general seismic and bathymetric survey of parts of rabaul

Cruise Report No. 69

of PE/PN.4

18 October 1982

GENERAL SEISMIC AND BATHYMETRIC SURVEY

OF PARTS OF RABAUL HARBOUR,

PAPUA NEW GUINEA

11 June 1982

by

D.L. Tiffin and J.G. Vedder

Prepared for:

COMMITTEE OF COORDINATION OF

JOINT PROSPECTING FOR MINERAL

RESOURCES IN SOUTH PACIFIC

OFFSHORE AREAS (CCOP/SOPAC)

WORK PROGRAMME CCSP-1/PN.4

As a contribution by:

UNDP Project Office

Project RAS/81/102

Investigation of

Mineral Potential of

t h e South Pacific

GENERAL SEISMIC AND BATHYMETRIC SURVEY OF PARTS OF RABAUL HARBOUR, PAPUA NEW GUINEA

11 June 1982

Cruise Report No. 69 of PE/PN.4

INTRODUCTION

This work was undertaken as par t of t h e CCOP/SOPAC Work Programme CCSP/PN.4 (Inshore and Nearshore Surveys related to Harbour Development and

Coastal Management). I t was accomplished as an adjunct t o a major hydrocarbon survey in Solomon Island waters by CCOP/SOPAC in cooperation with United States , Australia and New Zealand.

The ci ty of Rabaul, Papua New Guinea, is situated in a huge ancient caldera on t h e north coast of New Britain. The caldera is open to t h e sea on one side, forming

an excellent sheltered harbour. Several smaller ac t ive volcanoes are now si tuated

around t h e rim of t h e ancient caldera, and eruptions from these have been recorded

in historical time. During one eruption in 1937, 500 lives were lost. Rabaul now

has a population of close t o 25,000 people: a new eruption of any of t h e surrounding

volcanoes could prove t o be a major disaster for t h e c i ty and its people. The Rabaul

Volcanological Observatory (RVO) overlooks the harbour of Rabaul, and monitors seismicity and related volcanological events around Rabaul and elsewhere in northern

PNG with a network of seismic stations at s t ra teg ic places.

Over t h e last few years the frequency of seismic events noted on t h e Rabaul seismic network has increased several-fold. A t the same time, an area of gent le up-

l i f t has been noted and, since 1973, measured in some detai l for deformation changes by means of optical levelling, tilt-meters, and repeat readings on a network of gravity s tat ions aimed at monitoring secular changes. All measurements have shown consistent trends which indicate uplift of t h e area around Matupit Harbour. The largest change

is almost I meter on t h e south end of Matupit Island, but t h e pa t te rn of onshore

measurements suggest tha t even greater uplift may have occurred southeast of Matupit Island, under Blanche Bay near t h e ent rance t o Matupit Harbour. The maximum local

seismic act ivi ty is centered in the same general area.

The Director of the Rabaul Volcanological Observatory requested through his government tha t , if possible, CCOP/SOPAC, using t h e U.S. Geological Survey vessel

S.P. LEE, conduct bathymetric and other surveys during an intended four day port

s top in Rabaul.

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CRUISE OBJECTIVES

Originally, a detailed bathymetric survey was requested which was anticipated

t o outline any submarine uplift in the area southeast of Matupit Island. However, the S.P. LEE was not properly equipped to perform this type of detailed work, and is too large a vessel (1300 tons) to manoeuvre in tight quarters, or t o go into shallow

water. The proposed survey was therefore changed to provide both reconnaissance bathymetric da ta as well as a thorough testing of t h e performance of the Rabaul

seismic network using the airgun seismic sources of t h e survey ship. The latter would provide information on differences of sensitivities at individual stations, a factor

known to exist but without qualitative measurement. In addition, a sonobuoy refraction profile was desirable to obtain refract ion velocities in t h e upper crust of t h e

marine areas.

The basic objectives were therefore: a) to obtain bathymetric da ta t o determine if possible, where uplift is taking place under the bay; b) to obtain geological

d a t a to assist in determining the geological s t ructure; c) to test t h e response of t h e Rabaul seismic network t o artificial seismic waves from various locations in t h e

harbour; d) to check t h e accuracy of t h e locations provided by the seismic network; and e) to obtain seismic refraction velocities in t h e area north of Gazelle Peninsula

EQUIPMENT AND FACILITIES

The geological research vessel S.P. LEE is an American flag ship 208ft. in overall length, and 1300 registered tons, with a crew of 22 and scientific complement

of 18. The main capability of the ship is in multichannel seismic reflection surveys, for which i t is equipped with a 48-channel, 2.4km long seismic hydrophone streamer

and a tunable airgun array composed of f ive large air guns totalling 1300 cubic inch capacity. The ship is capable of performing bottom sampling operations such as coring

and dredging, and bottom photography.

The ship is also equipped with a 12kHz echo sounder, a 3.5kHz high resolution

sub-bottom profiler, and a powerful Uniboom sub-bottom profiling array capable of considerable sub-bottom penetration. A single channel air gun seismic system can

also be deployed, and sonobuoys a r e routinely used for collection of velocity data. Navigational facili t ies include two relat ive motion radars, Omega, Satel l i te Navigation receivers, Loran, Doppler Speed Logs and Gyro-compass heading information, all in- putting to an on-line computer navigational system which updates t h e trackline con-

tinuously and displays navigation information on monitors throughout t h e ship.

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Only selected equipment was operated during t he Rabaul Harbour Survey. As bathymetric information was a requirement, t h e 12kHz echo sounding system was

used. In addition, t he Uniboom system was deployed t o obtain sub-bottom information. EPC 19" graphic recorders were used for t he display. The sonobuoy recording system, including both t ape and graphic recorders, was operated over one sonobuoy profile

with one airgun of t he largest capacity (500 cu.in.) towed as t he sound source. Because of the shallow water, dangers t o navigation, and t he small survey area which required

a large number of turns, t he navigational functions normally carried out in t h e labor-

atory were placed on the bridge. A navigational officer manned each radar while

a third officer plotted positions on the char t a t 5 minute intervals. The captain conned t he ship from the bridge. The t ight quarters for a ship of this size precluded towing

much equipment over the side which would hamper her manouvreability or prevent her from going as tern if need be. The over-the-side equipment was therefore limited

t o one airgun off t he s tern and two short hydrophone arrays for t he Uniboom towed off the starboard side well forward of t he propellors.

A smoked cylinder recorder and a SSB radio were brought from RVO to record seismic shot-times and t o communicate directly from the ship to Observatory s taff manning t he seismic monitor network at RVO.

METHOD

Correspondence t o init iate this survey had taken place between RVO and CCOP/SOPAC a f t e r t he R/V S.P. LEE had begun its South Pacific survey programme. There was therefore no opportunity to include this work in t he ship's schedule, nor

was there t i m e t o plan a more thorough survey with equipment other than what was already on board. Since Dr H.G. Greene, USGS cruise co-ordinator, was already

scheduled t o visit Rabaul a few weeks before t he arrival of t he S.P. LEE, he was requested t o discuss t h e proposed survey with RVO in view of the capabilities and disadvantages of t he LEE for this type of work. During his visit, reconnaissance

t rack lines were proposed in Blanche Bay and Gree t (or Matupit) Harbour adjacent

t o Matupit Island to fulfill in par t t he objectives of t he original request.

A detailed bathymetric char t of Gree t Harbour was available from soundings carried ou t in 1962 by the Royal Australian Navy. I t was considered t h a t comparison

of new soundings in the harbour with values from the 1962 cha r t would help to determine t he ex ten t of uplift in t he harbour. Co-ordinates for t he proposed lines were

radioed to t h e ship in t he Solomon Islands where they were plotted on U.S. naviga-

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tional char t 82192 "Blanche Bay and Approaches". Other arrangements for t he survey were also completed by radio, and a rendezvous with a PNG harbour c r a f t was scheduled for 0600hrs June 11, to transfer RVO personnel and equipment to S.P. LEE.

At the appointed hour on June 11, four scientif ic personnel, th ree from RVO and one from Geological Survey of PNG, Port Moresby, joined the LEE off Cape Gazelle, a f t e r which t he survey equipment (one air gun and Uniboom hydrophone

streamers) was deployed over the side and the survey commenced. A t the end of t h e survey, a 'bar check' was made t o calibrate t he echo sounders using a measured wire lowered to t h e sea floor in about 16 fathoms of water.

SERVICES AND FACILITIES PROVIDED

The Government of Papua New Guinea, as a member of CCOP/SOPAC, was

a signatory t o t he Memorandum of Understanding governing t he conduct of t he surveys of t he S.P. LEE. The government kindly waived a l l port f ees for t he ship, provided

customs, immigration and pilotage services at no charge, and exempted t he vessel from customs duty on approximately 32,000 gallons of diesel fuel taken on board at Rabaul. They also provided a national f lag t o be used by the ship in PNG waters, and gave generous support in arranging provisioning and aiding other ship's business.

The s taff of RVO provided transportation where possible and arranged field t r ips to RVO and other areas of geological interest. Staff from RVO and PNG Geological

Survey participated in the survey. This support is greatly appreciated.

PRELIMINARY RESULTS

The tracklines covered, and five minute position fixes by simultaneous radar readings, are shown in Figures 1 and 2. The location of t he sonobuoy refraction profile is also indicated. Table 1 is a listing of t he five minute positions as taken from

the char t (82192).

Good quality bathymetric d a t a was obtained over t he whole survey. However,

several factors make determination of an uplift a r ea from the echo soundings very difficult. These factors were outlined t o RVO in a le t t e r prior t o t he survey

(NR/SOPAC/TECHSEC-345, 8 April 1982) which pointed out t h a t comparing two surveys t o determine small changes in water depth necessitates great ca r e in ensuring accuracy in navigation and in depth corrections in both surveys. Since much of t he

bottom over t he area covered by this survey is irregular, a small error (+_20m) in navigation could result in an error in depth of +_2m or more. The radar navigation

5

employed could easily have errors of this magnitude or greater , and additional large

errors could occur from the scale of t h e char t used for plotting (1:25,000). A t t h a t scale, 20 metres is less than .8 millimetres long, hardly larger than a pencil mark. Additional errors may accrue from errors in depth recorders and other factors,

including water velocities assumed. Nevertheless, i t is worthwhile to check as carefully as possible with previous da ta in t h e event that large changes in depth (+_10m)

may have occurred which could be documented with some reliability. For this purpose, arrangements were made to have tidal records available from Rabaul Harbour so t h a t

t h e tidal corrections could be applied t o t h e present survey data. The comparison of this da ta to previous surveys will be done by RVO staff in Rabaul where they

have access to t h e older survey data. The tidal da ta are shown in Figure 3 and Table 2.

The 'bar check' at the end of t h e survey showed a discrepancy of four metres

between t h e soundings from t h e wire and those from t h e echo sounder. Since t h e d ra f t of t h e vessel is this amount, t h e echo sounders were suspected to be unadjusted t o compensate for ship's draft, although t h e technical s taff thought they had been.

Upon further investigation in t h e electronic equipment, it was confirmed t h a t no transducer depth compensation had been applied, so t h a t all soundings must be increased by four metres to ref lec t t h e t rue water depth.

The high resolution data, also of excellent quality, was photographed by pola- roid camera and the photographs were l e f t with RVO. The seismic results show t h a t

large areas of thick sediments underlay parts of t h e bay, particularly under Matupit Harbour and southern Karavia Bay. The sediments under Karavia Bay are overlain

by possible pyroclastic or ash deposits which have l i t t l e or no stratification on t h e seismic records. These deposits appear t o extend onto the southern shoreline near t h e site of one of the seismic monitor stations. A further report on t h e geological

aspects of the survey will be provided later by H.G. Greene, USGS, and D. Tiffin.

The frequency and amplitude response of t h e seismometers employed in t h e Rabaul seismic network was shown t o b e adequate for most of the locations. Even

t h e weak signals (compared t o seismic ea r th tremors) generated by t h e airgun, which

has a higher frequency output than naturally generated seismic waves and therefore

is not centered in t h e response band of t h e seismometers, were picked up by those seismometers sited on bedrock. However, one seismometer s tat ion at Raluana Point

received t h e airgun impulse weakly, or not at all, even though t h e ship passed within 1km of this site. The stat ion is situated upon the sedimentary units noted on t h e

6

high resolution profiles t o underlay the southern shore of Karavia Bay. Evidently t h e sediments a t t enua te t h e seismic signal to a considerable extent compared to

bedrock.

I t is perhaps worth noting t h a t t h e seismic network response increased markedly

when t h e ship was in Matupit Harbour. This was apparently not only due to t h e proximity of t h e seismometer sites in this area, but possibly also due to increased coupling of t h e seismic pulse t o the underlying sea floor and to t h e greater reverberation usu-

ally generated in shallow water.

A calibration of t h e epicentre location programme of RVO using t h e ship's position at different areas within t h e seismic network will be made by RVO staff

using their computer programme. A composite f i lm record showing response of each stat ion throughout t h e course of t h e survey has been examined by RVO staff t o de ter-

mine sections of t h e record best suited for this purpose. I t should be noted t h a t t h e seismic source is about 60 metres astern of the navigation points determined from

t h e bridge radars, and approximately 10 metres below t h e water surface. These corrections should be applied.

The final objective of t h e work was accomplished with a sonobuoy profile about 15km in length north of Gazelle Peninsula (Figure 2). The sonobuoy da ta were pro- cessed by USGS staff a t Menlo Park, California, where slope corrections were applied.

Final refraction velocities and layer thicknesses are given in Figure 4.

CONCLUSIONS AND RECOMMENDATIONS

In order to provide more complete information to delineate the offshore area of uplift, a detailed survey will be necessary with carefully controlled positioning and bathymetry using shore based navigation. This work could be performed by

CCOP/SOPAC with t h e assistance of t h e government of Papua New Guinea in pro- viding a suitable small vessel, personnel to assist in t h e survey, properly prepared

shore sites and other facili t ies t o ensure tha t the accuracy required in vertical and horizontal control is attained. In t h e meantime, it is recommended t h a t RVO assess

the bathymetric results obtained in this survey by comparing with previous d a t a to determine if any area shows anomolous changes in depth not accountable t o survey inaccuracies. If fur ther surveys of this nature are desirable, they should be requested

of CCOP/SOPAC by Papua New Guinea.

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The siting of seismometer stations of the Rabaul network showing low or no response t o low level seismic impulses should be changed if possible to s i tes close

to, or preferably upon, bedrock.

I t should be possible t o improve epicentral location of events within t h e Rabaul seismic network by incorporating refraction velocities for t he marine refraction layers into t he computer programme used to calculate these epicentres.

LIST OF FIGURES

Figure 1: Track lines of survey vessel R/V S.P. LEE in Blanche Bay and Matupit Harbour, Rabaul.

Figure 2: Track line in to Rabaul Harbour showing location of sonobuoy profile.

Figure 3: Tidal curve for Rabaul Main Wharf 0500hrs to 1500hrs, June 11, 1982.

Figure 4: Final refraction velocities and layer thicknesses north of Cape Gazelle.

LIST OF TABLES

Table 1: List of Navigation Fixes for Survey of Blanche Bay and Matupit Harbour

Table 2: Da ta from Tidal Gauge at Main Wharf, Rabaul.

Table 1: List of Navigation Fixes for Survey of

Blanche Bay and Matupit Harbour

~ Latitude Longitude

0730 40 17.78' 1520 19.84'

0740 40 17.68' 1520 19.00'

0746 40 17.56' 1520 18.68'

0755 40 17.40' 1520 17.67'

0800 40 17.23' 1520 17.22'

0810 40 17.03' 1520 16.35'

0820 40 16.74' 1520 15.48'

0825 40 16.60' 1520 14.98'

0830 40 16.50' 1520 14.61'

0835 40 16.40' 1520 14.14'

0840 40 16.28' 1520 13.80'

0845 40 16.16' 1520 13.28'

0850 40 16.03' 1520 12.84'

0855 40 15.95 1520 12.35'

0900 40 15.84' 1520 11.91'

0905 40 15.76' 1520 11.46'

0912 40 15.92' 1520 10.98'

0915 40 16.22' 1.520 11.00'

0920 40 16.58' 1520 11.04'

0925 40 17.05' 1520 11.04'

0930 40 17.47' 1520 11.11'

0940 40 18.21' 1520 11.34'

0950 40 17.75' 1520 11.86'

0955 40 17.52' 1520 12.18'

1000 40 17.26' 1520 12.55'

1010 40 16.64' 1520 13.10'

1015 40 16.40' 1520 13.50'

1020 40 16.12' 1520 13.81'

1030 40 15.60' 1520 14.42'

1034 40 15.37. 1520 14.72

1038 40 15.24' 1520 14.66'

1042 40 15.12' 1520 14.33'

1045 40 15.28' 1520 14.11'

1055 40 16.23' 1520 13.65'

2

Table I (contd.)

~ Latitude Longitude

1100 40 16.68' 1520 13.39'

1105 40 17.09' 1520 13.22'

1108 40 17.08' 1520 12.95'

1115 40 16.44' 1520 12.81'

1120 40 15.86' 1520 12.70'

1125 40 15.58' 1520 12.56'

1130 40 15.30' 1520 12.49'

1137 40 14.82' 1520 12.40'

1141 40 14.56' 1520 12.40'

1144 40 14.41' 1520 12.32'

1150 40 14.23' 1520 12.04'

1152 40 14.05' 1520 12.02'

1155 40 13.93' 1520 12.08'

1158 40 14.00' 1520 12.22'

1203 40 14.34' 1520 12.28'

1208 40 14.86 1520 12.42'

1215 40 15.36' 1520 11.92'

1220 40 15.39' 1520 11.56'

1226 40 15.15' 1520 11.06'

1230 40 14.92' 1520 10.68'

1237 40 14.54' 1520 09.94'

1241 40 14.34' 1520 09.84'

1248 40 13.52' 1520 10.16'

1357 40 12.71' 1520 10.35'.

Table 2: Data from Tidal Gauge at Main Wharf

Data fran digital tide gauge at nein wharf Rabaul for the {:ericxi 0500-1500 LT11 June 1982.

0500 23.61 1000 22.09

0515 23.54 1015 21.99

0530 23.47 1030 21.92

0545 23.38 1045 22.00

0600 23.36 1100 21.87

23.26 21.83

23.20 21.73

23.10 21.80

0700 23.12 1200 21.68

22.93 21.68

22.86 21.59

22.77 21.60

0800 22.70 1300 21.53

22.58 21.57

22.51 21.56

22.46 21.53

0900 22.34 1400 21.55

22.29 21.51

22.20 21.53

22.21 21.48

1000 22.09 .1500 21.60

~ : Tide values are in feet I and the units and decim:il part of eachvalue corresponds with values obtainable fran the tide staff andfran a stevens analogue tide gauge which are both also at the neinwharf at Rabaul.

APPENDIX I

Personnel Part icipating

Scientific Personnel

Jack Vedder

Don Tiffin

Larry Beyer

Donna Blackman

Terry Bruns

Guy Cochrane

Jim Colwell

Alan Cooper

Frank Coulson

Gregory Lewis

Kay Kinoshita

Loren Kroenke

Michael Marlow

Raymond Wood

P. Lowenstein

P. Hill

P. d e Saint Ours

Benjamin Talai

Co-Chief Scientist, USGS, Menlo Park, California

Co-Chief Scientist, CCOP/SOPAC, Suva, Fiji

USGS Menlo Park, California




Bureau of Mineral Resources, Canberra, Australia


Solomon Islands Geological Survey, Honiara, Solomon Islands



Hawaii Insti tute of Geophysics, Honolulu Hawaii (CCOP/SOPAC)


New Zealand Geological Survey, Lower Hutt, New Zealand

Rabaul Volcanological Observatory, Rabaul

Geological Survey of Papua New Guinea, Por t Moresby



Technicians

Dave Hogg

Larry Kooker

Kevin O'Toole

Paul Wenberg

Electronics Technician, USGS Menlo Park, California

Electronics Technician, USGS Menlo Park, California

Marine Technician, USGS Menlo Park, California

Marine Technician, USGS Menlo Park, California

Ship's Master

Capt. Verne Pilgrim

and crew, R/V S.P. LEE

Rabaul Volcanological Observatory

Chris MacKee, RVO, manned t he Observatory monitors and SSB radio during t he survey.

*********

16 November 1982

Appendix II

Results of a CCOP/SOPAC - Rabaul Volcano Observatory (RVO) co-operative marine geophysical project in Blanche Bay, Rabaul, Papua New Guinea

by C McKee, P de Saint Ours, P Lowenstein, C Matupit, B Talai.

Introduction

The request by RVO for the marine geophysical work carried out in Blanche Bay by personnel of the USGS research vessel SP LEE, under the auspices of CCOP/SOPAC, was based on a number of objectives.

The initial objective was to obtain bathymetric data from a part of Rabaul caldera

which has shown uplift over the last 9 years. Harbour has been monitored in some detail since 1973 for deformation changes. techniques being applied are normal optical levelling, and a method of repeat gravity readings on a network of stations aimed at monitoring secular gravity changes. obtained, which indicate uplift of the area around Greet Harbour. change, at the southern extremity of Matupit Island, amounts t o almost 1 metre in the 9 years of measurement (McKee, 1982). The pattern of onshore measurements suggested that even greater uplift my have occurred south and southeast of Matupit Island, i.e. near the entrance to Matupi Harbour.

The area around Matupi (Greet) The

Both of these techniques have shown consistent trends in the measurements The largest

A detailed bathymetric chart of Matupi ut in 1962 by the Royal Australian Navy. new soundings in the harbour with values from the 1962 chart would determine the extent of uplift in the harbour.

Harbour was produced from soundings carried It was considered that comparison of

The second reason for requesting co-operative work with scientists aboard the SP LEE was that the "air-gun" seismic shot source on the boat provided a means of testing the performance of RVO's network of seismic stations around Blanche Bay,

and that registrations of the air-gun blasts at any time by three or more stations of the network would allow for testing of the methods of earthquake focal location used for Rabaul caldera earthquakes.

It was known that relative sensitivities of individual stations of the network were different, but only in a qualitative way. It was expected that a mobile, high

frequency seismic source of known intensity and location would assist in a better assessment of the relative sensitivities of the stations and of their frequency response. During the course of the work in Blanche Bay, radio communications

2/..

2

between RVO and the SP LEE enabled RVO personnel on the boat to be immediately informed of the response of the different stations of the seismic network.

1. Bathymetry in Blanche Bay.

A bathymetric chart (AUS 860, "Approaches to Simpson Harbour") prepared from RAN surveys in 1962, shows insufficient detail for useful comparison with traverses completed by the SP LEE in 1982. However, a detailed chart (mainly parallel north- south traverses at about 20m intervals) covering Matupi Harbour, from which part of the chart AUS 680 was prepared, is also available. This detailed chart extends to almost 1 km south of the entrance to Matupi Harbour.

Two sections of the SP LEE's track near the mouth of Matupi Harbour, entering and leaving the harbour, were selected for comparison with the detailed 1962 chart of this area. approximations were applied to the profiles taken from the 1962 chart and to the record from the SP LEE's bridge echo sounder. from the SP LEE were the only ones available at RVO. soundings can be determined from this poorly annotate3 record is qu i t e low (1 - 2 fathoms). received at RVO. m u s t be considered to be rough approximations.

However, before comparison could be made some corrections and

At the time of writing these soundings The accuracy to which

More accurate soundings were made but copies have not been Therefore the comparisons of 1962 and 1982 bathymetry made here

a) Corrections and approximations made in handling the data

(i) chart of Matupi Originally, the datum for soundings was assumed to be 8.23 feet below the centre of a RAN brass plaque set into a stone sea wall at the seaward end of Namanula Street (near Rabaul Yacht Club and Swimming Pool). However, this was believed to be too low, and the instruction printed on the chart stated that all soundings should be increased by 3.3 feet. AUS 680 is 4.93 feet below the RAN brass plaque, which shows the incorporation of this correction to the original datum. soundings of 0.5 fathoms, rather than 3.3 feet, introduces an error or 0.3 feet.

(ii) The zero level on the tide staff and the zero of the Steven's tide gauge, bath at the main wharf at Rabaul are nominally 1 foot below low water at ordinary spring tide. This tide level is stated to be 0.4 feet above the datum used for chart AUS 680, indicating that the zero level for both the tide

staff and Steven's tide gauge is 0.6 feet below the 1962 (AUS 680) datum. The digital tide gauge installed at the main wharf by personnel of the University

of Hawaii has been set so that the 20 foot level corresponds with the zero of

0.5 fathoms have been added to all soundings from the detailed 1962 Harbour in accordance with an instruction printed on the chart.

The datum given on chart

The correction made to the 1962

3/...

3

the other tide monitors at the main wharf.

The range of the tide level during the time interval covered by the two

sections of the SP LEE's track selected for study, viz. 1130-1144 LT and 1203-1220 LT on 11 June, was about 21.62 - 21.76 feet as read from the

University of Hawaii tide gauge, ie. staff. above the 1962 datum. However, because of the smallness of this difference, no tidal correction has been made to the 1982 soundings.

(iii) compensate for the ships draft of 4 metres, all soundings from the SP LEE must be increased by this amount. which results in an error of about 1 foot.

(iv) The approximations described in sections (i) - (iii) are all quite small when compared with the reading accuracy obtainable from the echo sounding record available at RVO. magnitude less than this reading accuracy.

about 1.7 feet above zero on the tide Therefore, the average tide level during this time was about 1.1 feet

Because the transducer for the SP LEE echo sounder was not adjusted to

In fact, the soundings were increased by 2 fathoms,

The approximations made are individually about an order of

b) Comparison of 1962 and 1982 bathymetry near the mouth of Matupi Harbour.

(i) The SP LEE's track at this time approximated a straight line oriented about 8' west of north, which allows for reasonable comparison with the detailed 1962 chart of Matupi Harbour. corrections were applied. The sections of the profiles inside Matupi slightly shallower depth of water in 1982, as do the sections at the shallowest part of the entrance to the harbour. profiles they begin to diverge at a point midway between the navigational fixes at 1137 and 1141, with the 1982 profile showing shallower water. not appear to be a steadily increasing divergence of the two profiles and for all of the southern halves of the profiles the difference in sea-floor elevation is 2-3 fathoms. indicates that the same parts of the sea-floor are being compared.

the difference in elevation is close to the reading accuracy appropriate to the 1982 echo sounding record studied, the consistency of the indicated sea- floor elevation change suggests it is real.

(ii) The track of the SP LEE during this time interval was more complicated, comprising three segments showing varying degress of curvature.

Ship's track 1130 - 144 LT

Figure 1 shows the two sea-floor profiles after Several aspects of these profiles are noteworthy.

Harbour show similar although

However, going southwards along these

There does

Reasonable agreement of the shapes of the profiles Although

Ship's track 1203 - 1220 LT

The first

4/....

4

segment, in the period 1203-1208 LT, followed a similar course to the northern half of the ship's track entering Matupi 1137-1144 LT. Harbour (1203-1208 LT) has a reasonable match with the 1962 topography (Fig 2), although shallower water (a few fathoms on average) is indicated from the SP LEE soundings in the area of the submerged ridge at the entrance to the harbour and in the harbpur itself. in the succeeding sections of the superimposed profiles (Fig 2). is indicated by the SP LEE soundings, a result which seems anomalous. factors which should be considered, however, are the probability that the SP LEE was following a curved course between the navigation fixes, and the possibly greater uncertainties of the 1962 soundings in this area which is at the southern extremity of the area of detailed survey.

Figure 2 shows that in the vicinities of the three navigation fixes at 1208, 1215, and 1220, the depth profiles show a fair match, but the intervening sections display large differences. southwards into deeper water, which may account, at least partially, for these differences .

Harbour ie. in the period The sea-floor profile of the part of cruise leaving Matupi

Marked differences in water depth are shown Deeper water

Two

A curving track would displace the ship

c) Summary of bathymetric study.

The results of the comparison of 1982 and 1962 soundings in the principal area of concern in Rabaul, ie. near the entrance to Matupi Harbour, are somewhat inconclusive, mainly because of uncertainties in data derived from the poorly annotated SP LEE bridge echo sounding record. more accurate bathymetric records become available at RVO, this study will be repeated.

Despite these uncertainties, comparison of 1962 bathymetry with the more reliable soundings made when the SP LEE was entering Matupi Harbour indicates a shallowing of water near the entrance to the harbour which suggests the need for further more-detailed bathymetric work in this area. Both sections of the 1982 soundings studied show slightly shallower water inside Matupi

which is consistent with known uplift (McKee, 1982) of Matupit Island and the area around Sulphur Point (see Fig 3).

If copies of the

Harbour

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2. Dynamic Response of RVO Seismic Network.

a) Frequency response (i) electronic filters were being employed separately in the two distinct groups

of seismic stations of the Rabaul network. RVO by buried telephone cables, incorporated a passive filter which reduced

higher frequencies gradually as frequency increased. The cut-off frequency was 7 Hz, and at frequencies greater than 7 Hz attenuation was 6 dB/octave.

The radio telemetry stations were fitted with a very effective, active, low

pass filter which had a sharp cut-off at about 10 Hz. than 10 Hz attenuation was about 20 dB/octave.

(ii) Response to SP LEE airgun blasts. For most parts of the Rabaul harbour traversed by the SP LEE with its airgun

operating, the frequency of the recorded blasts was 9-10 Hz (Fig 3). Essentially the same frequencies were recorded by all stations.

It is notable that when the ship was in shallow water slightly higher frequencies were recorded by some stations, usually the nearest ones. Specifically, when the ship was near Vulcan Point, station MTP registered a dominant frequency of 10-11 Hz; when the ship was in Matupi Harbour all stations registered frequencies of 11 - 12 Hz; when the ship was west of the Beehives, at the southwestern side of Simpson Harbour, frequencies of 10 - 11 Hz were registered by station WAN.

For the airgun used, the basic frequency of blasts is 11 Hz with a range of 9 - 13 Hz. frequencies produced by the blasts. of the blasts were attenuated in deeper water. coupling of the seismic energy to the solid earth took place and the recorded airgun blast frequency was close to the true dominant frequency produced by

Filters. At the time of the visit of the SP LEE, two types of

The "line" stations, connected to

At frequencies greater

It i s clear that the RVO seismic network faithfully recorded the It would appear that higher frequencies

In shallower water, better

this airgun.

b) Sensitivity of the RVO seismic network.

(i) The airgun blast energy (9.5 x 10 J) is equivalent to the explosive power of 227 g (0.5 1b) of chemical high explosive if the energy rating of the latter is accepted as 4.186 x 10 explosion energy is partitioned into energy of a number of forms, of which seismic energy is probably a minor component. Most of the energy of these shallow explosions would be converted to heating of the water and kinetic

Theoretical considerations of seismic energy generated by SP LEE airgun. 5

3 J g-1 (Nairn & Self, 1978) . However, this

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energy. explosions, the seismic energy may be of the order of 10-2 to 10-4 times the

kinetic energy of the blasts (Shimozuru, 1968). The details of the partitioning

of the blast energy into heat and kinetic energy are unknown, but in any case, the seismic energy will be a very small proportion of the available energy. the present, the seismic energy will be (perhaps generously) assumed to be 10-3 of the available energy.

An equivalent earthquake magnitude (ML) for an airgun blast of energy E can be calculated from the following relationship (Richter, 1958):

If the seismic efficiency of these blasts resembles that of volcanic

For

2 log E = 9.9 + 1.9 ML - 0.024 ML

2 5 If E is 9.5 x 10 J (10-3 of available energy 9.5 x 10 J) a value of ML = 0.1 is obtained for the equivalent magnitude of the airgun blasts. likely that the actual equivalent magnitude would be even lower than this as the assumed partitioning of available energy my be in error by one or two orders of magnitude in the case of seismic energy.

(ii) Registration of SP LEE airgun blasts by RVO seismographs. The SP LEE airgun blasts were not recorded at all by the Wood-Anderson seismograph which is situated in the RVO vault. The nominal magnification of this instrument (at its natural period of 1 second) at the time of the SP LEE visit was 2800.

The airgun blasts were, however, recorded by stations of the network of seismographs deployed around Rabaul caldera for volcano-seismic surveillance. maximum gain of this network is 30,000. sensitivity between individual stations of the network is known. sensitive stations, as determined from examination of the records of regional

earthquakes, are RAL and TKA. allowed a comparison of the sensitivities of some of the stations of the network. Figure 4 shows trace amplitudes of airgun blasts (measured from the Develocorder film record) plotted against distance between the seismic source and the stations. Blanche Bay show clear differences in sensitivity with the order of relative sensitivity from highest to lowest being RAL, WAN, MTP, TAV.

Station TKA is the most distant from the bay, and yet it surprisingly picked up some of the airgun blasts which other, closer stations, such as VUL and RPT, did not. Figure 4 indicates that TKA may be the most sensitive station of the

network.

However, it seems

The However, considerable variation in

The most

The mobile seismic source provided by the SP LEE

The four stations, WAN, RAL, TAV, MTP, around the northern shores of

However, anomalous behaviour of seismic waves travelling to TKA is

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suggested by apparently early first arrivals of the waves from some airgun blasts. The evidently high sensitivity of TKA and the early first arrivals suggest a zone of higher seismic velocity and of lower attenuation characteristics south of the caldera.

Despite the proximity of station RPT to some of the traverses of the SP LEE, this

station registered very few of the airgun blasts. be a combination of unconsolidated foundations at the RPT site (on beachsand) which would attenuate medium and high frequencies, and a Benioff seismometer (natural period 1 second) which has a poor response to the frequencies produced

by the airgun blasts.

Station VUL also showed a poor response to the SP LEE airgun blasts although this station is equipped with a seismometer sensitive to higher frequencies. The poor response may be an effect of distance, and in addition, this station is situated at about 330 m asl deposits which would have a markedly attenuating effect on higher frequency seismic waves.

Stations RAB and BOI did not register the SP LEE airgun blasts at all. case of station RAB, the low sensitivity may be an effect of local geology. is underlain by a large thickness of pyroclastic deposits. station BOI, the non-registration of the airgun blasts may have been caused by distance from the seismic source.

The cause of this appears to

and underlain by a thick sequences of pyroclastic

In the RAB

In the case of

3.

Two methods of earthquake location are used at RVO, viz. computer and graphical.

Both were employed using data from the mobile seismic source provided by the SP LEE in attempts to assess their accuracy. In both cases the results were generally

unsatisfactory although from some locations of the seismic source the graphical location method gave reasonable agreement with actual locations of the airgun blast sites.

a) Computer location method.

Assessment of RVO earthquake location methods.

The computer location method is routinely used at RVO for earthquakes with registrations at 5 or more stations of the network.

on fewer stations (3 or 4) could also be located using this method but uncertainties in the locations are unacceptably large in some cases.

applied in the present situation of attempting to locate selected SP LEE airgun

blasts, the computer method gave very poor results in 7 out of 8 cases.

Earthquakes registering

When

The

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computer locations differed from actual locations by about 3-6 km in most cases, and in one case when the SP LEE was near the Beehives (No 5 in Fig 5), the computer location differed by about 1 km.

The biggest mis-locations were for airgun blasts in Matupi of these computer locations being displaced about 6 km south of the blast sites is believed to be anomalously early registrations at TKA station.

No data from airgun blasts in the southern and eastern parts of Blanche Bay was used because of insufficient station registrations.

Harbour. The cause

b) Graphical location method

The graphical method of earthquake location used at RVO is a more subjective method than the computer one but it has been found useful for rapid, first approximations of earthquake locations. quakes registering on only 3 or 4 stations of the network.

The results of the application of this method to the SP LEE airgun blast data (Fig 6) can be separated into 3 groups. blasts from the southern part of the bay. 1 - 7 km.

For the central part of Blanche Bay and the central part of Simpson Harbour, mis-locations were generally in the range 1 - 2 km.

In Matupi locations was obtained.

It is routinely used for small earth-

Poor results were obtained for airgun Mis-locations ranged from about

Harbour reasonably good agrement between blast sites and graphical Mis-locations were usually less than 1 km.

c) Assessment of location methods

The test of RVO's earthquake location methods with the airgun blasts from the SP LEE was unsuccessful for a number of reasons:

(i) Slow seismic velocity in water resulted in anomalous seismic travel

times.

The small amount of seismic energy available in each airgun blast and

poor coupling of seismic energy to the solid earth resulted in non- registration of the blasts at some stations and weak registrations which are difficult to the precisely.

(ii)

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References

McKee, C 0 , 1982 - contribution on Rabaul, SEAN Bulletin, August 1982.

A, & Self, S , 1978 - Explosive eruptions and pyroclastic Nairn, I avalanches from Ngauruhoe in February 1975, Journal of Volcanology and Geothermal Research, V 3, No 1/2, 39-60.

Richter, C F , 1958 - Elementary Seismology. Freeman, San Francisco, California, 768 pp.

Shimozuru, D , 1968 - Discussion on the energy partition of volcanic eruption, Bulletin Volcanologique, 32, 383-394.

general seismic and bathymetric survey of parts of rabaul

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