<PAGE>

                                                                   EXHIBIT 10.18

                                 PROMISSORY NOTE

$3,040,000.00                     Houston, Texas              September 10, 2003

        FOR VALUE RECEIVED, the undersigned, OYOG OPERATIONS, LP, a Texas
limited partnership ("Borrower"), hereby promises to pay to the order of COMPASS
BANK an Alabama state banking corporation ("Lender"), at its designated office,
in lawful money of the United States of America, the principal sum of THREE
MILLION FORTY THOUSAND AND NO/100 DOLLARS ($3,040,000.00), together with
interest thereon at the rate set forth below.

        The outstanding principal balance hereof shall bear interest prior to
maturity at a varying rate per annum which shall from day to day be equal to the
lesser of (a) the Maximum Rate (hereinafter defined) or (b) the greater of (i)
the Libor Rate (hereinafter defined) in effect from day to day plus two percent
(2%) or (ii) three and three-fourths percent (3.75%) per annum, and each change
in the rate of interest charged hereunder shall become effective, without notice
to Borrower, on the effective date of such change in the Libor Rate or the
Maximum Rate, as the case may be; provided, however, if at any time the rate of
interest specified in clause (b) preceding shall exceed the Maximum Rate,
thereby causing the interest rate hereon to be limited to the Maximum Rate, then
any subsequent reduction in the Libor Rate will not reduce the rate of interest
hereon below the Maximum Rate until the total amount of interest accrued hereon
equals the amount of interest which would have accrued hereon if the rate
specified in clause (b) preceding had at all times been in effect. If an Event
of Default (hereinafter defined) has occurred and is existing, the principal
hereof shall bear interest at the Default Rate (hereinafter defined).

        Interest on the indebtedness evidenced by this Note shall be computed on
the basis of a year of 360 days and the actual number of days elapsed (including
the first day but excluding the last day) unless such calculation would result
in a usurious rate in which case interest shall be calculated on the basis of a
year of 365 or 366 days, as the case may be.

        Principal of and interest on this Note shall be due and payable in
eighty-four (84) installments as follows:

                (a) Three (3) monthly installments in the amount of accrued and
        unpaid interest on this Note shall be payable on the first of each month
        commencing October 1, 2003 and ending on December 1, 2003.

                (b) Eighty (80) monthly installments in the combined interest
        and principal amount of Twenty-Two Thousand One Hundred Seven and 56/100
        Dollars ($22,107.56) each shall be due and payable on the first day of
        each month, commencing January 1, 2004 until and including August 1,
        2010; and

                (c) A final installment in the amount of all outstanding
        principal, plus accrued and unpaid interest, shall be due and payable on
        September 1, 2010.

        At any time that there has been an increase in the Libor Rate, Lender
may reset the monthly combined principal and interest installment amounts set
forth above to an amount which would amortize this Note over a period of fifteen
(15) years (but with a final maturity date on September 1, 2010) taking into
account such increased Libor Rate as of the date of such increase in the Libor
Rate.

        This Note is (i) secured as provided in the Deed of Trust (hereinafter
defined) and (ii) the Assignment of Leases (hereinafter defined).

        Borrower may prepay the principal of and accrued interest on this Note
at any time without premium or penalty, provided that all such prepayments of
principal shall be applied to the principal payments due hereon in inverse order
of their maturities.

<PAGE>

        As used in this Note, the following terms shall have the respective
meanings indicated below:

                "Assignment of Leases" means the Assignment of Lessor's Interest
        in Lease and Security Agreement dated of even date herewith, executed by
        Borrower for the benefit of Lender, as the same may be amended,
        supplemented, or modified from to time.

                "Cooper Indemnity" means that certain Environmental Indemnity
        and Remediation Agreement dated as of September 10, 2003 among Cooper
        Industries, Inc., Borrower and Cooper Power Tools, Inc.

                "Debt" means for any Person (a) all indebtedness, whether or not
        represented by bonds, debentures, notes, securities or other evidences
        of indebtedness, for the repayment of money borrowed, including, with
        respect to Borrower, the indebtedness evidenced by this Note, and all
        other indebtedness of Borrower to Lender, (b) Rate Management
        Transaction Obligations, (c) all indebtedness representing deferred
        payment of the purchase price of property or assets, (d) all
        indebtedness under any lease which, in conformity with GAAP, is required
        to be capitalized for balance sheet purposes, (e) all indebtedness under
        guaranties, endorsements, assumptions or other contingent obligations,
        in respect of, or to purchase or otherwise acquire, indebtedness of
        others, (f) all indebtedness secured by a lien existing on property
        owned, subject to such lien, whether or not the indebtedness secured
        thereby shall have been assumed by the owner thereof, and (g) any
        obligation to redeem or repurchase any of such Person's capital stock,
        partnership or membership interests or other ownership interests as
        applicable.

                "Deed of Trust" means the Deed of Trust, Security Agreement,
        Assignment of Rents and Financing Statement dated of even date herewith,
        executed by Borrower for the benefit of Lender, as the same may be
        amended, supplemented, or modified from to time.

                "Default Rate" means the lesser of (a) the greater of (i) the
        sum of the Libor Rate plus five percent (5%) or (ii) eight percent (8%),
        or (b) the Maximum Rate.

                "Event of Default" each of the following shall constitute and be
        deemed an "Event of Default":

                (a) Borrower shall fail to pay this Note or any installment of
        this Note, whether principal or interest, within three (3) days of when
        due.

                (b) Any material representation or warranty made or deemed made
        by Borrower, Guarantor or any of their respective officers in any
        certificate, report, notice, or financial statement furnished at any
        time in connection with this Note or any Loan Document shall be false,
        misleading, or erroneous in any material respect when made or deemed to
        have been made.

                (c) Borrower or Guarantor, as applicable, shall fail to perform,
        observe, or comply with any covenant, agreement or term contained in
        this Note or any Loan Document and the same shall remain unremedied for
        thirty (30) days after the earlier of (i) knowledge of such failure by
        Borrower or Guarantor, as applicable or (ii) notice of such failure from
        Lender.

                (d) Borrower or Guarantor, as applicable, shall commence a
        voluntary proceeding seeking liquidation, reorganization, or other
        relief with respect to itself or its debts under any bankruptcy,
        insolvency, or other similar law now or hereafter in effect or seeking
        the appointment of a trustee, receiver, liquidator, custodian, or other
        similar official of it or a substantial part of its property or shall
        consent to any such relief or to the appointment of or taking possession
        by any such official in an involuntary case or other proceeding
        commenced against it or shall make a general assignment for the benefit
        of creditors or shall generally fail to pay its debts as they become due
        or shall take any corporate action to authorize any of the foregoing.

                                       -2-

<PAGE>

                (e) An involuntary proceeding shall be commenced against
        Borrower or Guarantor seeking liquidation, reorganization, or other
        relief with respect to it or its debts under any bankruptcy, insolvency,
        or other similar law now or hereafter in effect or seeking the
        appointment of a trustee, receiver, liquidator, custodian or other
        similar official for it or a substantial part of its property, and such
        involuntary proceeding shall remain undismissed and unstayed for a
        period of sixty (60) days.

                (f) Borrower or Guarantor shall fail to pay when due any
        principal of or interest on any debt for borrowed money (other than the
        obligations hereunder), or the maturity of any such debt shall have been
        accelerated, or any such debt shall have been required to be prepaid
        prior to the stated maturity thereof, or any event shall have occurred
        that permits (or, with the giving of notice or lapse of time or both,
        would permit) any holder or holders of such debt or any person acting on
        behalf of such holder or holders to accelerate the maturity thereof or
        require any such prepayment.

                (g) The occurrence or the existence of any default, Event of
        Default or similar condition or event (however described) with respect
        to any Rate Management Transaction

                (h) Any default, Event of Default or similar condition or event
        (however described) under the Cooper Indemnity.

                (i) This Note or any other Loan Document shall cease to be in
        full force and effect or shall be declared null and void or the validity
        or enforceability thereof shall be contested or challenged by Borrower,
        or Borrower shall deny that it has any further liability or obligation
        hereunder prior to payment in full of all obligations hereunder, or the
        lien created by the Deed of Trust shall cease to be a first priority
        lien.

                (j) Borrower or Guarantor shall fail to discharge within a
        period of thirty (30) days after the commencement thereof any
        attachment, sequestration, or similar proceeding or proceedings
        involving an aggregate amount in excess of $50,000.00 against any of its
        assets or properties.

                (k) Borrower or Guarantor shall fail to satisfy and discharge
        promptly any final judgement against it for the payment of money in an
        aggregate amount in excess of $50,000.00.

                "GAAP" means generally accepted accounting principles in the
        United States of America consistently applied.

                "Guarantor" means OYO Geospace Corporation, a Delaware
        corporation.

                "Guaranty" means the Guaranty Agreement executed by Guarantor in
        favor of Lender, as the same may be amended, supplemented, or modified
        from time to time.

                "Libor Rate" means, on any day, the London Interbank Offered
        Rate (Libor) for a one (1) month period as published in The Wall Street
        Journal on that day under the section "Money Rates." If such section of
        The Wall Street Journal reflects more than one rate as being the London
        Interbank Offered Rate (Libor) for a one (1) month period, then the
        highest rate shall be the Libor Rate. On days when The Wall Street
        Journal is not published, the Libor shall be the London Interbank
        Offered Rate (Libor) for a one (1) month period stated in the most
        recently published edition of The Wall Street Journal. In the event The
        Wall Street Journal ceases to be published altogether, or ceases to
        publish the London Interbank Offered Rate (Libor) for a one (1) month
        period, then Lender or its successors or assigns shall establish and use
        a new Libor Rate, in the exercise of its sole discretion, without any
        notice to the Borrower or any person being required. The Libor Rate
        shall automatically fluctuate, upward and downward, without notice to
        Borrower or any other person, as and in the amount the said published
        London Interbank Offered Rate (Libor) for a one (1) month period shall
        fluctuate. The Libor Rate is a reference rate and does not necessarily
        represent Lender's best or lowest rate or a favored rate, and Lender
        disclaims any statement, representation or warranty to the contrary.

                                       -3-

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                "Loan Documents" means this Note and all security agreements,
        deeds of trust, pledge agreements, assignments, letters of credit,
        guaranties, certificates and other instruments, documents, and
        agreements, if any, executed and delivered pursuant to or in connection
        with this Note, as such instruments, documents, and agreements may be
        amended, modified, renewed, extended, or supplemented from time to time.

                "Maximum Rate" means the maximum rate of nonusurious interest
        permitted from day to day by applicable law, including Chapter 303 of
        the Texas Finance Code (the "Code") (and as the same may be incorporated
        by reference in other Texas statutes). To the extent that Chapter 303 of
        the Code is relevant to any holder of this Note for the purposes of
        determining the Maximum Rate, each such holder elects to determine such
        applicable legal rate pursuant to the "weekly ceiling," from time to
        time in effect, as referred to and defined in Chapter 303 of the Code;
        subject, however, to the limitations on such applicable ceiling referred
        to and defined in the Code, and further subject to any right such holder
        may have subsequently, under applicable law, to change the method of
        determining the Maximum Rate.

                "Obligations" means all obligations, indebtedness, and
        liabilities of Borrower to Lender under this Note (including the payment
        of principal and interest hereon), the Rate Management Transaction
        Obligations and the other Loan Documents and all interest accruing
        thereon and all attorneys' fees and other expenses incurred in the
        enforcement or collection thereof.

                "Person" means any individual, corporation, limited liability
        company, business trust, association, company, partnership, joint
        venture, governmental authority, or other entity.

                "Rate Management Transaction" means any transaction (including
        an agreement with respect thereto) now existing or hereafter entered
        into between Borrower and Lender which is a rate swap, basis swap,
        forward rate transaction, commodity swap, commodity option, equity or
        equity index swap, equity or equity index option, bond option, interest
        rate option, foreign exchange transaction, cap transaction, floor
        transaction, collar transaction, forward transaction, currency swap
        transaction, cross-currency rate swap transaction, currency option or
        any other similar transaction (including any option with respect to any
        of these transactions) or any combination thereof, whether linked to one
        or more interest rates, foreign currencies, commodity prices, equity
        prices or other financial measures.

                "Rate Management Transaction Obligations" means any and all
        obligations, contingent or otherwise, whether now existing or hereafter
        arising, of Borrower to Lender arising under or in connection with any
        Rate Management Transaction.

                "Real Property" means the real property and interests in real
        property described in the Deed of Trust, and all improvements and
        fixtures thereon and all appurtenances thereto.

        The proceeds of this Note shall be used for the purchase of land and
building located at 7007 Pinemont, Houston, Texas for use in operations of
Borrower. Borrower may not use the proceeds of this Note for any other purpose
without first obtaining the consent of Lender.

        Borrower agrees to deliver to Lender:

                (a) Financial Statements for Borrower. As soon as available, and
        in any event within one hundred twenty (120) days after the end of each
        fiscal year of Borrower beginning with the fiscal year ending September
        30, 2003, a copy of the annual company prepared financial statements of
        Borrower for such fiscal year containing balance sheets and statements
        of income as at the end of such fiscal year and for the 12-month period
        then ended, all in reasonable detail, prepared in accordance with
        generally accepted accounting principals consistently applied, and
        certified by an officer of Borrower acceptable to Lender.

                                       -4-

<PAGE>

                (b) Financial Statements for Guarantor. As soon as available,
        and in any event within one hundred twenty (120) days after the end of
        each fiscal year of Guarantor, beginning with the fiscal year ending
        September 30, 2003, a copy of the annual audit report of Guarantor for
        such fiscal year containing balance sheets, statements of income and
        statements of cash flows as at the end of such fiscal year and for the
        12-month period then ended, in each case setting forth in comparative
        form the figures for the preceding fiscal year, all in reasonable
        detail, prepared in accordance with generally accepted accounting
        principals consistently applied, and audited and certified without
        qualification by independent certified public accountants of recognized
        standing acceptable to Lender.

                (c) Quarterly Financial Statements for Guarantor. As soon as
        available, and in any event within sixty (60) days after the end of each
        quarter of each fiscal year of Guarantor, a copy of the financial
        statements of Guarantor as of the end of such fiscal quarter and for the
        portion of the fiscal year then ended, containing balance sheets,
        statements of income and statements of cash flows in each case setting
        forth in comparative form the figures for the corresponding period of
        the preceding fiscal year, all in reasonable detail and certified by an
        officer of Guarantor acceptable to Lender to fairly and accurately
        present the financial condition and results of operations of Guarantor
        at the date and for the periods indicated therein.

        Borrower agrees with Lender that Borrower will not, without Lender's
prior written consent, (a) become a party to a merger, consolidation,
partnership or joint venture or purchase or otherwise acquire all or a
substantial part of the assets of any Person or any shares or other evidence of
beneficial ownership of any Person, (b) dissolve or liquidate, (c) sell, lease,
assign, transfer or otherwise dispose of substantially all of its assets, or (d)
enter into any agreement to do any of the foregoing.

        Borrower agrees with Lender that promptly after the commencement
thereof, Borrower will give Lender notice of all actions, suits and proceedings
before any court or governmental department, commission, board, agency or
instrumentality, domestic or foreign, affecting Borrower which could have a
material adverse effect on the financial condition of Borrower.

        Borrower agrees with Lender that Borrower will maintain with financially
sound and reputable insurance companies workmen's compensation insurance,
liability insurance and insurance on its property, assets and business, all at
least in such amounts and against such risks as are usually insured against by
Persons engaged in similar businesses and as required by the Deed of Trust. Each
insurance policy covering the Real Property shall name Lender as lender loss
Lender and provide that such policy will not be cancelled without thirty (30)
days prior written notice to Lender. Borrower will deliver to Lender copies of
all insurance policies required by this paragraph, together with loss payable
endorsements in favor of Lender with respect to all insurance policies covering
the Real Property.

        Borrower agrees with Lender that Borrower will comply in all material
respects with all laws and regulations and all agreements, contracts and
instruments binding on it or affecting its properties or business except where
the failure to comply could not reasonably be expected to have a material
adverse effect on the business or financial condition of Borrower.

        Borrower agrees with Lender that Borrower will pay or discharge at or
before maturity or before becoming delinquent (a) all taxes, levies, assessments
and governmental charges imposed on it or its income or profits or any of its
property and (b) all lawful claims for labor, material and supplies, which, if
unpaid, might become a Lien upon any of its property.

        Borrower agrees with Lender that Borrower will execute and deliver such
further instruments as may be requested by Lender to carry out the provisions
and purposes of this Note and the other Loan Documents and to preserve and
perfect the Liens of Lender in the collateral for this Note.

        In the event that an Event of Default exists and Lender decides to waive
such Event of Default or to forbear from taking action with respect to such
Event of Default, Borrower shall pay to Lender, immediately upon demand by

                                       -5-

<PAGE>

Lender, a waiver fee in an amount determined by Lender in its sole discretion.
The decision by Lender to waive any Event of Default shall be made by Lender in
its sole and absolute discretion, and Lender has no obligation whatsoever to
waive any Event of Default. The provisions of this paragraph shall not affect
Lender's other rights or remedies.

        If any payment of principal or interest on this Note is made ten (10) or
more days past the date when due, Borrower will pay, immediately upon demand by
Lender, and in addition to regularly accruing interest, a delinquency charge in
an amount equal to five percent (5.0%) of the amount which was not paid when
due. The provisions of this paragraph shall not affect Borrower's rights and
remedies arising as a result of such late payment or otherwise.

        Borrower represents and warrants to Lender that (a) Borrower is a Texas
limited partnership, duly organized and validly existing under the laws of the
state of Texas, (b) the execution, delivery and performance of this Note and the
other Loan Documents are within Borrower's powers, have been duly authorized by
all requisite action, and do not and will not contravene its partnership
agreement, other organizational document, any law or any agreement or
undertaking to which it is a party or by which it is bound, (c) this Note is a
legal, binding obligation of Borrower, enforceable against Borrower in
accordance with its terms, (d) there are no claims pending, or to Borrower's
knowledge threatened, which, if adversely determined, would have a material
adverse effect on the financial condition, operations or properties of Borrower,
and (e) except as may have been already obtained, no authorization or consent
of, and no filing or registration with, any court, governmental authority or
third party is or will be necessary for the execution, delivery or performance
by Borrower of this Note and the other Loan Documents.

        All notices and other communications provided for in this Note and the
other Loan Documents shall be in writing and may be telecopied (faxed), mailed
by certified mail return receipt requested, or delivered to the intended
recipient at the addresses specified below or at such other address as shall be
designated by any party listed below in a notice to the other parties listed
below given in accordance with this paragraph.

If to Borrower:                     OYOG Operations, LP
                                    12750 South Kirkwood, Suite 200
                                    Stafford, TX 77477
                                    Attention: Thomas McEntire
                                    Telephone No.: 281-494-8282
                                    Fax No.: 281-494-8310

        which address will change to the following after
        Borrower moves into its new headquarters:

                                    7007 Pinemont Drive
                                    Houston, Texas 77040

If to Lender:                       Compass Bank
                                    24 Greenway Plaza, Suite 1601
                                    Houston, Texas 77046
                                    Attention: Bruce Mercer
                                    Telephone No.: 713.966.2301
                                    Fax No.: 713.966.2388

Except as otherwise provided in this Note or any Loan Document, all such
communications shall be deemed to have been duly given when transmitted by
telecopy (fax), subject to confirmation of receipt, when personally delivered
or, in the case of a mailed notice, when duly deposited in the mails, in each
case given or addressed as aforesaid.

        Notwithstanding anything to the contrary contained herein, no provisions
of this Note shall require the payment or permit the collection of interest in
excess of the Maximum Rate. If any excess of interest in such respect is herein
provided for, or shall be adjudicated to be so provided, in this Note or
otherwise in connection with this loan transaction,

                                       -6-

<PAGE>

the provisions of this paragraph shall govern and prevail, and neither Borrower
nor the sureties, guarantors, successors or assigns of Borrower shall be
obligated to pay the excess amount of such interest, or any other excess sum
paid for the use, forbearance or detention of sums loaned pursuant hereto. If
for any reason interest in excess of the Maximum Rate shall be deemed charged,
required or permitted by any court of competent jurisdiction, any such excess
shall be applied as a payment and reduction of the principal of indebtedness
evidenced by this Note; and, if the principal amount hereof has been paid in
full, any remaining excess shall forthwith be paid to Borrower. In determining
whether or not the interest paid or payable exceeds the Maximum Rate, Borrower
and Lender shall, to the extent permitted by applicable law, (a) characterize
any non-principal payment as an expense, fee, or premium rather than as
interest, (b) exclude voluntary prepayments and the effects thereof, and (c)
amortize, prorate, allocate, and spread in equal or unequal parts the total
amount of interest throughout the entire contemplated term of the indebtedness
evidenced by this Note so that the interest for the entire term does not exceed
the Maximum Rate.

        Upon the occurrence of any Event of Default, the holder hereof may, at
its option, (a) declare the entire unpaid principal of and accrued interest on
this Note immediately due and payable without notice, demand or presentment, all
of which are hereby waived, and upon such declaration, the same shall become and
shall be immediately due and payable, (b) foreclose or otherwise enforce any
Lien granted to Lender to secure payment and performance of the Obligations, (c)
offset against this Note any sum or sums owed by the holder hereof to Borrower
and (d) take any and all other actions available to Lender under this Note, at
law, in equity or otherwise. Failure of the holder hereof to exercise any of the
foregoing options shall not constitute a waiver of the right to exercise the
same upon the occurrence of a subsequent Event of Default.

        If the holder hereof expends any effort in any attempt to enforce
payment of all or any part or installment of any sum due the holder hereunder,
or if this Note is placed in the hands of an attorney for collection, or if it
is collected through any legal proceedings, Borrower agrees to pay all
reasonable costs, expenses, and fees incurred by the holder, including all
reasonable attorneys' fees.

        THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS
OF THE STATE OF TEXAS AND THE APPLICABLE LAWS OF THE UNITED STATES OF AMERICA.
THIS NOTE IS PERFORMABLE IN HARRIS COUNTY, TEXAS.

        Borrower and each surety, guarantor, endorser, and other party ever
liable for payment of any sums of money payable on this Note jointly and
severally waive (except as otherwise expressly provided herein or in any other
Loan Document) notice, presentment, demand for payment, protest, notice of
protest and non-payment or dishonor, notice of acceleration, notice of intent to
accelerate, notice of intent to demand, diligence in collecting, grace, and all
other formalities of any kind, and consent to all extensions without notice for
any period or periods of time and partial payments, before or after maturity,
and any impairment of any collateral securing this Note, all without prejudice
to the holder. The holder shall similarly have the right to deal in any way, at
any time, with one or more of the foregoing parties without notice to any other
party, and to grant any such party any extensions of time for payment of any of
said indebtedness, or to release or substitute part or all of the collateral
securing this Note, or to grant any other indulgences or forbearances
whatsoever, without notice to any other party and without in any way affecting
the personal liability of any party hereunder.

        THIS NOTE, AND THE OTHER LOAN DOCUMENTS REFERRED TO HEREIN EMBODY THE
FINAL, ENTIRE AGREEMENT BETWEEN Borrower AND Lender WITH RESPECT TO THE SUBJECT
MATTER HEREOF AND THEREOF AND SUPERSEDE ANY AND ALL PRIOR COMMITMENTS,
AGREEMENTS, REPRESENTATIONS, AND UNDERSTANDINGS, WHETHER WRITTEN OR ORAL,
RELATING TO THE SUBJECT MATTER HEREOF AND THEREOF AND MAY NOT BE CONTRADICTED OR
VARIED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS OR
DISCUSSIONS OF BORROWER AND LENDER. THERE ARE NO ORAL AGREEMENTS BETWEEN
BORROWER AND LENDER.

                                       -7-

<PAGE>

                               OYOG OPERATIONS, LP

                               By: OYOG, LLC, its general partner

                                   By: OYO Geospace Corporation, its sole
                                       member

                                   By:    /s/ Gary D. Owens
                                         -----------------------------
                                   Name:  Gary D. Owens
                                   Title: President

                                       -8-