Helicopter TDEM

8/21/2019 Helicopter TDEM

1/16

DEVELOPM ENT OF A HELICOPTER TIME DOMAIN SYSTEM FOR

BATHYMETRIC M APPING AND SEAFLO OR CHARACTERISATION IN

SHALLOW W ATER

J ulian Vrbancich1*，lUchard Smith2Defence Science & Technology Organisation julian. vrbrncich@ dsto. defmce.gov.au1

Technicrl Images P ty Ltd m̂hh@ netconnft com. auj

Key Words: airborne-electromagnetic bathymetry marine-seismic inversion LIDAR,altimetry.

INTRODUCTION

Time domain airborne electromagnetic (AE M) data acquired from surveys over seawater in

Australian coastal waters can be interpreted to obtain seawater depths (Vrbancich and Fullagar

2007a; Wolfgram and Vrbancich 2007; Vrbancich 2009) and to identify the coarse features ofbedrock topography (Vrbancich and Fullagar 2007b; Vrbancich 2009) . A comparison ofderived water depths in shallow areas (< 50 m) with known bathymetry has shown that sub-mere depth accuracies can be achieved but these accuracies are not maintained over the entiresurvey region. Furthermore，the quantitative interpretation o f AEM data using 1D inversionmethods may require data rescaling (Vrbancich and Fullagar 2007a; Vrbancich 2009) and themeasured seawater conductivity as a known parameter. The rescaling coefficients in thesestudies were obtained from the slope and intercept o f linear fits between modelled and observeddecays at representative sites (control points) with known water depths. These restrictions lim itthe potential o f AEM for accurate bathymetric mapping.

A time domain helicopter AEM system (SeaTEM) is currently being developed for the DefenceScience and Technology Organisation for shallow water bathymetric mapping. This systemconsists o f a transmitter and receiver loop assembly mounted on a rigid structure referred to as a“bird” that is towed as a sling load below the helicopter. Instrument stability， calibration(Vrbancich and Fullagar 2007a; Brodie and Samondge 2006; Davis and Macnae 2008) andthe ability to accurately track both the swaying motion (i.e. bird swing) and the altitude o f theAEM sensor system over seawater during survey (Davis et al. 2006; Kratzer and Vrbancich 2007) are issues that need to be addressed in order to develop AEM as a reliable and accuratebathymetry mapping technique. System calibration self-response transmitter curentwaveform，and altimetry were investigated and preliminary findings are repored in this paper.

Before going airborne，the response of SeaTEM instrumentation over seawater was studied in a

controlled experiment designed to minimise the effect of bird swing and altimetry erors. Forthis purpose，the AE M system was floated above seawater using a circular rng，modified fromstructures used for open-sea fish faming，as the platform . This floating AEM system is referedto as the “sea-Hng”. Periodic E M measurements were made whilst the sea-Hng was beingtowed at about 2 knots in areas of known water depth. A marine seismic survey providedindependent estimates of sediment thickness. Sea-Hng data was interpreted to appraise theaccuracy of water depths and sediment thickness derived from AEM data and to identifycalibration erors.

The AEM bathymetry method also has the potential to provide water depths in the sur* zone，sothat bathymetry can be used to measure water depths on approaches to beaches (Vrbancich 2009) . The use of L ID AR to estimate the sea sur'ace topography in sur* zone areas is underinvestigation (Vrbancich unpublished) to suppor AEM bathymetric mapping. As well as

1


2/16

mapping the topography, the L ID AR data also provides accurate altimetry which may be morereHable than using a laser altimeter over seawater.

Airborne and ground EM measurements were conducted over resistive granite to study thesystem self response. The AEM measurements involved flying over a dosed loop o f knownelectrical properties placed on the ground (Davis and Macnae, 2008). The response o f theground loop combined with the flight path can be used to predict the AEM system response. The AEM transmitter current waveform was also measured directly and indirectly from theground loop data. Apart from the direct measurement o f the transmitter current waveform, theresults of these findings w ill be presented separately (Davis et al., 2009).

SEA RING (FLOATING AEM SYSTEM) EXPERIMENT

Instrumentation

The floating platform consists o f three concentric tubular rings, approximately 20 m indiameter, that provide a base for supporting four telescopic poles that in turn support the

transmitter and receiver loops. The elevation o f the poles can be adjusted to provide loopheights ranging between 〜 6 to 15 m above sea level, Figure 1.Three square loops were used,consisting o f a transmitter (Tx) loop (located centrally) and inner and outer receive (Rx) loops(Figure 2), referred to as Rx in and Rx out. The two inner loops are attached to the outer loop,thus adjusting the height of the outer loop automatically adjusts the heights o f the two innerloops.

Figure 1 :Sea- ring - ffoating A E M system，Proper B ay, P ort L incoln，South A us tia ia . Sev eral

sea- nngs used ib r fis h farm ing are show n in the background.

Three different loop heights were used for the EM measurements. The heights at each comer ofthe outer Rx loop could be measured whilst under tow. The heights o f the Tx loop and the innerRx loop at the comers adjacent to mast D1(F igure 3) could only be measured in very shallowwater by wading out to the area below these loops, and the vertical offsets (see inset in Figure 3)

2


3/16

were assumed to apply to the other comers o f the inner Rx and Tx loops for all 3 sets o f loopheights. Forty four EM measurements were recorded on the firs t day of sea trials and sixteenmeasurements for the second day. The outer Rx loop heights are shown in Figure 3: {...}，refersto records #1-#25, Dayl; (... )，refers to records #26 - #44 Dayl, and records #1- #7 Day2;[...】，refers to records #8 - #16 Day2.

F igure 2 : D i l showing three loops. The central loop is the transm itter loop, the inner and outer loops ae the receive loops (R x in，R x out).

F igure 3: Schematic diagram showing loop geom r̂y and heights. Inset thows relative heights

o floop comes a t mast #1.

3


4/16

The receiver channel times for the 21 windows, from start o f Tx current ramp-down, are givenin Table 1.Four turns were used for the inner loop, a single turn was used for the outer loop.

Rx inner Rx outerChannel No. Centre Time

(is)

Channel Width

(is)

Centre Time

(is)

Channel Width (is)

1 72.5 25 92 . 5 25

2 97.5 25 117 . 5 25

3 122.5 25 142 . 5 25

4 147.5 25 167 . 5 25

5 210 100 230 100

6 310 100 330 100

7 410.625 101.25 430.625 101.25

8 511.875 101.25 531.875 101.25

9 663.75 202. 5 683.75 202. 5

10 916.875 303.75 936.875 303.75

11 1271.25 405 1291.25 405

12 1726.875 506.25 1746.875 506.25

13 2283.75 607. 5 2303.75 607. 5

14 2941.875 708.75 2961.875 708.75

15 3701.25 810 3721.25 810

16 4561.875 911.25 4581.875 911.25

17 5523 . 75 1012 . 5 5543.75 1012. 5

18 6586. 875 1113.75 6606.875 1113 . 75

19 7856.719 1721.25 7876.719 1721.25

20 10181.25 2632. 5 10201.25 2632. 5

21 13218 . 75 3442. 5 13238 .75 3442. 5

Table 1 .Channel times and widths fo r inner and outer ree ive loops.

Transmitter Current Waveform and Ramp-down Time

The Tx current waveform comprises 5 ms bipolar pulses, transmitted at a fundamentalfrequency o f 25 Hz (15 ms off-time). The curent is supplied by a bank o f batteries. Thecurent pulse is quasi-trapezoidal with an exponential ramp-up time (750 is time constant),approximately 3.5 ms constant current, and a linear tum-off ramp. A measurement o f thecurrnt wavefom is shown in Figure 4, and a detailed section of the ramp-down measured withtwo currnt transducers is shown in Figure 5. Both measurements (Figures 4, 5) were madeover resistive granite in Western Australia with the transmitter and receiver loops paced on theground.

ΐοο ;$.oleiegyLi e VHVwpicouoicen

F igure 4: Experimental lrrnsm itter cuuent waveform. O n-time cuum tis 〜265A.

4


5/16

The ramp-down time was measured as 〜34 士 2 is , F igure 5. The Tx loop used at PortLincoln has a different dimension and hence a different inductance. Using the calculatedinductances o f the ground loop and the loop used in the sea-ring system, the predicted ramp-down time for the sea-ring loop is 〜 19 is for an on-time current of 〜216 A. This is in goodagreement with the measured ramp-down time o f 20 士 2 is obtained from direct measurements

over seawater. Accurate ramp-down times are important for EM modelling at early times.

ve rtical axis cunvnt: (A ). The blue and red curves re fer to cuuent sensors w ith bandwidths o f 100 kH z and 500 kH z respectively. The aqua (500 kH z) )ndyeU ow (100 kH z) symbolize these bandwidths depicting 2 is and W is ssm pling in te rvls . The slope o fthe steep curve on the le f

depicts the 100 A /is slew rate fo r both curnnt sensors.

Sea Trials

The sea-ring E M system was towed at a speed o f 〜2 knots (〜 1 m/s) with the towing vesselseparated from the sea-ring structure by about 100 m. Simultaneous Rx in and Rx outmeasurements were taken approximately every 5 to 7 minutes over a period o f at least 1minute.An algorithm was applied to the data to remove the effect o f bird-swing (i.e., induced voltagescaused by the swaying motion o f the receiver loops in the Earth’s magnetic field) and 1 s ,10 sand 60 s averages were obtained from stacked data.

The geographic locations o f EM recordings are shown in Figure 6. Forty four records wereobtained during the first day (olack dots); the sea-ring system was tethered at a mooringovernight and measurements resumed the next day. F ifeen records (white dots) were obtainedat the start of the second day with the firs t 11 measurements recorded whilst the sea-ring wastethered at several locations on the mooring. The sixteenth recording on the second dayconsisted o f a continuous measurement, divided into twenty one 60 s contiguous blocks as thesea-ring passed through the passage between Grantham Island and the mainland (blue dots). Thered dots locate the marine seismic events.

The sea state for the second day was very calm. The sea state for the first day was choppy,resulting in a relatively larger bird swing component compared to that observed for the secondday. Water depths were recorded directly at each o f the 44 EM measurement locations using ahand-held sonar instrument attached to the sea-ring. Bathymetric data was also obtained fromtwo independent sources: a multi-beam sonar survey that covered the area between B illy Lights

5


6/16

Point and Grantham Island, and a single-beam survey that covered most o f the survey area northof B illy Lights P oint, Figure 6. Seawater conductivity measurements were made at periodicintervals during both days and varied from 5.09 to 5.15 S/m. A value of 5.1 S/m was used forall locations.

F igure 6: Location o f recordings a t P ort L inco ln overlaid on a section o f chart AUS134 (see inset fo r loca tion on AustmHa ?s coastline). The coloured g rid images show the extent o fthe two bathymetry datasets obtained from sonar soundings， which overlap a t B illy L ights P oint. P redicted tid a l com ctions were applied to these bathym etric soundings. This chart section is not to be used fo r navigation purposes.

MARINE SEISMIC REFELECTION SURVEY

A marine geophysical survey was conducted by a contractor (Golder Associates P ty Ltd) to mapthe thickness o f unconsolidated sediment overlying marine bedrock. Images o f the subsuracestratigraphy and top o f bedrock were acquired with a low frequency seismic reflection profilingsystem having a bandwidth o f 400 to 800 Hz. The refection signals were received using a 10-dement hydrophone aray. The assumed compressional velocity in unconsolidated sedimentwas 1550 m/s.

The estimated sediment thickness was obtained at the locations shown in Figure 6 (red dots).

These locations indude several tie lines ringing approximately perendicular to the shore in thenorhem region and redundant transects along the norh-south transects for data validation. Thehighest data quality was obtained in the norhem area where calm weather conditions prevailed.

6


7/16

Strong water currents and adverse sea conditions resulted in reduced data quality in the southernarea. In some areas, there were horizontal discontinuous reflectors and on occasions, theseshallow reflectors, or other surface conditions such as coarse-grained sediment, limited orprevented acoustic penetration and therefore masked the reflection from the top o f bedrock. The bedrock was characterized as an acoustically-hard, high amplitude, discontinuous, seismicreflector with relatively high topographic relief, associated with onshore promontories andpeninsulas. Figure 7 depicts a schematic diagram of sediment thickness that includes thepassage between Grantham Island and the mainland. The interpreted sediment thickness fromthe marine seismic survey was gridded in the northern area and sampled at the sea-ringlocations. The resulting sediment thickness was subsequently combined with known waterdepths to provide an estimate o f the depth to bedrock, which was used to appraise the accuracyof depth to bedrock obtained from EM measurements.

DISTANCE ALONG TRACKLINE

0 200 400 600 800 1000 1200 1400 1600 1800

F igure 7: sediment thickness (white) and bedrock topography (grey) estimated from m ane seismic reflection measurements adjacent to Grantham Island. R efemng to F igure 6， distance along track (m) commences approximately from the south-west entrance to the passage between Grantham Islm d and the mainland (sW t o f blue dots) m d fimshes aaproximatey where the th ird white dot is located nooh-east o f Grantham Island. (Source: G older Associates L td)

INTERPRETATION OF SEA RING DATA

Water and sediment depths were interpreted via ID inversion o f EM data using program Amity(Fullagar Geophysics %y Ltd). A two-layer model of seawater and sediment over resistivebasement was used for all inversions. The conductivity o f the seawater, sediment layer andresistive basement was fixed at 5.1 1.25 and 10-4 S/m respectively. The starting depth for theseawater layer and sediment layer was 10 and 20 m respectively.

Inversion of Raw Data

Figure 8 shows the depths o f the first layer, d1,which represents the seawater depth, and thedepth o f the second layer, d2 (relative to the surface) which represents the depth to bedrock,obtained from inversion o f data averaged over 60 s for the inner Rx loop using all 21 channels(Figure 8a, 8b) and using only channels 10 to 21(Figure 8c, 8d) The results for the outer Rxloop (not shown) show better agreement with known bathymetry and worse agreement withestimated depths to bedrock.

Generally, the d1 and d2 depths show very poor agreement with known depths, and only in afew cases, as shown in F igure 8d, is there is an improvement if the early channels are omittedfrom the inversion process. Other starting depths and sediment conductivities also gave pooragreements with known depths. A significant improvement in the agreement between d1 andknown water depths was however found in all cases (i.e., using data averaged over 1,10 and 60s for both inner and outer receiver loops) using a lower seawater conductivity o f 4.6 S/m. Asimilar improvement was also found in a previous study where HoistEM data was used to

7


8/16

F igure 8: d l and d2 using E M data averaged over 60 s from inner R x loop， compared with known bathym etiy and estimated depth to bedrock. B lu r: d l) b) d2 ) d); p ink (a b): measured water depth; red, aqua ) b): water depths from gridded bathymetiy datasets; the three sets o f water depths (pink red aqua) are added to the estimated sediment thickness from m ane seismic to obtain the depths to bedrock in c，d (pink wd，aqua curves).

Data Rescaling

The raw EM data contains errors arising from system calibration (e.g. gain errors, errors inassumed loop geometry etc.) and self-response. The system self-response cannot be separated

from the data because， unlike AEM data, it is not possible to obtain any high altitudemeasurements. Data rescaling was performed by comparing the observed (stacked raw)

(B) Day 1 Rx_ln [60 s ave] depth to bedrock

(D) Day 1 Rx_ln [60 s ave] depth to bedrock Ch 10-21

determine water depths in Sydney Harbour - in this case, Vrbancich and Fullagar (2004) usedan unrealistically low seawater conductivity to obtain a reasonable agreement between inverteddepths and known bathymetry.

(A) Day 1 Rx_ ln [60 s ave] bathymetry

(C) Day 1 Rx In [60 s ave] bathymetry Ch 10-21

record number

E·

}

£d θ ρ

8


9/16

voltages with those obtained from forward modelling using a 2-layer over basement model atselected control points. The control points were carefuUy chosen where the marine seismicevent locations (red dots, Figure 6) were closely co-located with the sea-ring measurements(blue, white and black dots, Figure 6). Slopes and intercepts were obtained from linearregression analysis o f the scatter plots o f observed voltages (x-axis) and model voltages (y-axis)for all 21 channels, separately for each receiver loop. The slopes are shown in Figure 9.

F igure 9: S lope o f Unear G t between observed and modelled data using 24 selected contwJ points. P ink: outer receiver loop; blue，inner receiver loop.

A slope o f unity (with zero intercept) implies that the observed voltage at a given channel(Table 1)agrees with the model voltage. The data rescaling process essentially serves as a first-order correction as there is no certainty that the sediment conductivity (1.25 S/m) and depths tobedrock used to model the EM response at the control points are accurate. As shown in Figure9, for most channels, the slope is within 10% of unity, however for the early channels, the

difference increases, approaching 40% for the second channel of the inner receiver loop. Theselarger differences for the early channels would contribute to the discrepancies in agreementbetween d1 and known water depth when all 21 channels are used in the inversion o f raw EMdata, e.g., Figures 8a, 8c. The observed voltages for all sea-ring measurements were rescaledusing both slope and intercept:

V— rescaled、= slope* Vobserved+ intercept

Inversion Using Rescaled Data Day 1

Figures 10 and 11 show the inverted depths d1 and d2 obtained from 10 and 60 s averaged EMdata recorded from the cuereceiver loop, during the firs t day, compared with the measured seadepth and depth to bedrock obtained by adding the sediment thickness to the measured depth.Figure 12 shows the equivalent depths obtained from 60 s averaged data recorded from the inner receiver loop. The d1 and d2 profiles are in very good agreement. The inverted depths are alsoindependent o f the transmitter loop heights used for this experiment, which is 9.91 m for records1 to 25, and 8.77 m for records 26 to 43. A t such low altitudes, a 1 m variation in heightsignificantly affects the EM response.

The inverted depths d1 in Figures 10a,11a and 12a generally show very good agreement withmeasured water depths, typically to within 0.5 to 1m and represents a significant improvementcompared to using raw EM data (Figures 8a, 8c). This highlights the necessity to remove theself-response and to minimise calibration errors. We suspect that the slope of the line o f best f t

is related to a calibration error and that the offset is related to the self-response. The inverteddepths to bedrock (d2) also show a reasonable agreement with depths estimated from marineseismic measurements (Figures 10b,11b and 12b). (Records 7 -12, 31,32 and 34 were not

9


10/16

(A) Day 1 Rx out 60 s average Bathymetry

geographically close enough to a seismic measurement to obtain a reliable estimate of the depthto bedrock from the seismic data.) This level o f agreement was not found with previous studiesthat employed rescaled EM data to estimate the coarse features o f the bedrock topography(Vrbancich and Fullagar, 2007b; Vrbancich 2009); this may arise because o f the inferior qualityof the rescaled AEM data caused by uncertainty in altitude and orientation, compared to the searing system with a stable, fixed transmitter altitude and longer data acquisition times.

F igure 10: inverted depths， 10 s averaged R x out E M data, Day 1 . (a): Blue, d l; pink

measured depth. ( ) : Blue d2; pink，estimated depth to bedrock. d2(record 44)= -62m.

F igure 1 1 :inverted depths， 60 s averaged R x out E M data， D y 1 . ( ) : Blue d l; pink

measured depth. (b): Blue d2; pink，estimated depth to bedrock.

(B) Day 1 Rx_ out 60 s average De pth to Bedrock

) s d a p

s d

a p

050505050

1

1

2

2

3f

-

j

>

4

4

5

)

£d a p

10


11/16

(A) Day 2 Rx_in 1 s average Bathymetry

10 1

record number

(B) Day 1 Rx in 60 s average Bathymetry

(A) Day 1 Rx in 60 s average Bathymetry

F igure 12: inverted depths，60 s avemged R x in E M data, Day 1 .(a): B lue, d l; pink，measured depth. ( ) : Blue d2; pink，estimated depth to bedrock. d2(rmord 44)= -60m.

Inversion Using Rescaled Data Day 2: Tethered

The depths d l and d2 were obtained from inversion o f EM data recorded whilst the sea-ringsystem was tethered at two locations either side of a large sea-ring mooring. Thesemeasurements were made in very calm waters. Records 1,3 to 7 (all tethered) were made witha Tx altitude o f 8.77 m, the loops were then lowered to a new Tx altitude o f 5.85 m which

remained fixed for records 8 to 16. Record 8 is ignored as the sea-ring was adrift, and records 9-1 1 were recorded at another tethered location. These locations can be seen as a duster ofwhite dots in Figure 6. Records 12 to 15 were made whilst under tow heading for the passageadjacent to Grantham Island.

Figures 13 and 14 show the inverted depths d1 and d2 obtained from 1 and 60 s averaged EMdata recorded from the inner and outer receiver loops respectively. Apart from the scatter o f thed1 and d2 depths obtained from 1 s averaged EM data at each record location, the depths arevery similar to depths obtained from 10 s and 60 s averaged EM data, for both inner and outerreceiver loops. These results show the consistency of the inverted depths: d1 is essentiallyconstant at each o f the two tethered locations and all d1 values agree very well with the knownbathymetry. The d2 values are also fairly constant at the tethered locations and appear tounderestimate the depth to bedrock by about 5 to 10 m for records 13 -15.

}£d a p

s d a p

11


12/16

(B) Day 2 Rx_o ut 60 s average De pth to B edrock

(A) Day 2 Rx_ou t 60 s average Ba thymetry

(B) Day 2 Rx_in 1 s average De pth to B edrock

F igure 13: inverted depths， 1 s averaged R x in E M data，D ay 2. (a): B lue, d l;p in k，measured depth; aqua tide-convcted multi-beam sonar (b): Blue d2; pink， estimated depth to bedrock. (R ecord 2 was aboned).

F igure 14: inverted depths，60 s averaged R x_oui E M data，D ay 2. (a): Blue d l;p in k，measured depth; aqua tide-conected multi-beam sonar, (b): Blue d2; pink， estimated depth to bedrock.

Inversion Using Rescaled Data Day 2: Grantham Island

Figure 15 shows the inverted depths d l and d2 obtained from 1 s averaged EM data recorded

from the inner receiver loop，with the Tx altitude fixed at 5.85 m. A continuous recording wasmade as the sea-ring system was towed through the passage between Grantham Island and themainland. The total recording was split into 21 records，each representing 60 s of data. Thistraverse passes through some very shallow water. Very similar d l and d2 depth profiles wereobtained from Rx out data，and from ls ， 10 s and 60 s averaged EM data. The d l values forrecords 1and 2 appear anomalous; however they merely reflect the rapid change in water depthduring the 60 s interval that spans each record. The agreement between d1 and measuredbathymetry is typically less than about 0.3 m.

The estimate o f depth to bedrock from seismic data for records 1 to 8 (Figure 15b pink) isinaccurate because the locations o f the EM records are not in close proximity (i.e. not within〜50 m) to the seismic events (red and blue dots in F igure 6). In this case，the d2 depths may bemore reliable around records 6 and 7 than the estimated depths from marine seismic data. For

£· ) s d a p

0 1 2 1 4

^ I / I

- H

1 !—

丨

-5

-10

-15

-20

-25

-30

-35

-40

-45

-50

12


13/16

(B) Day 2 Rx_in 1 s average De pth to B edrock

(A) Day 2 Rx_in 1 s average Bathymetry

the remainder of the transit (records 10 - 21), there is reasonable agreement between the twodepth to bedrock profiles.

F igure 15: inverted depths， 1 s averaged R xin E M d a ，Day 2, G rantham Island passage. (a): Blue d l; aqua，measured depth. ( ) : Blue d2; pink，estimated depth to bedrock.

ALTIMETRY OVER SEAWATER

Fixed-wing flight trials were performed using a 2D laser scanner (i.e. a LIDAR system) andinertial navigation equipment that will be fitted to the SeaTEM rig for AEM bathymetricmapping. The area surveyed was located at the mouth o f the Murray R iver in the Coorongregion，south o f Adelaide，South Australia. These trials demonstrated that the LID AR systemcould measure reflections from seawater in the surf zone to detect small variations intopography (Vrbancich et a l. 2008) A fter processing the LIDAR and inertial navigation data,the sea surface topography is finally obtained by subtracting the altitude o f the laser scannerfrom the height above a reference datum for each point on the sea surface scanned by the laser.However the altitude is also o f interest because the 2D altimetry surface acquired from the laserscanner would be considered to be significantly more reliable than single beam measurementsacquired from a laser altimeter. In addition, slant range errors arising from bird swing would

overestimate the altitude if the AEM system is not fitted with inertial navigation. For AE Mbathymetry applications,，he quasi-2D altimetry would be averaged over an area representativeof the EM footprint.

An example of sea surface topography and associated LID AR altimetry is given in Figure 16. The top image shows the sea surface close to the beach with maximum wave heights o f 〜 lm. The lower image shows the corresponding quasi-2D altimetry surface which varies by about 30m. The furrows in the altimetry map correspond to relatively small variations in altitude causedby the waves. These results demonstrate that LID AR methods can provide very accuratealtimetry for AEM applications in a marine environment.

13


14/16

、

F igure 16: LID A R images o fsea sudace topography (top) and associated ̂ trn r̂y (bottom ) in a s u rfzone. W idth:〜 170 m ;le ngth :〜1850 m. Topography scale: pink， S 0.9 m ; blue -0.4 m. A litm etry scale: pink 190m; blue， 160 m.

CONCLUSIONS

This report has demonstrated the usefulness o f a foating “airborne” EM system to provide a

stable platform where transmitter and receiver loops are positioned at a fxed altitude. Inaddition, EM data can be recorded and stacked over longer time intervals, compared to airborneEM, to improve data quality. The disadvantage of this system is that the loop geometry is notfxed and is susceptible to the prevailing sea state which causes the platform to bob up anddown. The removal of the bird swing component combined with longer averages significantlyreduces this problem. Another disadvantage is that the self-response o f the EM instrumentationcannot be measured as, unlike airborne EM, it is not possible to take recordings at highaltitudes.

Water depths were obtained from direct measurements and from two overlapping sonar datasetsto provide a ground truth for appraising the accuracy o f water depths derived from EM data. Amarine seismic survey was undertaken to provide estimates o f sediment thickness, whichcombined with accurate water depth gives an estimate o f the depth to bedrock. These depthswere compared with depths to bedrock obtained from EM data assuming a model consisting ofseawater and sediment overlying a resistive basement.

Using raw stacked EM data, it possible to obtain accurate bathymetry and in some cases, areasonable depth to bedrock, however this can only be achieved by using an underestimatedvalue o f the measured seawater conductivity. In addition, even better bathymetry agreementcan be obtained using a one-layer model for inversion (i.e., ignoring a sediment layer) for thisdataset. The purpose o f these trials was to minimise the uncertainties caused by altitude errorsand identify calibration errors. Rescaling the EM data based on known water depths andestimated depths to bedrock at selected control points within the survey effectively accounts for

calibration errors and self-response in the EM instrumentation. These errors are still underinvestigation. The water depths derived from rescaled EM data show excellent agreement withknown water depth, typically witmn 0.5 m. The depths to bedrock also show good agreement ingeneral, with depths obtained from marine seismic data. These conclusions demonstrate that' airborne” E M data can be used to (i) measure water depth accurately, and (ii) measure thetopographic features o f a resistive basement, consistent with results obtained from marineseismic data. These findings also highlight the need for accurately calibrated EMinstrumentation.

The application o f L IDAR methods for mapping altimetry over a sea surface was demonstratedwith measurements taken over a surf zone with 〜1 m wave heights. The resulting quasi-2Daltimetry surface can be averaged over the approximate size o f the E M footprint to obtain an

altitude that is expected to be s ignificatly more reHable than single-shot laser altimeterreadings, that in addition, may also be susceptible to s la t range errors caused by bird motion.

14


15/16

ACKNOWLEDGEMENTS

We acknowledge Geoff Hall (Geosolutions Pty Ltd) for his concept o f using a sea ring structure,adapted from similar structures used for fish farming, and for constructing the rigging thatsupported the EM loops. We also acknowledge Keith Mattews (Kayar P y Ltd) and GrahamBoyd (Geosolutions P y Ltd) for supporting this project. J V also acknowledges Flinders Portsand the Australian Hydrographic Office (AHO) for the bathymetry datasets, and the AHO forpermission to use a segment o f chart AUS134. J V also acknowledges Peter Fullagar (FullagarGeophysics P y Ltd) for supporting this project and development o f specialized inversionsoftware. J V also acknowledges 化 chard Graham (Golder Associates P y Ltd) who undertookthe marine seismic survey and Dick Sylwester (Golder Associates P ty Ltd) who interpreted thedata to provide sediment thickness.

REFERENCES

Brodie, R., and Sambridge, M. 2006, A holistic approach to inversion o f frequency-domainairborne EM datt. G eophysics，', G301-G312.

Davis, A.C ., Macnae, J ., and Robb, T., 2006, Pendulum motion observed in HEM systems:

Explor. Geophys. 37, 355-362.

Davis, A.C ., and Macnae, J ., 2008, Quantirymg AEM system characteristics using a ground

loop: Geophys., 73, F179-F188.

Davis, A.C., Macnae, J ., Vrbancich, J ., and Smith, R., 2009, Monitoring the current waveform

of the SeaTEM system: Adelaide, Sth. Australia: 20th Intemat. Geophys. Conf. ASEG,

Extended Abstracts.

Kratzer, T., and Vrbancich, J ., 2007, Real-time kinematic tracking o f towed AEM birds: Explor.

Geophys. 38,132-143.

Vrbancich, J ., and Fullagar, P .K., 2007a, Improved seawater depth determination using

corrected helicopter time domain electromagnetic datt: Geophys. Prosp. 55, 407-420.

Vrbancich, J ., and Fullagar, P .K., 2007b, Towards remote sensing o f sediment thickness and

depth to bedrock in shallow seawater using airborne TEM: Explor. Geophys. 38, 77-88.

Vrbancich, J ., 2009, An investigation of seawater and sediment depth using a prototype airborne

electromagnetic instrumentttion system (SeaTEM) - a case study in Broken Bay, Australia:

Geophys. Prosp. in press.

15


16/16

Vrbancich, J . and Fullagar, P .Κ., 2004, Towards seawater depth determination using the

helicopter HoisTEM system: Explor. Geophys. 35, 292-296.

Vrbancich, J , L ieff W., and Hacker, J , 2008, unpublished.

Wolfgram, P., and Vrbancich, J , 2007，Layered earth inversion of AEM data incorporating

aircraft attitude and bird offset - a case study o f Torres Strait: E xplor. Geophys. 38,144-149.

16

Helicopter TDEM

Documents

Transcript of Helicopter TDEM