their figure 4). In this situation, the relatively large nodule groups may act as large spheres, thereby generating a stronger backscatter signal than individual nodules. This is, indirectly, predicted by Mitchell's calculations, leading him to suggest that GLORIA is most suited for mapping large nodules.

In summary, we believe that GLORIA is capable of mapping the extent of Fe-Mn nodule fields, but would repeat our original contention that higher frequency sonar surveys will be required for economic assessment of such fields. Further study of nodule fields at low frequencies is required to explain some apparently anomalous backscatter characteristics at these frequencies. GLORIA remains the best tool for reconnaissance surveys of large areas (e.g. entire Exclusive Economic Zones), even when mapping of Fe-Mn nodule fields is the main survey objective, because the speed of survey outweighs any disadvantages in terms of nodule field assessment. Such assessment can only proceed efficiently after the extent of any nodule field has been mapped.

Acknowledgements

Discussions with M. L. Somers, Q. J. Huggett and T. P. LeBas greatly assisted in the preparation of this reply. Reviews by D. Twichell and W. Danforth improved the manuscript.

Scanlon, K.M. and Masson, D.G., 1992. Fe-Mn nodule field indicated by GLORIA, north of the Puerto Rico Trench. Geo-Marine Letters 12: 208-213.

Weydert, M.M.P., 1990. Measurements of the acoustic backscatter of selected areas of the   deep seafloor and some implications for the assessment of manganese nodule resources. Journal of the Acoustical Society of America 88: 350-366.

Figure (click on image for closer view)