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                                                                   EXHIBIT 10.25

DEBENTURE - FLOATING RATE

Issued to:     RoyNat Inc.
               4500 Canterra Tower
               400 - 3rd Avenue S.W.
               Calgary, Alberta  T2P 4H2
               Facsimile No. 269-7701

                              Venture Seismic Ltd.
                             3110 - 80th Avenue S.E.
                                Calgary, Alberta
                                     T2C 1J3

                                    DEBENTURE

$900,000.00    April 29, l997

1.             Venture Seismic Ltd. (the "Company") for value received hereby
promises to pay to RoyNat Inc. ("RoyNat"), at its address specified above, the
principal amount of Nine Hundred Thousand Dollars ($900,000.00) in the manner
hereinafter provided, together with all other moneys which may from time to
time be owing hereunder or pursuant hereto.

2.             Principal Payments. Subject to the provisions of this Debenture,
the principal amount of this Debenture shall become due and payable by 50
monthly instalments, each in the amount of $18,000.00, payable on the 15th day
of each month from and including May 15, l997 to and including June 15, 2001 and
the balance of the said principal amount, together with interest thereon and all
other moneys owing hereunder, shall become due and payable on the 15th day of
June, 2001.

3.             Interest. Interest shall be payable on the 15th day of each month
on the balance from time to time outstanding of the principal amount of this
Debenture, any overdue interest and any other moneys due and payable hereunder,
both before and after maturity, default or judgment, at the rate of interest per
annum (the "Loan Rate") which is 2.75% greater than RoyNat's Floating Base Rate
for the Applicable Period calculated and compounded monthly, computed from the
respective dates of advance of the moneys by RoyNat to the Company until payment
in full of all moneys owing hereunder. RoyNat shall notify the Company at least
five days prior to each interest payment date of RoyNat's Floating Base Rate for
the Applicable Period but the non-receipt of any such notice by the Company or
the failure of RoyNat to give such notice shall in no way limit or negate the
obligation of the Company to pay interest on such payment date. The first
interest payment date shall be May 15, l997.

4.             Redemption.  The Company may redeem this Debenture prior to
maturity either in whole at any time or, when not in default hereunder, in part
from time to time on not less than 30 days' written notice at a price equal to
the principal amount being redeemed plus an additional amount equal to three
months' interest on such principal amount at the Loan Rate in effect on the
date fixed by the Company for redemption and together in each case with accrued
and unpaid interest on such principal amount to the date fixed for redemption
and, in the case of redemption in whole, all other moneys owing hereunder.  The
Company shall have no other right of prepayment.


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               Notwithstanding the foregoing paragraph, the Company may, when
not in default hereunder, on not less than 10 days' written notice, redeem up to
15% of the principal amount outstanding on the date fixed by the Company for
redemption at a price equal to the principal amount being redeemed, together
with accrued and unpaid interest on such principal amount to the date fixed for
redemption. This right may be exercised once during each 12 month period
following the date of the initial advance of funds hereunder, and shall be
non-cumulative.

4A.             Conversion to Fixed Interest Rate. The Company may at any time 
request that RoyNat provide a quotation as to the availability of fixed interest
rates on the principal of this Debenture. Within 60 days after receipt of such
request, RoyNat shall provide a written offer to the Company, offering specified
rates of interest for a selection of fixed interest periods as RoyNat may then
be able to provide and specifying a date from which the conversion would be
effective (the "Conversion Date"). Such rates of interest shall be 2.75% above
RoyNat's Term Base Rate for the relevant periods. The offer shall stipulate any
extensions of the term of this Debenture that RoyNat may require. If the Company
accepts such offer by selecting the applicable interest rate and fixed interest
period, the rate so specified and accepted shall be the "Loan Rate" for all
purposes of this Debenture from and after the Conversion Date for the fixed
interest period selected. If such period expires before the end of the agreed
extended term, the rate of interest on this Debenture shall revert to the
variable rate, provided in Section 3 hereof for the balance of the agreed
extended term, but the Company may thereafter avail itself again of the
provisions of this Section 4A. The Company shall forthwith execute and deliver
such documentation, if any, as RoyNat shall request to give effect to any
interest rate conversion or extension of the term of this Debenture. The Company
shall have no right to redeem this Debenture after conversion to a constant
interest rate except as may be specified in the offer from RoyNat accepted by
the Company, notwithstanding the provisions of Section 4 hereof.

               For the purposes of this Section:

    (a) "RoyNat's Term Base Rate", means, at any time, the annual rate of
    interest which RoyNat establishes at its principal office in Toronto as the
    reference rate of interest which RoyNat will charge for closed fixed rate
    term loans in Canadian dollars made to its customers in Canada for varying
    durations and which it refers to as "RoyNat's Term Base Rate" for that
    duration of loan; and

    (b) "Government of Canada Bond Yield" on any date means the yield to
    maturity, assuming semi-annual compounding, which a non-callable Government
    of Canada bond would carry if issued, in Canadian dollars in Canada at 100%
    of its principal amount on such date with a term to maturity equal to the
    fixed interest rate period being offered by RoyNat, as quoted to RoyNat by a
    money market dealer or Canadian Charter Bank.

5.             Partial Payments.  In case less than the total principal amount
of this Debenture is redeemed at any time, the principal amount so redeemed
shall be credited against the principal payable hereunder in inverse order of
maturity.

6.             Security. As security for the payment of the principal, interest
and all other moneys from time to time payable under this Debenture, and the
performance by the Company of all its obligations hereunder, but subject to
Permitted Encumbrances and to the exception as to leaseholds


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hereinafter contained, the Company hereby grants a security interest in and
grants, assigns, mortgages and charges, as and by way of a first, fixed and
specific mortgage and charge to and in favour of RoyNat all furniture,
machinery, equipment, vehicles, accessories and other tangible personal property
(other than Inventory) now owned or hereafter acquired by the Company, together
with any proceeds of sale or disposition thereof, and including without
limitation the property described in Schedule "B" hereto.

               And for the same consideration and purposes and subject to the
same exceptions, the Company hereby charges as and by way of a first floating
charge to and in favour of RoyNat the undertaking of the Company and all its
property and assets for the time being, both present and future, and of
whatsoever nature and wherever situate (other than property and assets from time
to time effectively subjected to the fixed and specific mortgages and charges
created hereby or by any instrument supplemental hereto).

               Provided that such floating charge shall not prevent the Company
from time to time until the security hereby constituted shall have become
enforceable from selling, leasing or otherwise disposing of the property, rights
and assets included in such floating charge or from making expenditures with a
view to the expansion of its business or from giving security constituting
Permitted Encumbrances, all in the ordinary course of its business and subject
to the provisions of this Debenture. In particular, the Company may give
security to its bankers on its Inventory or by way of assignment of its accounts
receivable (except to the extent such accounts receivable represent proceeds of
the sale or disposition of property specifically mortgaged or charged hereunder
or under any instrument supplemental hereto) and such security if validly
perfected shall rank prior to the lien hereof on such assets without further
action by RoyNat. Notwithstanding any other provision of this Debenture except
as provided in the foregoing sentence, the security interests constituted hereby
and by any supplemental security granted to RoyNat shall not be subordinate to,
nor is there any intention to subordinate such security interests to, any
Permitted Encumbrances or security interests held by others.

               All security interests created by this Debenture attach
immediately upon execution of the Debenture. The attachment of the floating
charge has not been postponed and the floating charge shall attach to any
particular property intended to be subject to it as soon as the Company has
rights in such property.

               All property and assets of the Company whether specifically
charged or subjected to the floating charge are hereinafter referred to as the
"Mortgaged Premises".

7.             Exception as to Leaseholds. It is hereby declared that the last
day of any term of years reserved by any lease or sublease, verbal or written,
or any agreement therefor, now held or hereafter acquired by the Company is
excepted out of the Mortgaged Premises, but the Company shall stand possessed of
any such reversion upon trust to assign and dispose thereof as RoyNat may
direct.

8.             Payments and Notice. Any payments not received by RoyNat by two 
o'clock p.m. on a Business Day shall be deemed to have been received on the next
Business Day. Any notice required or desired to be given hereunder or under any
instrument supplemental hereto shall be in writing and may be given by personal
delivery, by facsimile or other means of electronic communication or by sending
the same by registered mail, postage prepaid, to RoyNat or to the Company at
their respective addresses set out above and, in the case of electronic
communication, to


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the facsimile numbers set out above. Any notice so delivered shall be
conclusively deemed given when personally delivered and any notice sent by
facsimile or other means of electronic transmission shall be deemed to have been
delivered on the Business Day following the sending of the notice, and any
notice so mailed shall be conclusively deemed given on the third Business Day
following the day of mailing, provided that in the event of a known disruption
of postal service, notice shall not be given by mail. Any address for notice or
payments herein referred to may be changed by notice in writing given pursuant
hereto.

9.             Covenants. (a)  This Debenture is issued subject to and with the
benefit of all the covenants, terms and conditions in Schedule "A" hereto which
Schedule forms a part hereof.

    (b) In addition to such covenants, terms and conditions, the Company
    covenants with RoyNat that so long as this Debenture remains outstanding the
    Company shall:

             (i)   execute and deliver all such documents as may be necessary
                   to maintain in force the pre-authorized payment system
                   specified in the Offer of Finance; and

             (ii)  not permit Consolidated Working Capital to fall below a ratio
                   of 1:1 excluding the current portion of term debt.


10.            Offer of Finance. This Debenture is being issued by the Company 
to RoyNat pursuant to the terms of a certain letter agreement between the
Company and RoyNat dated April 14, 1997 (such letter agreement including any
amendments thereto being herein called the "Offer of Finance"). All terms and
conditions of the Offer of Finance shall remain in full force and effect, except
to the extent inconsistent with the provisions of this Debenture.

11.            Maximum Recovery. If any amounts, whether on account of interest,
fees, bonus or additional consideration, becomes payable to or is received by
RoyNat pursuant to this Debenture, the Offer of Finance, any other security
document or other agreement which would exceed the maximum amount recoverable
under applicable law on moneys advanced by RoyNat:

    (a)      any amounts so payable shall be reduced and are hereby limited to
             the maximum amount recoverable under applicable law;

    (b)      any amounts so received by RoyNat shall, at RoyNat's option, either
             be returned to the Company or, notwithstanding Section 4 hereof, be
             deemed to have been received by RoyNat as a partial redemption of
             this Debenture and shall be credited against principal payable
             hereunder in inverse order of maturity; and

    (c)      if paragraph (a) requires the reduction in an amount or amounts
             payable to RoyNat, RoyNat in its sole discretion shall determine
             which amount or amounts shall be reduced to ensure compliance with
             this Section 11.

12.            Extensions and Amendments. Any agreement for the extension of the
time of payment of the moneys hereby secured or any part thereof made at, before
or after maturity, and prior to the execution of a discharge or release of this
Debenture, or any agreement for altering the term, rate


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of interest (whether increased or decreased), the amount of the principal
payments hereunder or any other covenant or condition hereof, need not be
registered in any office of public record but shall be effectual and binding
upon the Company and upon every subsequent mortgagee, chargee, encumbrancer or
other person claiming an interest in the Mortgaged Premises or any part thereof
when executed by the Company and delivered to RoyNat.

13.            Receipt and Waiver. The Company hereby acknowledges receipt of a
true copy of this Debenture and, to the extent permitted by law, waives all
rights to receive from RoyNat a copy of any financing statement or financing
change statement filed, or any verification statement received, at any time in
respect of this Debenture or any supplemental or collateral security granted to
RoyNat.

14.            Binding Effect, Governing Law and Headings. These presents are 
binding upon the parties hereto and their respective successors and assigns.
This Debenture shall be governed by and construed in accordance with the laws of
the Province of Alberta and the laws of Canada applicable therein. The division
of this Debenture into sections and the insertion of headings are for
convenience of reference only and shall not affect the construction or
interpretation of this Debenture.

15.            Invalidity, etc. Each of the provisions contained in this 
Debenture is distinct and severable and a declaration of invalidity, illegality
or unenforceability of any such provision or part thereof by a court of
competent jurisdiction shall not affect the validity or enforceability of any
other provision of Debenture.

16.            Waiver of Certain Legislation.  The Company hereby declares and
covenants that:

    (a)      THE LAND CONTRACTS (ACTIONS) ACT (Saskatchewan) shall have no
             application to any action as defined in THE LAND CONTRACTS
             (ACTIONS) ACT with respect to this Debenture, any of the Mortgaged
             Premises or any supplemental or collateral security granted to
             RoyNat; and

    (b)      THE LIMITATION OF CIVIL RIGHTS ACT (Saskatchewan) shall have no
             application to: (i) this Debenture; (ii) any indenture, instrument
             or agreement entered into by the Company, at any time hereafter,
             supplemental, collateral or ancillary hereto or in implementation
             of this Debenture or the Offer of Finance and involving the
             payment by the Company of money or the liability of the Company to
             pay money; (iii) any mortgage, charge or other security for the
             payment of the money made, given or created by this Debenture;
             (iv) any instrument or agreement entered into by the Company at
             any time hereafter, renewing or extending or collateral to this
             Debenture or any other security given to RoyNat by the Company; or
             (v) the rights, powers or remedies of RoyNat or a Receiver or any
             other person under this Debenture or under any other security
             granted by the Company to RoyNat or instrument or agreement
             collateral, supplemental or ancillary hereto or referred to in
             this Debenture; and

17. (a)      Right of Consolidation. The right of consolidation shall apply
             to this Debenture notwithstanding Section 27 of the PROPERTY LAW
             ACT of British Columbia or any similar statutory provision in force
             from time to time.


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    (b)      For the purposes of Section 198.1 of the LAND TITLE ACT (British
             Columbia), the floating charge hereby created over the Borrower's
             real and immovable property, both freehold and leasehold, shall
             become a fixed charge thereon upon the earlier of:

             (i)   the occurrence of an event described in clause 9(e), (f) or
                   (g) of Schedule "A" hereto; or

             (ii)  RoyNat taking any action pursuant to clause 10 of Schedule
                   "A" hereto to enforce and realize on the security hereby
                   constituted.


               IN WITNESS WHEREOF the Company has executed this Debenture.

                                    VENTURE SEISMIC LTD.



                                    By:   /s/ BRIAN KOZUN
                                          ---------------------------
                                          Name:  BRIAN KOZUN
                                          Title: President & CEO
      c/s


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                                  SCHEDULE "A"

1.             THE COMPANY HEREBY DECLARES, COVENANTS AND AGREES THAT IT:

    (a)  As to Title - is the sole legal and beneficial owner of the Mortgaged
         Premises and owns the same free of all encumbrances other than
         Permitted Encumbrances;

    (b)  Future Acquisitions - shall at its expense on the request of RoyNat,
         execute and deliver to RoyNat such further assurances and documents as
         RoyNat may require to perfect RoyNat's security on all or any part of
         the Mortgaged Premises, or to specifically charge any or all of the
         property then subject to the floating charge created hereby;

    (c)  Pay Costs - shall pay all costs and expenses (including legal fees on a
         solicitor and own client basis) of RoyNat incidental to or which in any
         way relates to this Debenture or its enforcement including:

         (i)    the preparation, execution and filing of this Debenture and any
                instruments postponing, discharging, amending, extending or
                supplemental to this Debenture or any security required by the
                Offer of Finance ("RoyNat's Security");

         (ii)   perfecting and keeping perfected RoyNat's Security;

         (iii)  maintaining the intended priority of RoyNat's Security on all
                or any part of the Mortgaged Premises;

         (iv)   taking, recovering or possessing the Mortgaged Premises;

         (v)    taking any actions or other proceedings to enforce the remedies
                provided herein or otherwise in relation to this Debenture or
                the Mortgaged Premises, or by reason of a default hereunder or
                the non-payment of the moneys hereby secured;

         (vi)   taking proceedings, giving notices and giving responses required
                under any applicable law concerning or relating to RoyNat's
                Security, including compliance with the provisions of applicable
                bankruptcy, insolvency, personal property security and mortgage
                enforcement legislation;

         (vii)  any inspections required to be made to the Mortgaged Premises,
                or the review of any plans, specifications or other
                documentation which may require the approval or consent of
                RoyNat;

         (viii) obtaining the advice of counsel and other advisors in relation
                to the foregoing; all such costs and expenses and other monies
                payable hereunder, together with interest at the Loan Rate,
                shall be payable on demand and shall upon being incurred by
                RoyNat be secured hereby and constitute a charge on the
                Mortgaged Premises and any proceeds of realization;


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    (d)  To Pay Rents and Taxes - shall pay all rents, taxes and assessments
         lawfully imposed upon the Mortgaged Premises or any part thereof and
         upon the income and profits of the Company when the same become due and
         payable, shall show to RoyNat on request receipts for such payment, and
         shall not suffer any builders', construction, statutory or other liens
         or rights of retention, other than Permitted Encumbrances, to remain
         outstanding upon any of the Mortgaged Premises;

    (e)  To Maintain Corporate Existence and Security  - shall maintain its
         corporate existence, shall keep the Mortgaged Premises in good
         condition and repair, shall maintain the security hereby created as a
         valid and effective security at all times so long as any moneys are
         outstanding hereunder, shall carry on and conduct its business in a
         proper and efficient manner and in accordance with all applicable law,
         shall not materially alter the kind of business carried on by it, shall
         advise RoyNat promptly in writing of any proposed change in its name,
         shall observe and perform all of its obligations under leases, licences
         and other agreements to which it is a party so as to preserve and
         protect the Mortgaged Premises and the income therefrom, and shall keep
         proper books of accounts with correct entries of all transactions in
         relation to its business;

    (f)  Not to Sell  - shall not, except as otherwise permitted hereunder,
         remove, destroy, lease, sell or otherwise dispose of any of the
         Mortgaged Premises; provided that the Company may sell or otherwise
         dispose of furniture, machinery, equipment, vehicles and accessories
         which have become worn out or damaged or otherwise unsuitable for their
         purposes on condition that it shall substitute therefor, subject to the
         lien hereof and free from prior liens or charges, property of equal
         value so that the security hereby constituted shall not thereby be in
         any way reduced or impaired;

    (g)  To Hold Proceeds of Unauthorized Sale in Trust - in the event the
         Mortgaged Premises or any part thereof are sold or disposed of prior to
         the full discharge of this Debenture by RoyNat, in any manner not
         authorized by this Debenture, shall hold all proceeds of such sale or
         disposition received by the Company as trustee for RoyNat until the
         Company has been fully released from this Debenture by RoyNat;

    (h)  To Insure  - shall keep insured the Mortgaged Premises with reputable
         insurers approved by RoyNat in such amounts against loss or damage by
         fire and other causes or perils as RoyNat may reasonably require and
         shall pay all premiums necessary for such purposes as the same shall
         become due.  All policies of insurance issued in respect of the
         Mortgaged Premises and all proceeds thereof are hereby assigned to
         RoyNat as security for the Company's obligations hereunder.  Each
         policy of insurance shall show RoyNat as loss payee, as its interest
         may appear, shall contain such mortgage clauses as RoyNat may require,
         shall be in terms satisfactory to RoyNat and, at the request of RoyNat,
         shall be delivered to and held by RoyNat;

    (i)  To Furnish Proofs - shall forthwith on the happening of any loss or
         damage furnish at its expense all necessary proofs and do all necessary
         acts to enable RoyNat to obtain payment of the insurance moneys;


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    (j)  Inspection by RoyNat  - shall allow any employees or authorized agents
         of RoyNat at any reasonable time to enter the premises of the Company
         to inspect the Mortgaged Premises, including without limitation the
         right to undertake soil, ground water, environmental or other tests,
         measurements or surveys in, on or below the Mortgaged Premises, and to
         inspect the books and records of the Company and make extracts
         therefrom, and shall permit RoyNat prompt access to such other persons
         as RoyNat may deem necessary or desirable for the purposes of
         inspecting or verifying any matters relating to any part of the
         Mortgaged Premises or the books and records of the Company, provided
         that any information so obtained shall be kept confidential, save as
         required by RoyNat in exercising its rights hereunder.  The Company
         shall pay all costs and expenses of agents retained by RoyNat for
         purposes of inspection under this clause (j);

    (k)  Deliver Financial Statements  - shall deliver to RoyNat within 120 days
         of the close of each financial year of the Company one copy of the
         audited annual financial statements for that year, including the
         balance sheet and statements of income, retained earnings and changes
         in financial position accompanied by the report of the Company's
         auditors, and such other statements or reports as may be required by
         RoyNat in the Offer of Finance, and within 45 days after the first half
         of each of the Company's financial years, one copy of the interim
         financial statements signed by an authorized officer of the Company,
         all of which financial statements shall be prepared in accordance with
         generally accepted accounting principles; and shall at the same time
         deliver to RoyNat copies of all management reports prepared by the
         auditors of the Company together with any other statements stipulated
         in the Offer of Finance;

    (l)  Not to Create Certain Charges  - without the prior written consent of
         RoyNat, which consent shall not be unreasonably withheld, shall not
         create or suffer to exist any charge or encumbrance over all or any
         portion of the Mortgaged Premises ranking or purporting to rank prior
         to or pari passu with the charges hereof, other than Permitted
         Encumbrances, and shall not permit any subsidiary to mortgage, charge
         or otherwise encumber any of its property or assets or issue any bonds,
         debentures, shares or other securities, except to the Company;

    (m)  Not to Remove  - prior to the removal of any of the Mortgaged Premises
         from the Provinces of Alberta, British Columbia, Saskatchewan or
         Manitoba, the Northwest Territories or the States of Texas, Louisiana,
         Arkansas, Alabama, Mississippi, Montana or North Dakota or to leasehold
         property, the Company shall effect such further registrations and
         obtain such other consents and give such other security, at the sole
         cost and expense of the Company, as may be required or desirable to
         protect or preserve the security hereby created, and the Company shall
         forthwith notify RoyNat of the intended removal and the action proposed
         to be taken;

    (n)  No Actions - has received no notice of and has no knowledge of any
         pending, potential or threatened litigation or claim for judicial or
         administrative action which would adversely affect the Mortgaged
         Premises or their use or market value;

    (o)  Compliance with Environmental Laws -


<PAGE>   10

         (i)    shall conduct and maintain its business, operations and the
                Mortgaged Premises so as to comply in all respects with all
                applicable Environmental Laws, including obtaining all necessary
                licenses, permits, consents and approvals required to own or
                operate the Mortgaged Premises and the business carried out on,
                at or from the Mortgaged Premises;

         (ii)   except as specifically permitted by RoyNat in writing, the
                Company shall not permit or suffer to exist, Contaminants or
                dangerous or potentially dangerous conditions in, on or below
                the Mortgaged Premises including, without limitation, any
                polychlorinated biphenyls, radio-active substances, underground
                storage tanks, asbestos or urea formaldehyde foam insulation;

         (iii)  has no knowledge of the existence of Contaminants or dangerous
                or potentially dangerous conditions at, on or under the
                Mortgaged Premises or any properties in the vicinity of the
                Mortgaged Premises which could affect the Mortgaged Premises or
                the market value thereof;

         (iv)   has not given or received, nor does it have an obligation to
                give, any notice, claim, communication or information regarding
                any past, present, planned or threatened treatment, storage,
                disposal, presence, release or spill of any Contaminant at, on,
                under or from the Mortgaged Premises or any property in the
                vicinity of the Mortgaged Premises, including any notice
                pursuant to any Environmental Laws or any environmental report
                or audit.  The Company shall notify RoyNat promptly and in
                reasonable detail upon receipt of any such claim, notice,
                communication or information or if the Company becomes aware of
                any violation or potential violation of the Company of any
                Environmental Laws and shall describe therein the action which
                the Company intends to take with respect to such matter;

         (v)    upon the request of RoyNat shall establish and maintain a system
                to assure and monitor continued compliance with, and to prevent
                the contravention of, Environmental Laws, which system shall
                include periodic reviews of such compliance system;

         (vi)   shall at the Company's expense, if reasonably requested by
                RoyNat in writing, retain an environmental consultant acceptable
                to RoyNat, acting reasonably, to undertake environmental tests
                and to prepare a report or audit with respect to the Mortgaged
                Premises and deliver same to RoyNat for its review; and

         (vii)  shall indemnify and save harmless RoyNat, its officers,
                directors, employees, agents and shareholders from and against
                all losses, damages or costs suffered by RoyNat arising from or
                relating to any breach by the Company of the foregoing
                covenants in this clause 1(o) and any breach by the Company or
                any other person now or hereafter having an interest in the
                Mortgaged Premises which is asserted or claimed against
                RoyNat.  This indemnity shall survive the 


<PAGE>   11

                payment in full of all amounts secured hereunder and the 
                discharge of this Debenture. RoyNat shall hold the benefit of
                this indemnity in trust for those indemnified persons who are
                not parties to this Debenture.

2.             Waiver of Covenants. RoyNat may waive any breach by the Company 
of any of the provisions contained in this Debenture or any default by the
Company in the observance or performance of any covenant or condition required
to be observed or performed by the Company hereunder, provided that no such
waiver or any other act, failure to act or omission by RoyNat shall extend to or
be taken in any manner to affect any subsequent breach or default or the rights
of RoyNat resulting therefrom.

3.             Performance of Covenants by RoyNat. If the Company shall fail to
perform any covenant on its part herein contained, RoyNat may in its absolute
discretion perform any such covenant capable of being performed by it. If any
such covenant requires the payment of money or if the Mortgaged Premises or any
part thereof shall become subject to any charge ranking in priority to the lien
hereof, RoyNat may make such payment and/or pay or discharge such charge, but
shall be under no obligation to do so. All sums so paid by RoyNat, together with
interest at the Loan Rate, shall be payable on demand and shall constitute a
charge upon the Mortgaged Premises. No such performance or payment shall relieve
the Company from any default hereunder or any consequences of such default.

4.             Appointment of Monitor. If in the opinion of RoyNat, acting 
reasonably, a material adverse change has occurred in the financial condition of
the Company, or if RoyNat in good faith believes that the ability of the Company
to pay any of its obligations to RoyNat or to perform any other covenant
contained herein has become impaired or if an event of default has occurred,
RoyNat may by written notice to the Company, appoint a monitor (the "Monitor")
to investigate any or a particular aspect of the business and affairs of the
Company for the purpose of reporting to RoyNat. The Company shall give the
Monitor its full co-operation, including full access to facilities, assets and
records of the Company and to its creditors, customers, contractors, officers,
directors, employees, auditors and agents. The Monitor shall not exercise any
decision making or other management responsibilities in connection with the
affairs of the Company. The Monitor shall have no responsibility for the affairs
of the Company nor shall it participate in the management of the Company's
affairs. The Monitor shall act solely on behalf of RoyNat and shall have no
contractual relationship with the Company as a consultant or otherwise. The
appointment of a Monitor shall not be regarded as an act of enforcement of this
Debenture. All reasonable fees and expenses of the Monitor shall be paid by the
Company upon submission to it of a written invoice therefor.

5.             Application of Insurance Proceeds. Any insurance moneys received 
by RoyNat pursuant to clause 1(i) above may at the option of RoyNat be applied
to rebuilding or repairing the Mortgaged Premises, or be paid to the Company, or
any such moneys or any insurance moneys received by RoyNat upon the death of any
person whose life is insured under any policy of insurance assigned to RoyNat as
security for the obligations of the Company hereunder may be applied in the sole
discretion of RoyNat, in whole or in part, to the repayment of the principal
amount hereby secured or any part thereof whether then due or not, with any
partial payments to be credited against principal instalments payable hereunder
in inverse order of their maturity dates.


<PAGE>   12

6.             No Merger or Novation. The taking of any judgment or the exercise
of any power of seizure or sale shall not operate to extinguish the liability of
the Company to perform its obligations hereunder or to pay the moneys hereby
secured, shall not operate as a merger of any covenant herein contained or
affect the right of RoyNat to interest at the Loan Rate in effect from time to
time hereunder, and the acceptance of any payment or other security shall not
constitute or create any novation. The execution and delivery of this Debenture
or of any instruments or documents supplemental hereto shall not operate as a
merger of any representation, warranty, term, condition or other provision
contained in any other obligation or indebtedness of the Company to RoyNat or
under the Offer of Finance.

7.             Security in Addition. The security hereby constituted is in 
addition to any other security now or hereafter held by RoyNat. The taking of
any action or proceedings or refraining from so doing, or any other dealings
with any other security for the moneys secured hereby, shall not release or
affect the charges created hereby.

8.             Partial Discharges. RoyNat may in its sole discretion grant 
partial discharges in respect of any of the Mortgaged Premises on such terms and
conditions as it shall deem fit and no such partial discharges shall affect the
remainder of the security constituted hereby nor shall it alter the obligations
of the Company hereunder.

9.             Events of Default. The whole of the principal balance remaining 
unpaid together with interest and all other moneys secured by this Debenture
shall, at the option of RoyNat, become immediately due and payable and the
security hereby created shall become enforceable in each of the following events
(each event being herein called an "event of default"):

  (a) if the Company defaults in payment of the principal of or interest on this
      Debenture or on any other indebtedness of the Company to RoyNat when the
      same becomes due;

  (b) if the Company defaults in the performance or observance of any of the
      covenants contained in clause 9(b) of the Debenture or in clauses 1(f),
      1(j), 1(l) or 1(m) of this Schedule or section 4 of this Schedule;

  (c) if the Company defaults in the performance or observance of any other
      covenant or condition herein contained and such default shall continue for
      15 days after written notice thereof to the Company by RoyNat;

  (d) if there is any material misrepresentation or misstatement contained in
      any certificate or document delivered by an officer or director of the
      Company in connection with this Debenture;

  (e) if the Company institutes any proceeding or takes any corporate action or
      executes any agreement or notice of intention to authorize its
      participation in or commencement of any proceeding (i) seeking to
      adjudicate it a bankrupt or insolvent, or (ii) seeking liquidation,
      dissolution, winding up, reorganization, arrangement, protection, relief
      or composition of it or any of its property or debt or making a proposal
      with respect to it under any law relating to bankruptcy, insolvency,
      reorganization or compromise of debts or other similar laws


<PAGE>   13

      (including, without limitation, any application under the COMPANIES' 
      CREDITORS ARRANGEMENT ACT or any reorganization, arrangement or compromise
      of debt under the laws of its jurisdiction of incorporation);

  (f) if any proceeding is commenced against or affecting the Company:

      (i)     seeking to adjudicate it a bankrupt or insolvent;

      (ii)    seeking liquidation, dissolution, winding up, reorganization,
              arrangement, protection, relief or composition of it or any of its
              property or debt or making a proposal with respect to it under any
              law relating to bankruptcy, insolvency, reorganization or
              compromise of debts or other similar laws (including, without
              limitation, any reorganization, arrangement or compromise of debt
              under the laws of its jurisdiction of incorporation); or

      (iii)   seeking appointment of a receiver, trustee, agent, custodian or
              other similar official for it or for any part of its properties
              and assets, including the Mortgaged Premises or any part thereof;

  (g) if an encumbrancer or secured creditor shall appoint a receiver or agent
      over any part of the Mortgaged Premises, or take possession of any part of
      the Mortgaged Premises or if any execution, distress or other process of
      any court becomes enforceable against any of the property of the Company,
      or a distress or like process is levied upon any of such property unless
      such distress or process is being diligently and with merit defended;

  (h) if the Company takes any corporate proceedings for its dissolution,
      liquidation or amalgamation with another company or if the corporate
      existence of the Company shall be terminated by expiration, forfeiture or
      otherwise;

  (i) if a default occurs under any agreement supplemental hereto or under any
      other security previously, now or hereafter granted to RoyNat by the
      Company or any guarantor of the obligations of the Company or should any
      party to any agreement supplemental or collateral hereto fail to carry out
      or observe any covenant or condition on its part to be observed or
      performed and such default continues for 15 days after written notice
      thereof to the Company by RoyNat;

  (j) if Voting Control of the Company shall change without the prior written
      consent of RoyNat.

10.            Enforcement.  Upon the happening of any event of default, RoyNat
shall have the following rights, powers and remedies in addition to any other
rights, powers and remedies which otherwise may be available to it in law:

  (a) to enter upon and take possession of all or any part of the Mortgaged
      Premises;

  (b) to preserve and maintain the Mortgaged Premises and make such
      replacements thereof and additions thereto as it shall deem advisable;


<PAGE>   14

      (c) to exercise all powers necessary to the performance of all functions
          provided for herein including without limitation the powers to 
          purchase on credit, to borrow money in the Company's name or in its 
          own name and to advance its own money to the Company at such rates 
          of interest as it may deem reasonable;

      (d) to sell, for cash or credit or part cash and part credit, lease or 
          dispose of all or any part of the Mortgaged Premises whether by 
          public auction or by private sale or lease in such manner as RoyNat 
          may determine, provided that it shall not be incumbent on RoyNat to 
          sell, lease or dispose of the said property but that it shall be 
          lawful for RoyNat peaceably to use and possess the same without 
          hindrance or interruption by the Company, or any other person or 
          persons whomsoever, and to receive income from such property and to 
          convey, transfer and assign to a purchaser or purchasers the title 
          to any undertaking, property and assets so sold and provided further 
          that in the case of a sale on credit RoyNat shall only be liable to 
          account to the Company, any subsequent encumbrancers and others for
          moneys actually received by RoyNat; and

      (e) to appoint by instrument any person or persons to be a Receiver of 
          all or any portion of the undertaking, property and assets hereby 
          charged, to fix the Receiver's remuneration and to remove any 
          Receiver so appointed and appoint another or others in his stead.

11.            Powers of Receiver.

  (1) Any Receiver shall have all of the powers of RoyNat set out in Section 10
      of this Schedule and, in addition, shall have the following powers:

      (a) to carry on the business of the Company and to enter into any 
          compromise or arrangement on behalf of the Company; and

      (b) with the prior written consent of RoyNat to borrow money in his name 
          or in the Company's name, for the purpose of carrying on the 
          business of the Company and for the preservation and realization of 
          the undertaking, property and assets of the Company including, 
          without limitation, the right to pay persons having prior charges or 
          encumbrances on properties on which the Company may hold charges or 
          encumbrances, with any amount so borrowed and any interest thereon 
          to be a charge upon the Mortgaged Premises in priority to this 
          Debenture;

  (2) Any Receiver appointed pursuant to the provisions hereof shall be deemed
      to be an agent of the Company for the purposes of:

      (a) carrying on and managing the business and affairs of the Company, and

      (b) establishing liability for all of the acts or omissions of the 
          Receiver while acting in any capacity hereunder and RoyNat shall not 
          be liable for such acts or omissions,


<PAGE>   15

provided that, without restricting the generality of the foregoing, the Company
irrevocably authorizes RoyNat to give instructions to the Receiver relating to
the performance of its duties as set out herein.

12.            Application of Moneys.  All moneys actually received by RoyNat 
or by the Receiver pursuant to Sections 10 and 11 of this Schedule shall be 
applied:

  (a) first, in payment of claims, if any, of secured creditors of the Company,
      including any claim of the Receiver pursuant to clause 11(1), ranking in
      priority to the charges created by this Debenture;

  (b) second, in payment of all costs, charges and expenses of and incidental to
      the appointment of the Receiver (including legal fees on a solicitor and
      its own client basis) and the exercise by the Receiver or RoyNat of all or
      any of the powers granted to them under this Debenture, including the
      reasonable remuneration of the Receiver or any agent or employee of the
      Receiver or any agent of RoyNat and all outgoings properly paid by the
      Receiver or RoyNat in exercising their powers as aforesaid;

  (c) third, in or towards the payment to RoyNat of all moneys due to it by the
      Company in such order as RoyNat in its sole discretion may determine; and

  (d) fourth, subject to applicable law, any surplus shall be paid to the
      Company.

13.            Restriction on Company and its Officers and Directors. Upon the 
Company receiving notice from RoyNat of the taking of possession of the
Mortgaged Premises or the appointment of a Receiver, all the powers, functions,
rights and privileges of each of the directors and officers of the Company with
respect to the properties, business and undertaking of the Company shall cease
unless specifically continued by the written consent of RoyNat.

14.            Discharge and Satisfaction. Upon payment by the Company to RoyNat
of all moneys hereby secured, these presents shall cease and become null and
void and the Mortgaged Premises shall revest in the Company without any
acknowledgement or formality, but RoyNat shall upon the request and at the
expense of the Company, execute and deliver to the Company a full release and
discharge.

15.            No Obligation to Advance.  Neither the issue and delivery of this
Debenture nor the advance of any funds hereunder shall obligate RoyNat to
advance any further funds hereunder.

16.            Limited Power of Attorney. The Company hereby appoints RoyNat as 
the Company's attorney, with full power of substitution, in the name and on
behalf of the Company, to execute, deliver and do all such acts, deeds, leases,
documents, transfers, demands, conveyances, assignments, contracts, assurances,
consents, financing statements and things as the Company has agreed to execute,
deliver and do hereunder, under the Offer of Finance or otherwise, or as may be
required by RoyNat or any Receiver to give effect to this Debenture or in the
exercise of any rights, powers or remedies hereby conferred on RoyNat or any
Receiver, and generally to use the name of the Company in the exercise of all or
any of the rights, powers or remedies hereby conferred on RoyNat or any
Receiver. This appointment, being coupled with an interest, shall not be revoked
by the insolvency, bankruptcy, dissolution, liquidation or other termination of
the existence of the Company or for any other reason.


<PAGE>   16

17.      Interpretation.  As used herein the following expressions shall have
the following meanings:

  (a) "Applicable Period", with respect to any Interest Period, means the period
      commencing on the 8th day of the month in which such Interest Period
      commences and ending on the 7th day of the following month, except that if
      the rate of interest hereunder is being determined:

      (i)     for the purpose of redemption by the Company, the Applicable
              Period shall end on the 7th day preceding the redemption date; or

      (ii)    for any other purpose, other than the payment of interest on the
              day following an Interest Period, the Applicable Period shall end
              on the day preceding the day on which the rate is being determined
              and the following Applicable Period shall commence on such day.

  (b) "Business Day" means any day except Saturday, Sunday or a statutory
      holiday.

  (c) "Contaminant" means any solid, liquid, gas, odour, heat, sound, smoke,
      waste, vibration, radiation or combination of any of them resulting
      directly or indirectly from human activities that may cause: (i)
      impairment of the quality of the natural environment for any use that can
      be made of it, (ii) injury or damage to property or to plant or animal
      life, (iii) harm or material discomfort to any person, (iv) an adverse
      affect on the health of any person, (v) impairment of the safety of any
      person, (vi) rendering any property or plant or animal life unfit for use
      by man, (vii) loss of enjoyment of normal use of property, or (viii)
      interference with the normal conduct of business, and includes any
      pollutant or contaminant as defined in any applicable Environmental Laws
      and any biological, chemical or physical agent which is regulated,
      prohibited, restricted or controlled.

  (d) "Environmental Laws" means the common law and all applicable federal,
      provincial, local, municipal, governmental or quasi-governmental laws,
      rules, regulations, licences, orders, permits, decisions or requirements
      concerning Contaminants, occupational or public health and safety or the
      environment and any other order, injunction, judgment, declaration, notice
      or demand issued thereunder;

  (e) "Interest Period" means each monthly period commencing on the 15th day of
      a month and ending on the 14th day of the following month.

  (f) "Inventory" means property of the Company held for sale including products
      purchased for resale, finished goods, work in process and raw materials
      but not including any property not intended to be directly incorporated in
      finished goods or products to be sold.

  (g) "Loan Rate" means the rate of interest specified in Section 3 of the
      Debenture.


<PAGE>   17

  (h) "Receiver" shall include one or more of a receiver, receiver-manager or
      receiver and manager of all or a portion of the undertaking, property and
      assets of the Company appointed by RoyNat pursuant to this Debenture.

  (i) "RoyNat" means RoyNat Inc., its successors and assigns and, where
      applicable, includes those for whom it acts as nominee or agent.

  (j) "RoyNat's Floating Base Rate for the Applicable Period" means, with
      respect to any Applicable Period, the arithmetic average (rounded to three
      decimal places) of the annual rate of interest which is the rate
      determined as being the arithmetic average of the "BA 1 month" rate
      applicable to Canadian Dollar bankers' acceptances displayed and
      identified as such on the "Reuters' Screen CDOR Page" (as defined in the
      International Swap Dealer Association, Inc. definitions, as modified and
      amended from time to time) as at approximately 10:00 a.m. (Toronto,
      Ontario time) on each Business Day during such Applicable Period, plus
      .50%; provided, however, if such rate does not appear on the Reuters'
      Screen CDOR Page as contemplated on any Business Day during such
      Applicable Period, then the rate on such Business Day shall be the prime
      lending rate of The Bank of Nova Scotia as at approximately 10:00 a.m.
      (Toronto, Ontario time) on such Business Day.

  (k) "Voting Control" means the ownership of a sufficient number of outstanding
      shares of a corporation to elect a majority of its directors; and "Voting
      Control of the Company" means the Voting Control of the Company stated in
      the Offer of Finance or such different Voting Control as shall have been
      effected with the prior written consent of RoyNat.

  (l) "Working Capital" of a company means the excess of its current assets over
      its current liabilities calculated in accordance with generally accepted
      accounting principles with any dissent as to the calculation thereof being
      conclusively resolved by RoyNat; and "Consolidated Working Capital" means
      the Working Capital of the Company and all its subsidiaries calculated on
      a consolidated basis.

  (m) "Permitted Encumbrances" means any of the following:

      (i)     liens for taxes, assessments, governmental charges or levies not
              at the time due;

      (ii)    easements, rights of way or other similar rights in land existing
              at the date of this Debenture which in the aggregate do not
              materially impair the usefulness in the business of the Company of
              the property subject thereto;

      (iii)   rights reserved to or vested in any municipality or governmental
              or other public authority by the terms of any lease, licence,
              franchise, grant or permit, or by any statutory provision, to
              terminate the same or to require annual or other periodic payments
              as a condition to the continuance thereof;

      (iv)    any lien or encumbrance the validity of which is being contested
              by the Company in good faith and in respect of which either there
              shall have been deposited with 


<PAGE>   18

              RoyNat cash in an amount sufficient to satisfy the same or RoyNat
              shall be otherwise satisfied that its interests are not prejudiced
              thereby;

      (v)     any reservations, limitations, provisos and conditions expressed
              in any original grant from the Crown;

      (vi)    title defects or irregularities which, in the opinion of counsel
              to RoyNat, are of a minor nature and in the aggregate shall not
              materially impair the usefulness in the business of the Company of
              the property subject thereto; and

      (vii)   validly perfected security given by the Company to its bankers on
              its Inventory or under assignments of its accounts receivable,
              except to the extent that such accounts receivable represent
              proceeds of the sale or disposition of property specifically
              charged, mortgaged or assigned under this Debenture or any
              security supplemental hereto;

      (viii)  builder's carrier's, warehouseman's mechanic's or other similar
              liens incidental to construction or operations which have not at
              such time been filed pursuant to law and relate to obligations not
              due or delinquent or related to obligations being contested at the
              time in good faith by the Borrower, provided that the Borrower has
              posted security therefor with the Lender or with the lienholder in
              an amount sufficient to discharge same;

      (ix)    purchase money Security Interests, upon or in any property
              acquired or held by the Borrower in the ordinary course of
              business, to secure the purchase price of such property or to
              secure indebtedness incurred solely for the purpose of financing
              the acquisition of such property and Security Interests existing
              on such property at the time of its acquisition (other than any
              such Security interest created in contemplation of such
              acquisition); provided, however, that the aggregate principal
              amount of the indebtedness secured by the Security Interests
              referred to in this paragraph (ix) shall not exceed $500,000 in
              the aggregate in any fiscal year.

      (x)     Security Interests incurred or, trusts or deposits arising in
              connection with workers' compensation, unemployment insurance
              pension, employment or other social benefit laws or regulations or
              any judgment under such laws or regulations; provided however, (i)
              the Borrower' obligations thereunder are not at the time due or
              delinquent, or (ii) the validity of which is being contested by
              the Borrower in good faith and by appropriate proceedings,
              provided that no execution in respect of such Security Interest,
              trust or deposit has been initiated or, if initiated, such
              execution has been stayed, and (iii) all such failures in the
              aggregate have no material adverse effect on the financial
              condition of the Borrower;

      (xi)    Security Interests securing the performance of bids, tenders,
              leases, contracts (other than for the repayment of borrowed
              money), statutory obligations, surety and appeal bonds and
              performance bonds and other obligations of like nature, incurred
              as an incident to and in the ordinary course of business, and
              judgment liens,


<PAGE>   19

              provided, however, that all such Security Interests (i) in the
              aggregate have no material adverse effect on the financial 
              condition of the Borrower and (ii) do not secure directly or 
              indirectly judgments in excess of $50,000; and

      (x)     Security interests previously granted to RoyNat Inc.


<PAGE>   20

                                  SCHEDULE "B"



to the Debenture of Venture Seismic Ltd.

dated the 29th day of April, 1997


  1 -- Central Recording Station (CRS) with 2 (3480 Cartridge Drives)

 70 -- Acquisition Remote Units (SAR) without antenna bags

  4 -- RF Blasters

  3 -- Battery Chargers

250 -- 3 @ 31-1/4 meter Opseis cables

  2 -- Sets of manuals

  1 -- Power Unit Capacity Tester

  6 -- Deployment Aid Model 710